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  Acoustic Glossary

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Generally, the sensitivity of human hearing is restricted to the frequency range of 20 Hz to 20,000 Hz, with greatest sensitivity centered in the 500 to 8,000 Hz frequency range. Above and below this range, the ear becomes progressively less sensitive. To account for this feature of human hearing, sound level meters apply filtering of acoustic signals according to frequency. This filtering is called A-weighting. Sound pressure level values obtained using this weighting are referred to as A-weighted sound pressure levels and are signified by the identifier dBA.

Absolute pitch.
The exact pitch value of a musical note (for example, middle C) as opposed to its position relative to other pitches.

Acoustic center.
The point in space of the origin of sound. For a sound emitting transducer (e.g., a loudspeaker), the point from which the spherical waves appear to diverge as observed at remote points. (See also acoustic origin.)

Acoustic environment.
The complete set of all objects and their respective physical properties having an influence on the sound field that surrounds a listener. The acoustic environment is a major determinant of perceived sound quality because most of the sound emitted by a source (e.g., a loudspeaker) typically arrives at the listener through a multiplicity of paths. (A single bounce off an object is termed a "first-order" reflection, two bounces a "second-order reflection," and so on.) Each time a sound reflects off an object, the object's material properties affect how much each frequency component of the sound wave is absorbed and how much is reflected back into the environment. Sounds can also pass through objects, including such "substantial" objects as walls, ceilings, floors and windows. An object's material properties and its geometry?its corners, edges, openings, shape, size, etc.?often influence sound in ways more complex than just reflection, including diffraction, refraction and diffusion.

Acoustic origin.
The point in time at which the signal originates. (See also acoustic center.)

Acoustical Masking.
Any sound of 6db+ that masks others of similar frequency.

The measured percentage of Articulation Loss of Consonants by a listener. %ALCONS of 0 indicates perfect clarity and intelligibility with no loss of consonant understanding, while 10% and beyond is growing toward poor intelligibility, and 15% typically representing the maximum loss acceptable. %ALCONS can be measured by acoustic analyzers such as TEF.

In room acoustics, early reflections and reverberation. The audible sense of a room or environment surround a sound source.

A parameter of sound related to the extent of oscillation of a vibrating body, of sound pressure, or of an analog voltage.

Amplitude envelope.
The function describing how the maximum amplitude of a sound waveform evolves over time. The amplitude envelope is often characterized as consisting of four parts: The attack portion (i.e., the part during which the amplitude is rapidly increasing); the decay portion (i.e., the "backside" of the attack, during which the amplitude is rapidly diminishing); the sustain portion (i.e., the part during which the amplitude is relatively stable); and the release portion (i.e., the final part during which the amplitude diminishes into silence).

Amplitude modulation.
A change in amplitude according to a periodic or aperiodic function. If the modulation is done periodically, its effects on the carrier tone can be described in two equivalent ways. The first is by simply describing the result as a repeating change in the amplitude of the carrier. The second is to describe it as a mixture of a fixed intensity carrier with a number of additional fixed intensity tones, called "side bands."

A general term referring to the impairment of musical abilities due to damage to one or both cerebral hemispheres.

Analytic listening.
The ability of a listener to perceptually isolate individual elements of a complex sound or sequence, such as frequency components in a complex sound or individual events in rapid sequences. In synthetic listening the tendency is to perceive sound complexes or temporal sequences in a global fashion.

Literally, without echo. An anechoic chamber is a low-noise, highly absorptive environment, often used in acoustical testing, that allows the direct sound of the device under test (e.g., a loudspeaker) to be measured without contamination from reflections off the chamber's walls, floor or ceiling.

(See Isochrony.)

A general term referring to the impairment of language abilities following damage to the left hemisphere of right-handed people.

Apparent motion.
If two lamps at two different locations in space are flashed in close succession, the viewer obtains an impression of motion between them.

Apparent source width (ASW).
Discovered and developed by A. H. Marshall, ASW is a subjective parameter of spaciousness in concert halls, and is related to the level, at the listenerís ears, of lateral reflections in the first 50 to 80 milliseconds after the arrival of the direct sound. Increasing the ratio of this reflected energy to the direct sound increases the sense of spaciousness. Narrow, rectangular, ìshoebox-shapedî halls like the famous Musikvereinsaal in Vienna and Symphony Hall in Boston tend to foster strong, early-arriving reflections from the side walls, subjectively broadening the sound source and imparting body and fullness to the music.

(From the Italian appoggiare meaning to learn.) A short-duration tone that is a neighboring note (a semitone or whole tone higher or lower) of the principal note which it precedes.

(See chord.)

Articulation loss of consonants.
A measure of speech intelligibility. The percentage of consonants heard incorrectly, strongly influenced by noise or excessive reverberation. (See also %ALCONS.)

A type of very soft noise appearing in speech sounds. It occurs in the phoneme "h" in English, or with less duration after the release of an unvoiced consonant, for example, after the "p" in "pie."

(See tonal system.)

(See amplitude.)

Attenuate, attenuation.
The lessening of sound signal level due to divergence, absorption, reflection, refraction, diffraction, etc., typically expressed in decibels.

Auditory agnosia.
A general term referring to impairments in recognizing auditory objects, events, and sequences that usually follow damage to both temporal lobes.

Auditory beats.
The sensation of periodic fluctuation that results when two simultaneous components are very close to one another in frequency. Listeners hear the fluctuation pattern as consisting of beats when their auditory system lacks enough frequency resolution to distinguish the component frequencies.

Auditory environment.
The perceived image of the acoustic environment; the way the acoustic environment is perceived. (See also virtual auditory environment.)

Auditory event.
What is auditorially perceived, in contrast to a sound event, which is a physical phenomenon of vibrations and waves in air or other elastic medium. The relationship between sound events and auditory events is the subject of psychoacoustics.

Auditory Gestalts.
(See Gestalt.)

Auditory stream.
A mental description of a physical (or virtual) sound source and its behavior through time. Auditory stream segregation refers to the process of perceptual organization of sound that accomplishes the construction of this description.

Auditory tube.
(See Eustachian tube.)

Augmented triad.
(See chord.)

The technique of using computer-based mathematical models of an acoustic environment and 3-D sound processing methods to make audible the sound field of a source in the modeled space. Somewhat analogous to building and viewing a scale model of a contemplated building, auralization enables an acoustician or sound designer to build a computer model of a listening space and then "play" the room's sound through headphones. See also article, "Virtual Acoustic Reality."

Backward recognition masking (also called informational masking).
The reduction in the ability to recognize a sound pattern due to the subsequent presentation of another sound pattern with similar information content. This kind of masking is thought to result from a process different from that or normal (sensory) masking.

Basilar membrane.
The organ of hearing. More specifically, a membrane that runs the length of the cochlea which is a bony, fluid-filled spiral in the inner ear. The basilar membrane performs a kind of frequency analysis of the incoming acoustic signal: different locations along the membrane vibrate preferentially in response to different frequencies. The hair cells connected to each part of the membrane thus preferentially send neural information about the presence of those frequencies to the brain. The spatial pattern of activity along the basilar membrane thus encodes the frequency content of the signal.

(See auditory beats.)

Beat period.
(See tempo.)

Blind spot.
(See scotoma.)

In the home entertainment context, pertaining to presentations involving the visual and auditory sensory modalities.

Pertaining to two ears. A presentation of sound is binaural when both ears are presented with the sound. Binaural sound also refers to a specific sound playback technology, used mainly in headphones-based research and virtual reality applications, in which an individual's HRTFs are determined and synthesized to enable 3-D auditory experiences that are indistinguishable from reality, or nearly so.

BR (bass ratio).
In concert hall acoustics, the ratio of the average reverberation times at 125 and 250 Hz to the average of the RT's at 500 and 1000 Hz. It is determined only for a hall when fully occupied.

In concert hall acoustics, a bright, clear, ringing sound, rich in harmonics, is called ìbrilliant.î In a brilliant sound the treble frequencies are prominent and decay slowly. This means that the high frequencies are diminished only by the natural absorption of the sound in the air itself.

C80(3) or clarity factor.
In concert hall acoustics, the ratio, expressed in decibels, of the energy in the first 80 milliseconds of an impulse sound arriving at a listener's position divided by the energy in the sound after 80 milliseconds. The divisor is approximately the total energy of the reverberant sound. The symbol (3) indicates the average of the C80 values in the 500, 1000 and 2000 Hz bands. . More generally, clarity refers to the degree to which the separate strands in a musical performance perceptually stand apart from one another; see also definition.

In musical research, a unit of pitch change equal to 0.01 semitones.

The simultaneous sounding of a group of notes, usually three or more. In Western music, chords of three notes consisting of the first, third and fifth degrees of a scale are called triads. Major triads consist of intervals of a major third (four semitones) and perfect fifth (seven semitones) with respect to a reference pitch (the root). The third is minor (three semitones) in a minor triad. The third is major and the fifth is augmented (eight semitones) in an augmented triad. The third is minor and the fifth is diminished (six semitones) in a diminished triad. When the notes of a chord are played in ascending or descending succession, the melodic figure is called an arpeggio.

Chromatic scale.
(See scale.)

Going around the perimeter of the pinna; said of certain headphones.

The snail-shaped cavity, approximately 1-1/4 inches long, 3/8 inches wide and 2 inches high, in the temporal bone that contains the basilar membrane which is the organ of hearing.

Cocktail party effect.
A form of auditory stream segregation by which a listener's ability to localize sound sources (see localization) can increase intelligibility. So called because at a cocktail party, a listener can focus on and understand a conversation while dozens or even hundreds of other conversations occur all around. If a conventional 2-channel high-resolution recording were made and subsequently played back, the listener would not be able to understand individual conversations because they have been spatially blended into the two speakers.

Comb filter.
A sequence of evenly spaced peaks or dips in the frequency response when viewed on linear scale caused by two or more identical signals which combine at near equal amplitudes but at slightly different time intervals. So called because the frequency response plot resembles the teeth of a comb.

Combinatorial explosion.
A problem for virtual reality designers (who by definition must add interactivity to immersiveness), because when a person can probe environment, the VR designer must provide for nearly infinite possible simulations.

Complex tone.
A tone composed of two or more pure tones. (See also spectrum.)

In acoustics, the portion of a sound wave in which air molecules are pushed together, forming a region with higher-than-normal atmospheric pressure. The opposite of rarefaction. In audio signal processing, the reduction in dynamic range caused by a compressor.

(See pinna.)

Connectionist network.
(See neural net.)

Critical band.
A range of frequencies surrounding the frequency of a designated pure tone. When other pure tones whose frequencies are within this range are played at the same time as the designated tone, the auditory system does not hear the two completely independently. The designated tone may be masked (see masking), beats may be heard, or other forms of interaction may occur. The size of the critical band increases for higher frequency tones, ranging from about 100 Hz for low-frequency tones to above 2 kHz for very high ones.

Critical distance.
The distance from a sound source at which direct sound and reverberant sound are at the same level.

Abbreviation of decibel.

See A-weighting.

(See amplitude envelope.)

A unit of the intensity of sound. The decibel (abbreviated dB) is a relational measure, expressing the relative intensity of the described sound to a reference sound. The decibel is a logarithmic measure, specifically 10 times the logarithm of the ratio of two voltages, currents or sound pressures. A difference of 20 dB between two sounds means that the more intense one has 10 times the amplitude (100 times the power) of the softer. A single decibel is commonly thought to be the smallest change in sound pressure level that the trained human ear can detect.

A trademark term of Keith Yates Design Group referring to highly immersive entertainment characterized by the depth and multiplicity of sensory modalities presented to the audience.

In concert hall acoustics, definition, like clarity, refers to the degree to which individual strands in a musical presentation can be differentiated from each other. There are two kinds of definition: horizontal, which applies to tones played in succession; and vertical, in which tones are played simultaneously. Horizontal definition refers to the degree to which sounds that follow one another stand apart. Composers can specify certain musical factors that determine the horizontal definition, such as tempo, repetition of tones in a phrase, and the relative loudness of successive notes. Performers can vary the horizontal definition by the manner they choose to phrase a passage. Acoustical factors that affect horizontal definition are the length of the reverberation and the ratio of the loudness of the early sound to that of the reverberant sound--the same two factors that determine fullness of tone, but in inverse relation. Vertical definition refers to the degree to which sounds that occur simultaneously are heard separately. Composers specify vertical definition by choosing simultaneous tones and their relation to the tones surrounding them, and the choice of instruments on which theyíre played. Performers can alter vertical definition by varying the dynamics of their simultaneous sounds and through the precision of their ensemble. Acoustical factors such as the energy ratio of early sound to reverberant sound also affect vertical definition.

The bending of a wave front around an obstacle in the sound field.

Diffuse field.
Sound field in which the sound pressure level is the same everywhere and the flow of energy is equally probable in all directions.

The spatial and/or temporal scattering of sound energy.

An acoustical device designed to spread sound reflections.

Diminished triad.
(See chord.)

Directional transfer function (DTF).
(See head-related transfer function.)

Directivity factor (Q).
The ratio of the sound pressure squared, radiated directly ahead of a sound source, to the sound pressure squared radiated in all directions.

The perception of fine distinctions or differences between stimuli.

In Western tonal music, the fifth degree of the diatonic scale or the triad (see chord) built on it. This is an important degree from the standpoint of the tonal hierarchy since, as its name indicates, it dominates the other degrees (excepting the tonic). (See also tonal system.)

Duplex theory.
(See localization.)

Early decay time (EDT).
In concert hall acoustics, the measurement, expressed in seconds, taken in the same fashion as reverberation time except that EDT is the time it takes for a signal to decay from 0 to -10 dB relative to its steady-state value. A multiplying factor of 6 is necessary to make the EDT time comparable to RT. Short decay times cause music and speech to sound dry or muffled. Long decay times make speech difficult to understand or even unintelligible. ,

An individual's unique head-related transfer function (HRTF), typically derived for each ear by placing a tiny probe microphone inside the meatus, placing a loudspeaker at a known location relative to the listener, playing a test signal through the loudspeaker and recording the microphone signal. By comparing the original test signal to the signal received by the probe microphone, the filter function of a sound source at that position, and for that ear, is known. The loudspeaker is then moved to another location and the process is repeated until an entire, spherical map of filter sets has been devised.

A sound wave which has been reflected or otherwise returned with sufficient magnitude and delay (typically >90 milliseconds) to be perceived as distinct from that directly transmitted.

Echoic memory.
A hypothetical preperceptual sensory register within which auditory information is temporarily stored without being recorded. The function of this memory would be to preserve sensory information during the time needed for higher-level processing mechanisms to extract useful information. Echoic memory does not last more than a few seconds. It corresponds to iconic memory in the visual modality.

Energy-Time Curve (ETC).
In TEF measurements, a display of all the energy returned during a specified time span. Time is displayed on the abscissa (x axis) and energy on the ordinate (y axis). An ETC reveals how energy is released from a system or room or device after it is hit with a sudden application of input energy confined to a given frequency band.

In concert hall acoustics, envelopment is the second component of spaciousness, and generally describes a listener's impression of the strength and directions from which the reverberant sound appears to arrive. Listener envelopment (abbreviated LEV) is judged highest when the reverberant sound seems to arrive at a person's ears equally from all directions--forward, overhead and behind.

eQuake (tm).
Trade name of an infrasonic floor-motion system developed by Keith Yates and produced and marketed under the Immersive Technologies brand name. The eQuake system relies on several proprietary elements, including a real-time processor that takes a subwoofer-output audio feed from a surround-sound processor, and synthesizes a sub-20 Hz signal that is output to a below-floor excitation system. The eQuake is the world's first residential system to add realistic, infrasonic, haptic content to conventional audio/video playback, thereby marking the transition from a bimodal to a trimodal sensory experience.

Eustachian tube.
Also known as the auditory tube, the Eustachian tube is an approximately 1-1/2 inch long conduit that serves to equalize air pressure on both sides of the tympanic membrane (eardrum), and to allow for drainage of the middle ear by serving as a portal into the nasopharynx (a region of the alimentary canal).

False alarm.
Judging that a signal is present when it is not or that a change occurred when none did. Also called a false-positive response.

Far field.
The distribution of sound energy at a very much greater distance from a sources than the linear dimensions of the source and in which the sound waves can be considered to be plane waves.

A device that can change the relative amplitudes and phases of the frequency components in the spectrum of a signal. A high-pass filter attenuates low frequencies and lets the high ones pass through. A low-pass filter does the opposite.

Fission boundary.
(See temporal coherence boundary.)

Flutter echo.
In room acoustics, a series of specific reflective returns caused by large surfaces being parallel to each other.

Flutter sense.
(See mechanoreceptor.)

(See resonance structure.)

Fourier analysis.
A mathematical analysis of waves, discovered by the French mathematician Fourier (1768-1830). Fourier proved that any periodic sound, or any non-periodic sound of limited duration, could be represented (Fourier analysis) or created out of (Fourier synthesis) the sum of a set of pure tones with different frequencies, amplitudes and phases.

Fourier transform.
A mathematical description of the relationship between functions of time and corresponding functions of frequency; a map for converting from one domain to the other. For example, if we have a signal that is a function of time--an impulse response--then the Fourier Transform will convert that time domain data into frequency data, for example, a frequency response.

The central portion of the retina where visual acuity, or the ability to distinguish small objects and details, is greatest. Only about half a millimeter in diameter, the fovea is the retina's "rod-free zone" and is densely packed with cones. (See also retina.)

Free field.
An environment in which there are no reflective surfaces within the frequency region of interest.

A measure of the rate at which something repeats. This term usually refers to the repetition rate of a periodic waveform and is expressed in Hz (cycles per second) or kHz (thousands of cycles per second). The period is the inverse of frequency, or the amount of time a single cycle lasts. (See also harmonicity.)

Frequency component.
(See harmonic.)

A speech sound produced by frication, that is, by forcing air through a constriction in the vocal tract. Examples are "s" and "f."

Fundamental frequency.
(See harmonic.)

G (strength factor).
In concert hall acoustics, the ratio, expressed in decibels, of the sound energy at a seat in a hall that comes from a non-directional source (usually located successively at one to three difference positions on the stage) to the sound energy from the same source when measured in an anechoic room at a distance of 10 meters. G is measured in six frequency bands: 125, 250, 500, 1000, 2000 and 4000 Hz.

Gmid (mid-frequency strength factor).
Same as G (strength factor), except that the decibel levels are the average of the G's measured in the 500 and 1000 Hz bands.

Glow (low-frequency strength factor).
Same as G (strength factor), except that the decibel levels are the average of the G's measured in the 125 and 250 Hz bands.

>From the German word for "form" or "shape." The central idea of Gestalt psychology is that the properties of a whole form cannot be derived by simply summing the properties of its individual parts. The constitution of these forms obeys the perceptual laws (or principles) that were demonstrated for visual perception by the Gestalt psychologists in the early decades of the 20th century, but which have in general been confirmed for auditory perception as well. These principles include the grouping into forms of elements on the basis of their proximity, similarity, continuity, symmetry and closure. A configuration of elements that obeys one or more of these principles may be considered to be "well formed" and as such is a preferred way of experiencing the sensory input. (See also auditory stream.)

In concert hall acoustics, if the side walls or the surfaces of hanging panels are flat and smooth and are positioned to produce strong early sound reflections, the sound from them may take on a brittle or harsh quality, analogous to optical glare. Acoustical glare can generally be prevented by adding irregularities to these surfaces or by curving them. In the 18th and 19th centuries, fine-scale irregularities on sound-reflecting surfaces were provided by baroque carvings or plaster ornamentation.

(See auditory stream, segmentation.)

Hair cells.
(See basilar membrane.)

Pertaining to the sense of touch, from the Greek word haptein, to grasp. There are four types of sensory neurons (mechanoreceptors) involved in the haptic modality. The haptic, or tactile, sensory modality is the only active sense that can be used to explore our environment; vision and hearing are passive senses since they cannot act upon the environment [no e-mail regarding the Heissenberg Uncertainty Principle, please!].

One component (or partial, or overtone) of a complex tone whose component frequencies are all integer multiples of a common fundamental frequency (see frequency). The intervals between components of the harmonic series are defined by harmonic ratios (i.e., ratios of simple integer numbers). The term "harmonic ratios" can also be applied to very low frequency rates of repetition as are found in rhythms.

The state of being harmonic or periodic. Periodicity is mathematically synonymous with harmonicity, though the former refers to a regularity in the sound's time description while the latter refers to a regularity in its frequency description. Contrasting terms to this one include inharmonicity or aperiodicity (usually for complex tones composed of inharmonically related partials) and randomness (usually employed to refer to noise waveforms).

Haas effect.
(See precedence effect.)

Head-related transfer function (HRTF).
The frequency response between the point in space where a sound source is located, and the ear, due to anatomical features of the head, upper torso and pinnae. These features shape the response in such a way as to allow the ear to localize a sound source in space. (Also known as head transfer function [HTF], pinnae transform, outer ear transfer function [OETF], and directional transfer function [DTF]. See also localization.)

(See pinna.)

Helmholtz, Hermann von.
Scientist who, during the second half of the 19th century, contributed to our knowledge about almost every topic in the fields of perception and sensory processes. Helmholtz argued that perception was based upon a process of inference, in which, through past experience, we infer from the sensations we receive at a given time the nature of the object or event that they probably represent.

Heschl's gyri.
(See temporal lobe.)

The organization of a set of elements into subsets according to relations of dominance and subordination. Each element of a subset is subordinate to the subset as a whole which itself is subordinate to the superset of which it is an element, and so on. In a strict hierarchy no element can be a member of more than one subset at a given level of the hierarchy.

Abbreviation for head-related transfer function. (See also earprint and localization.)

Abbreviation for heating, ventilation and air conditioning.

Abbreviation of Hertz. (See frequency.)

IACCA (interaural cross-correlation coefficient).
The measure of the difference in the sounds arriving at the two ears of a listener facing the performing entity in a hall. IACC is usually measured by recording on a digital tape recorder the outputs of two tiny microphones located at the entrances to the ear canals of a person or a dummy head, and quantifying the two ear differences with a computer program. IACCA is determined with a frequency bandwidth of about 100 to 8000 Hz and for a time period of 0 to about 1 second. No frequency weighting is used.

The interaural cross-correlation coefficient determined for a time period of 0 to 80 milliseconds. It is the average of the values measured in the three octave bands with mid-frequencies of 500, 1000 and 2000 Hz. It has been shown to be a sensitive measure for determining the apparent source width (ASW) of a performing entity as heard by a person seated in the audience.

The interaural cross-correlation coefficient determined by averaging the values in the 500, 1000 and 2000 Hz bands, for a time period of 80 to 750 milliseconds. It correlates approximately to the state of sound diffusion in a concert hall.

The ability to retrieve from memory a name or concept associated with an object or event.

Abbreviation for inside-the-head localization.

Abbreviation for Impact Isolation Class.

Pertaining to "immersion," or the feeling of being present in a mediated world rather than the immediate physical environment. The success of the phenomenon is thus dependent on the absence of, or the ability to block out, sensory cues associated with the immediate environment (the "real world"), and the degree to which the cues supplied by the mediated world are both deep (i.e., rich in informational content) and broad (i.e., correlated across multiple sensory modalities; see also haptic and eQuake). The mediated environment can be purely fictional or a temporally and/or spatially distant real environment. The question isn't whether the created world is as real as the physical world, but whether the created world is real enough for you to suspend your disbelief for a period of time. The introduction of perspective in painting by Masaccio in the 1420s took a first step toward immersion by creating a sense of depth that integrated the spectator into the pictorial space. But because the medium of painting simulates depth on a flat surface the spectator cannot break through the canvas and walk into the pictorial space. (See also DeepEntertainment(tm).)

Immersive Technologies Corporation.
A California company founded by Keith Yates in 1998 to develop and manufacture and/or license technologies to increase the immersive power of movie and music playback experiences. See also eQuake.

Impact Isolation Class (IIC).
A measure or specification of isolation effectiveness of building structures from impact noises such as slammed doors, dropped objects, footfalls, shuffled furniture, etc. The higher the IIC rating, the better such isolation. Impact noises can be transmitted through walls, floors, and ceilings throughout a building and re-radiated at distant locations. Careful design and special construction materials (floating floors, isolation pads, resilient channels, spring rails, flexible connectors and hangers, for example) can help improve IIC ratings, which may be thought of as the structure-borne equivalent of the airborne noise ratings addressed by STC.

Impulse response.
A measurement of sound pressure versus time, showing how a device responds to an impulse.

Information processing.
A key concept in cognitive psychology. Drawing on the image of the way computers work, information resulting from stimulation of the sense organs is analyzed and transformed by a number of serial and parallel processors (see neural net) each of which takes as input the information output by another processor.

Informational masking.
(See backward recognition masking.)

Pertaining to frequencies below the audible range, i.e., sub-20 Hz. Note: Sound in the 2-5Hz range played at 100-125dB may produce difficulty in swallowing and slight post-exposure headache. Sound in the 2-5Hz range played at 125-137dB may produce chest wall vibration; difficulty in speaking and voice modulation; swaying sensations; lethargy and drowsiness; and post-exposure fatigue and headaches. Sound in the 5-15Hz range played at 125-137dB may produce middle-ear pain; difficulty in speaking and voice modulation; severe chest wall vibration; severe abdomen vibration and associated feelings of nausea; a falling sensation; lack of concentration and drowsiness; tinnitus; and severe post-exposure fatigue and headaches. According to some researchers, 7Hz is possibly the most disturbing frequency, being close to the natural resonance frequency of many of the internal body organs and being the same frequency as the alpha brainwaves. Sound in the 15-20Hz range played at 125-137dB may produce severe middle ear pain; respiratory difficulties (gagging sensations); nasal cavity vibration; persistent eye watering; tinnitus; sensation of fear; excessive perspiration and shivering; and severe post-exposure fatigue and headaches.

Inharmonic tone.
A tone composed of partial that are not all integer multiples of a common fundamental.

Initial time delay gap (ITDG).
See t1.

Inner ear.
The deepest part of the ear. It is contained within a system of spaces and canals, known as the osseous or bony labyrinth, in the temporal bone. These spaces and canals are divided into three sections: the vestibule, which contains two balance organs, the utricle and saccule; the semicircular canals, located behind the vestibule, and the cochlea. The spaces between the bony walls of the osseous labyrinth and the membranous labyrinth are filled with one of several types of fluid, which deliver nutrients to the cells of the inner ear; provide the chemical environment needed for transfer of energy from a vibratory stimulus to a neural signal; and function as the medium to carry vibratory stimuli from the oval window to the sensory structures along the cochlear partition. ,

Inside the head localization (IHL).
(See lateralization.)

The name given to the physical energy with which a sound is present. It contrasts with "loudness," which is the perceptual experience approximately correlated with that physical intensity.

Intimacy (or presence).
In concert hall acoustics, a venue is said to have ìacoustical intimacyî if music played in it gives the impression of being played in a small hall. In the language of the recording and broadcast industries, an intimate hall is said to have "presence." See also t1 (initial time-delay gap).

Intonation, musical.
(See scale.)

A sequence of events is called isochronous if the time separating each pair of successive events is strictly equal. The absence of isochrony is called anisochrony.

Abbreviation for just noticeable difference.

Just noticeable diference (jnd).
The smallest change in a stimulus parameter (frequency, intensity, duration) that can be detected by a listener at a predefined level of performance (e.g., 71 percent of the time). (See also Weber's law.)

Key distance.
Perceptual proximity of the keys of the Western tonal system. Keys sharing more pitches are considered to be more closely related than those with fewer pitches in common.

Abbreviation of kiloHertz. (See frequency.)

Lateral Energy Fraction.
(See LFE4.)

Lateral geniculate body.
A peanut-sized area of the brain to which the output of the retina is sent. Each lateral geniculate body (there are two, one on each side of the brain) routes it output to the visual cortex.

The identification of a sound that is presented over headphones is described as "lateralization" rather than localization in recognition of the fact that sound playback over headphones is generally not "externalized," i.e., it is experienced as coming from somewhere between the two ears rather than from somewhere in the surrounding environment. Lateralization is the identification of the position of the sound on the left-right dimension. Also referred to as inside-the-head localization (IHL). ,

Abbreviation for listener envelopment.

The lateral energy fraction determined by the ratio of the output of a figure-8 microphone with its null axis pointed to the source of the sound, divided by the output of a non-directional [i.e., omnidirectional] microphone at the same position. LFE4 is determined for the time period of 0 to 80 milliseconds and is the average of the LF's in the four frequency bands, 125, 250, 500 and 1000 Hz. It is equal to the ratio of the weighted energy in the sound that does not come from the direction of the source to that which comes from all directions including that of the source. LFE4 also correlates with the apparent source width (ASW).

Listener envelopment (LEV).
In concert hall acoustics, a component of spaciousness referring to a listener's impression of the strengths and directions from which the reverberant sound seems to arrive. Listener envelopment is judged highest when the reverberant sound seems to arrive equally from all directions--forward, overhead, behind.

In concert hall acoustics, a subjective quality related primarily to the reverberation times at the middle and high frequencies, those above about 350 Hz. A hall can sound "live" and still be deficient in bass. If a room is sufficiently reverberant at low frequencies, it is said to sound "warm."

The judgment of the place of spatial origin of a sound. Humans localize sounds based on two primary cues: interaural intensity difference (IID), and interaural time difference (ITD). IID refers to the fact that a sound is louder at the ear it is closer to (the "ipsilateral" ear) for two reasons: because sound intensity diminishes with distance traveled; and because the head itself blocks the sound path to the more distant ("contralateral") ear). ITD refers to the fact that a sound will arrive at the ipsilateral ear before the contralateral ear. Generally speaking, the ear-brain system uses ITD cues to determine the spatial origin of low-frequency sounds, and IID cues to determine the spatial origin of higher frequency sounds. The IID/ITD keys to localization were first proposed by Lord Rayleigh in the first decade of the 20th century, and are sometimes referred to as the duplex theory of localization. About 60 years later researchers discovered that, in addition to IID and ITD information, the brain processes information about the sound source's location based on how its energy has been accentuated or attenuated in the mid- and high-frequency ranges by minute time delays caused by the folds and depressions in the listener's pinnae (and at lower frequencies by the shoulders and upper torso): Because of the pinna's asymmetry, different angles of sound incidence produce different characteristic filtering. (The spectral-shaping influence of the pinnae can be readily verified by trying to localize sound after filling their cavities with putty.) The effect of filtering by the pinna and upper body is termed the head-related transfer function (HRTF) and is unique for each individual, similar to a fingerprint. (In fact, an individual's HRTF is sometimes called his or her earprint.) Localization accuracy in humans is most precise for sound sources located in front of the listener and at ear level. Localization is not simply an auditory process, but includes higher order brain functions which combine learned responses, complex pattern matching, and cross referencing with other senses in the brain, resulting in a unified (though not always correct) perception of the location of a sound source. (See also lateralization and visual capture.) ,

Logarithmic scale.
A scale in which the logarithm of the physical variable is used instead of the raw value. This has the effect that equal steps along the scale represent equal ratios between the raw values. Examples in audition are the decibel scale and the scale of musical pitch.


Major scale.
(See scale.)

Major triad.
(See chord.)

The "hammer" bone of the middle ear.

The process by which one sound (the masker) affects the threshold of audibility of another sound (the target or probe) when played at the same time. More intense sounds mask less intense ones. The amount of masking depends on the proximity of the frequency components (see critical bands, frequency and harmonic) of the two sounds, as well as on the global intensity of the masker. The greater the level, the greater the extent to which a given masker frequency can mask target components at higher frequencies (see backward recognition masking).

Meatus (also called the external auditory meatus).
The ear canal, leading from the concha to the tympanic membrane (eardrum). Approximately 1 inch long, the outer one-third of the meatus is cartilaginous; the remaining two-thirds is bony. Ceruminous (wax) and sebaceous (oil) glands are plentiful in the cartilaginous segment, and are also found on the posterior and superior walls of the bony canal. The wax and oil lubricate the canal and help keep it free of debris and foreign objects.

Mechanoreceptors are the receptors involved in the haptic (tactile) sensory system and come in four distinct types: Merkel's receptors and Meissner's corpuscles, both with relatively small receptive fields and located in the dermal papillae (superficial skin); and pacinian corpuscles and Ruffini corpuscles, both with larger receptive fields and located deeper in the skin, i.e., subcutaneously. The smaller receptive fields of the Merkel's and Meissner's structures allow them to resolve finer spatial details that the pacinian and Ruffini structures. The four mechanoreceptor types respond to different intensity and frequency ranges of mechanical stimuli. Meissner's corpuscles are most sensitive to low-frequency (< 100 Hz) sinusoidal mechanical stimuli; their excitation is felt as a gentle fluttering in the skin, sometimes termed flutter sense. In contrast, pacinian corpuscles are maximally sensitive to higher frequency (50-500 Hz) stimuli, which evoke a diffuse, humming sensation in the deeper tissue. Ruffini corpuscles and Merkel's receptors respond to indentation of the skin. The spatial distribution of mechanoreceptors is not uniform; the densest distribution can be found in the fingertips. (See also sensory experience.)

Meissner's corpuscle.
(See mechanoreceptor.)

Melodic contour.
The pattern of ascending and descending pitch changes in a melody.

Mental representation.
A hypothetical pattern of mental or brain activity that represents some feature of the world, of the person, or of the interaction between the person and the world.

Mental schema.
A mental program or formula that has been proposed by Jean Piaget and other psychologists as a means by which people represent the world and regulate their interactions with it. The concept implies more of an active control mechanism than the concept of mental "representation."

Merkel's receptor.
(See mechanoreceptor.)

The group of phenomena related to the musical measure. It consists of the hierarchical ordering of the piece of music into units of equal duration (beats; see also hierarchy). This ordering is indicated by the time signature at the beginning of the score. From a phenomenological point of view, the presence of a metric organization in the heard piece is evidenced by the fact that one can tap one's foot or dance in synchrony with the music.

Middle ear.
A six-sided cavity between the outer ear and the inner ear, principally containing the ossicles (often called the "hammer" [malleus], "anvil" [incus] and "stirrup" [stapes], the three smallest bones in the body); two muscles, the tensor tympani and the stapedius; and the opening to the Eustachian tube. Sound is transformed at the middle ear from acoustical energy at the eardrum to mechanical energy at the ossicles; the ossicles convert the mechanical energy into fluid pressure within the inner ear via motion at the oval window. ,

Minor scale.
(See scale.)

Minor triad.
(See chord.)

Missing fundamental.
The phenomenon of the "missing fundamental" is one in which the listener, presented with a harmonic tone in which the fundamental is absent, hears the same pitch as would be heard if the fundamental had been present. Therefore, only some of the harmonics are needed to hear the pitch. The pitch that is heard when the fundamental is absent is called periodicity pitch because the period of the wave is the same whether the fundamental is present or not.

Mode, musical.
(See scale.)

Near field.
That part of a sound field, usually within about two wavelengths from a sound source, where there is no simple relationship between sound level and distance.

Neural net.
A system composed of many simple processing units, formally mimicking the operation of nerve cells, which are connected together in complex patterns of excitation and inhibition and propagate activation to other units by way of these connections. The current state of a given unit and the degree to which it excites other units can be influenced by the success it has had in activating them. Propagated activity among cells can lead the system to stable states in which the activity of the units remains relatively constant. These states constitute the "response" of the system to a given stimulation by the (external or internal) environment. The main hypothesis concerning this kind of architecture (also called connectionist or parallel distributed processing networks), is that it is better suited to modeling the microstructure of cognition than more classical data flow or serial processing models: processing, representation and memory are postulated to be distributed over units in the net rather than being constrained to specific storage locations and processing routines.

Neural spectrogram.
(See spectrum.)

A nerve cell. A neuron's job is to take in information from the cells that feed into it; to integrate (sum up) that information; and to deliver that integrated information to the next neuron. The information is usually conveyed in the form of brief nerve impulses. In a given cell, one impulse is the same as any other; they are "stereotyped" events. Impulse rates vary from one every few seconds to about 1000 per second. Anatomically, the nerve cells consists of a globular-shaped cell body with a nucleus, mitochondria and other organelles; a cylindrical-shaped, signal-transmitting nerve fiber called an axon; and a number of branching and tapering fibers called dendrites, typically under one millimeter in length. The entire nerve cell-the cell body, axon and dendrites-is encased in the cell membrane. The cell body and dendrites receive information from other nerve cells; the axon, which may be anywhere from less than a millimeter to more than one meter in length, transmits this information from the nerve cell to other nerve cells. Near the point where they end, an axon typically splits into many smaller branches whose ends come very close to, but do not touch, the cell bodies or dendrites of other nerve cells. At these regions, called synapses, information is conveyed from one nerve cell, called the presynaptic cell, to the next, called the postsynaptic cell. Neural signals originate at a point near where the axon joins the cell body, and travel down the length of the axon, away from the cell body and toward the terminal branches. At a terminal, the information is transferred across the synapse to the next cell or cells by a process called chemical transmission.

A random waveform whose frequency spectrum contains all audible frequencies, called white noise. A noise signal that contains all frequencies with equal energy per octave is called pink noise, commonly used to test loudspeakers. A noise signal that is filtered, removing higher and lower frequencies and just letting through a small band of frequencies, is called narrow-band or band-pass noise. Filtering out the high frequencies starting from a certain cut-off frequency gives low-pass noise. Taking a noise waveform over a certain time period and then repeating this segment gives what is called frozen noise. [1

Noise criteria (NC) curves.
A measure of background noise in rooms. Each NC curve is defined by its sound pressure level at eight octave-band center frequencies: 63, 125, 250, 500 1000, 2000, 4000 and 8000 Hz. The lower the NC rating, the lower the background noise level. The preferred range of NC performance for sound-critical spaces (e.g., home theaters, home media rooms, home listening rooms, concert and opera halls, recital halls and broadcasting and recording studios) is < NC-20. Factors that must be addressed in achieving satisfactory NC performance typically include mechanical (HVAC) design and the construction detailing of the room's envelope, i.e., its walls, ceiling, floor, windows and doors, in order to reduce noise infiltration from areas exterior to the room. (See also Room Criteria, Sound Transmission Class (STC) and Impact Isolation Class (IIC).)

Objective tinnitus.
(See tinnitus.)

One of the pitch intervals in music. Physically, a note that is an octave higher than another has a frequency that is twice that of the lower one.

Octave equivalence.
(See pitch.)

Not directly in front of a microphone or loudspeaker.

Pertaining to the ear; aural.

Inflammation of the ear, which may be marked by pain, fever, hearing abnormalities, deafness, tinnitus, and vertigo.

Outer ear.
The external structure of the ear, consisting of the pinna and meatus.

Outer ear tranfer function (OETF).
(See head-related transfer function.)

(See harmonic.)

Pacinian corpuscle.
(See mechanoreceptor.)

A major clue to the perception of depth in vision, parallax arises from the relative motions of near and far objects that is produced when the viewer moves his or her head up and down or from side to side. See also stereopsis.

Parallel distributed processing.
(See neural net.)

(See harmonic.)

Passing tone. Ornamental notes melodically interleaved between two notes that are part of the triad (see chord) of the principal key.

What the perceiver sees or hears as a result of stimulation, as opposed to the physical reality of the stimulation. The percept may be considered the "object" of study in perceptual psychology.

Perceptual centration.
The fixing of one's gaze for sometimes very short periods of time in specific areas as one explores a visual form. These fixation points constitute the zones of perceptual centration. This term was applied to auditory perception by Frances to designate the auditory information upon which listeners focus their attention at a given moment.

Perceptual invariance.
The impression of perceiving the same object, event or pattern in spite of variations in stimulus structure, due, for example, to being played louder or softer, faster or slower, higher or lower, or in different acoustic environments.

(See frequency.)

The phase is the particular point in a wave that is passing a position in space at a certain instant of time. Phase is measured in units of degrees, with 360 degrees representing one complete cycle of the wave. If two tones have the same period and are occurring at the same time, the temporal lag of one with respect to the other can be described in terms of phase. If two waves are out of phase by 180 degrees, the later one is lagging by one-half a period.

The basic classes of sounds used to form the words of a language. Examples in English are "k," "oo," and "th." They are often represented by single written letters.

Phonemic restoration.
A hypothetical active process by which a speech sequence that is interrupted by a noise sound in place of a given phoneme results in the listener's impression of having heard the phoneme. This effect does not occur if a silent gap is left at the place where the phoneme normally occurs.

Pink noise.
(See noise.)

The external, visible, largely cartilaginous appendage of the outer ear. Its perimeter is demarcated by a ridge-like rim called the helix, which curves down to the earlobe (lobule) at its bottom. Roughly in the middle is a relatively large, cup-shaped depression called the concha.

Pinna transform.
(See head-related transfer function.)

The auditory attribute on the basis of which tones may be ordered on a musical scale. Two aspects of the notion of pitch can be distinguished in music: one related to the frequency (or fundamental frequency) of a sound (measured in Hz) which is called pitch height, and the other related to its place in a musical scale which is called pitch chroma. Pitch height varies directly with frequency over the range of audible frequencies. This "dimension" of pitch corresponds to the sensation of "high" and "low." Pitch chroma, on the other hand, embodies the perceptual phenomenon of octave equivalence, by which two sounds separated by an octave (and thus relatively distant in terms of pitch height) are nonetheless perceived as being somehow equivalent. This equivalence is demonstrated by the fact that almost all scale systems in the world in which the notes are named assign the same names to notes that are roughly separated by an octave, i.e., the labeling system cycles at every octave. Thus pitch chroma is organized in a circular fashion, with octave-equivalent pitches considered to have the same chroma. Chroma perception is limited to the frequency range of musical pitch (50-4000Hz).

Polar ETC.
In TDS acoustical measurements, Polar Energy-Time Curves (ETC) measure the magnitude and time of arrival of reflections, and, importantly, display the direction of the reflecting surface relative to the microphone placement. Polar ETC's can thus allow the operator to pinpoint the location of one or many reflecting surfaces in a concert hall, auditorium, theater, recording studio or residential playback venue.

Polar pattern.
The characteristic sound radiation pattern of a microphone and loudspeaker, usually plotted to show sound sensitivity or output, respectively, at various angles of sound incidence.

The positive or negative direction of an electrical, acoustical or magnetic force. Two identical signals in opposite polarity are 180 degrees apart at all frequencies. Polarity is not frequency dependent.

Precedence effect.
An effect in which the human auditory system suppresses early reflections of a direct sound, i.e., it "fuses" the direct sound and its early reflections and localizes the source on the basis of the earlier (i.e., direct) sound. The basis for the distinction is that the reflections arrive with a certain delay compared to the direct sound. Precedence effect is sometimes referred to as the law of the first wavefront or the Haas effect.

Gradual and biologically normal loss of acute hearing with advancing age.

Primary auditory cortex.
(See temporal lobe.)

The travel of sound waves through a medium (e.g., air).

The sense of body position.

A notion introduced by Rosch to designate an abstract representation of a whole class of objects, of which the prototype would constitute the central tendency.

The study of the relationship between physical measures of sound (e.g., amplitude and frequency) and the perception of them.

Pure tone.
A tone with a sinusoidal waveform is called a pure tone because it is considered to be the simplest form of tone and sounds pure when played in isolation.

(See directivity factor.)

The portion of a sound wave in which air molecules are spread apart, forming a region with lower-than-normal atmospheric pressure. The opposite of compression.

Recency effect.
An increase in correct recall rate for the most recently presented items of a list compared with those presented earlier in the list.

The impression that an object, event or sequence has been experienced before or is familiar.

In acoustics, the bouncing or return of a sound wave from an object larger than one-quarter wavelength of the sound. When the object is one-quarter wavelength or slightly smaller, it also causes diffraction of the sound.

The change in direction of a sound wave that occurs when sound passes from one medium to another (e.g., from air to glass to air, or through layers of air with different temperatures.

Relative phase.
The phase of one sine wave compared to another.

Relative pitch.
(See absolute pitch.)

Resonance structure.
A resonance structure can be described in terms of the relative level produced at each frequency by a resonating object. Most physical objects (membranes, bars, air columns, strings) have several modes of vibration that resonate at different frequencies, thus constituting a complex resonance structure. In the case of speech, these resonance regions are called formants. The placement of the formants is a major clue to the identity of a vowel. The way resonant frequencies change rapidly over time is a clue to the identity of several classes of consonants.

Technically a part of the brain and located on the inner surface of the eyeball, the retina translates light into nerve signals, which are then routed via the optic nerve to the lateral geniculate body. The retina consists of three layers of nerve-cell bodies. The layer at the back of the retina contains roughly 125 million light receptors, the rods and cones. Rods, which considerably outnumber cones, are responsible for our vision in dim light and are out of commission in bright light. The three types of cones do not respond to dim light but are responsible for our ability to see fine detail and for color vision; cones are "tuned" to absorb long, medium or short wavelengths of light, loosely corresponding to red, green and blue. The distribution of rods and cones varies considerably over the surface of the retina; in the center, where fine-detail vision is best, is the fovea, which is densely packed with cones. The retina's middle layer contains three types of nerve cells: bipolar cells, which receive input from the receptors (i.e., rods and cones); horizontal cells, which link receptors and bipolar cells; and amacrine cells, which link bipolar cells and retinal ganglion cells. The layer at the front of the retina contains approximately 1 million of the aforementioned retinal ganglion cells, whose axons pass across the surface of the retina, collect in a bundle, and leave the eye to form the optic nerve.

Reverberant sound field.
A sound field made of reflected sounds in which the time average of the mean square sound pressure is everywhere the same and the flow of energy in all directions is equally probable. This requires an enclosed space with essentially no acoustic absorption, e.g., a reverberation chamber.

In concert hall acoustics, reverberation refers to sound that persists in a venue after a tone is suddenly stopped. A hall that is reverberant is called a "live" hall. (See also liveness.) A room that is not reverberant is called a "dead" or "dry" room.

Reverberation time (RT).
Defined as the time, multiplied by a factor of 2, that it takes for the sound in a hall to decay from -5 to -35 dB below its steady-state value. The factor of 2 is necessary because RT must conform to the original definition of sound decay which was from 0 to -60 dB. Roughly speaking, RT is the time it takes for a loud sound to decay to inaudibility after its source is cut off. RT is usually measured in octave or one-third octave bands. The source of sound may be a pink noise or a sound impulse. Originally, RT was determined from a plot of sound pressure level vs. time as recorded on the moving paper of a graphic level recorder. Today it is determined by the Schroeder (1965) method which involves computer integration of a backward-played tape recording of the decaying signal. The mid-frequency RT is the average of the RTs at 500 and 1000 Hz. The measurement is generally made in both occupied and unoccupied halls, at two positions when occupied or at 8 to 24 positions when unoccupied. The data in each frequency band at the various positions are averaged. A least-squares fit to the -5 to -35 dB portion of the decay curve is used in setting the value of RT for each band and position. The RTs of the largest stone cathedrals can be nearly 10 seconds; the world's most renowned concert halls typically fall in the range of 1.8 to 2.2 seconds; opera houses typically fall in the 1.2 to 1.6 second range; aggressively damped home theaters can exhibit RTs below 0.25 seconds. A venue's use and its RT must be consonant: A home theater with a 6 second RT would render movie dialog unintelligible, while a cathedral with a 0.3 second RT would deflate its sonic grandeur. ,

Rhythm pattern.
A sequence of events having a specific set of time intervals between the onsets of successive events. Sequences having different onset-to-onset intervals are said to have different rhythmic structures or temporal structures.

Rise time.
The time taken for a signal to rise from silence to full intensity. The tones of different instruments can be distinguished by their rise time, the tones of percussive instruments like the piano rising very rapidly and others like the tuba, more slowly. In music, "rise time" is called "attack" (see amplitude envelope). ,

Room criteria (RC) curves.
A measure or specification of background noise from HVAC systems according to measured sound pressure level at 10 octave-band center frequencies: 16, 31.5, 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz. Room Criteria curves were derived for use in office spaces and are more demanding than Noise Criteria curves at low frequencies.

Room modes.
Frequencies at which sound waves in a room resonate (in the form of standing waves), based on the room dimensions.

(See chord.)

Root mean square (rms).
The effective DC voltage of an AC signal. The square root of the mean value of the squares of the instantaneous values of a varying quantity.

(see Reverberation time.)

Ruffini corpuscle.
(See mechanoreceptor.)

In acoustics, a unit of absorption equal to the absorption of 1 square foot of surface which is totally sound absorbent. Named after Wallace Clement Sabine, the Harvard professor honored as the "father of architectural acoustics" for his investigations into concert hall sound at the turn of the century.

The normal, but largely unnoticed rapid darting of the eyes from one fixed point to another.

Scale, musical.
A set of pitches (or notes) arranged with certain intervals among them within the span of an octave (see also pitch). The scale pattern generally repeats in each octave. Each note constitutes a degree of the scale. Each diatonic scale consists of intervals between adjacent notes that are either minor or major seconds (one or two semitones, respectively). The different arrangements of major and minor seconds yield different modes. The two most important modes in Western tonal music are the major and minor modes. The chromatic scale contains all twelve semitone steps within an octave. Another kind of scale which does not fall within the tonal system but which was used extensively in the music of Debussy and Ravel is the whole-tone scale, which has only six notes, all separated by whole tones. Intonation (or tuning system) refers to the exact tuning of the notes of a given scale system. The most widely used tuning system in Western music is the equal-tempered system in which all intervals can be expressed as integer multiples of a standardized semitone. This system was brought to Europe from China and adopted during the 17th century.

Schroeder integration of reverberation.
In acoustics, an integration of reverberant data in which the last energy is integrated first and the initial arrival is integrated last, all of which is normalized by the total. The integration simulates the effect of taking many time measurements and averaging them together.

The "blind spot" in human vision corresponding to the region where the optic nerve enters the eye, i.e., the oval-shaped area about 2 millimeters in diameter with no rods or cones. You can "map" your blind spot simply by closing one eye and gazing at a small object across the room. Hold a Q-Tip at arm's length directly in front of the object and slowly move it out to the right exactly horizontally. The white cotton will vanish when it is about 18 degrees out. Now, if you place the stick so that it runs through the blind spot, it will appear as a single, continuous stick, without any gap. (This feature is referred to as "completion.") You are not normally aware of your blind spot, and cannot be, unless you test for it. You don't see black or white or anything there; you see nothing. ,

The process by which speech signals are divided into phonemes, syllables or words. It consists of creating boundaries between groups of elements. In music, segmentation refers to the process of dividing an event sequence into distinct groups of sounds. The factors playing a role in segmentation are similar to the principles of grouping addressed by Gestalt psychology.

The smallest standard musical interval (i.e., step in pitch) in the Western equal-tempered pitch system (see scale). All other intervals can be described as containing an integer number of semitones, e.g., the octave contains 12 semitones, the perfect fifth 7 semitones, etc. A tone that is a semitone higher than another is approximately 6 percent higher in frequency. There is a semitone separation between any black key on the piano and its nearest white neighbor or between adjacent white keys that have no black keys between them.

Sensory experience.
Sensory experiences occur when stimulus energies excite one or more types of receptor neurons, of which there are five specialized types in animals: chemoreceptors, mechanoreceptors, thermoreceptors, photoreceptors and nociceptors. These receptors transduce (change) the form of input energy into a neural (electro-chemical) signal. A single photon or micrometer of mechanical displacement is sufficient to excite photoreceptors in the retina or mechanoreceptors in the skin, respectively. Receptors selectively relay certain features of the stimulus to the central nervous system. Individual receptors are tuned to one or several stimulus features. Localization of a sensation is a function of the size of the receptive field of the receptor. The duration of a sensation is related both the duration of the stimulus and the perceived intensity. The intensity of a sensation is mediated by two mechanisms: Stimuli of increasing intensity evoke progressively more activity in a receptor, and recruit additional receptors with higher activation thresholds.

Signal-to-noise ratio (S/N).
The ratio in decibels between signal and noise. An audio component with a high signal-to-noise ratio has relatively little background noise accompanying the signal; a component with a low signal-to-noise ratio is noisy.

Sine wave.
The simplest form of periodic wave motion, expressed by the equation y = sin x, where x is degrees and y is voltage or sound pressure level. All other forms can be created by adding (mixing) a number of sine waves. The wave form of a "pure tone" is a sine wave.

Having the shape of a sine wave.

In acoustics, a unit of loudness. Defined as the loudness of a 1000 Hz tone 40 dB above threshold. A millisone is one-thousandth of a sone and is often called the loudness unit.

Energy that transmitted by pressure waves in air or other materials and is the objective cause of the sensation of hearing. Longitudinal vibrations in a medium in the frequency range 20 Hz to 20 kHz.

Sound Transmission Class (STC).
In acoustics, a single number rating for describing sound transmission loss of a wall or partition.

In concert hall acoustics, a hall is said to be "spacious" if the music performed in it appears to the listener to emanate from a source wider than the visual width of the actual source, and if the listener is noticeably enveloped by the reverberant sound. The former attribute is often referred to as apparent source width (ASW); the latter attribute is often referred to as listener envelopment (LEV). ,

Spectral envelope.
(See spectrum.)

(See spectrum.)

A description of the frequency content of a sound waveform, usually presented as a graph with frequency on the abscissa (x axis) and amplitude on the ordinate (y axis). A pure tone would have a single vertical line at the appropriate frequency with a height indicating its amplitude. A complex sound (see complex tone) would have several such lines, indicating the multiple components. Drawing a curve through the tops of the lines would describe the spectral envelope. A spectrogram is another representation of a spectrum in which the time component is reintroduced: time is represented on the abscissa, frequency on the ordinate, and amplitude is coded as the darkness of the trace at a given frequency and time. In an auditory neural spectrogram, instead of a continuous signal, the probability of occurrence of nerve spikes at a given moment in time is represented. The frequency axis is replaced by a frequency-specific auditory nerve channel (see basilar membrane). A third type of spectral representation called a time-frequency perspective plot is drawn in three dimensions, with time along the x axis, amplitude along the y axis, and frequency along the z axis.

Specular reflection.
A mirror-like reflection of sound from a flat surface; reflections that do not spread out.

Speech intelligibility.
A measure of sound clarity that indicates the ease of understanding speech. It is a complex function of psychoacoustics, signal-to-noise ratio of the sound source, and direct-to-reverberant energy within the listening environment.

Speed of sound.
In air, approximately 1130 feet per second at 20 degrees Centigrade.

Square wave.
A square wave is one in which there are only two values of the displacement of the wave from the neutral position, a positive displacement and an equally large negative displacement. The wave moves instantaneously form one state to the other and remains equally long in each state. Its spectrum contains odd harmonics only, whose intensities are inversely proportional to the harmonic number.

In concert hall acoustics, the measure of the degree of support that the hall, including the walls and ceiling of the hall and of the enclosure immediately surrounding the players, give to the players on stage. It is the difference, in decibels, between the impulse sound energy from an omnidirectional sound source that arrives at a player's position within the first 10 milliseconds, measured at a distance of 1 meter from the sound source, and that which arrives in the time interval between 20 and 100 milliseconds at the same position. The sound arriving in the later interval has been reflected from one or more surfaces surrounding the player's position on the stage, and its strength, minus the strength of the sound in the first 10 milliseconds, is made with the chairs, music stands and percussion in place, except that those near the source and receiver are set aside. The measurements are made at several positions and the data are averaged.

Standing wave.
In acoustics, an apparently stationary waveform created by multiple reflections between opposite room surfaces. At certain points along the standing wave, the direct and reflected waves cancel, and at other points the waves add together or reinforce each other. These are sometimes called room modes.

The smallest muscle in the body, located in the middle ear. Contraction of the stapedius pulls the stapes, altering the mechanical efficiency of the ossicular chain. ,

(See Sound Transmission Class.)

Steady state.
(See amplitude.)

Stereopsis. The most important mechanism for assessing depth in human vision. First enunciated in 1838 by Sir Charles Wheatstone (who also invented the "Wheatstone bridge" in electricity), stereopsis depends on the slight differences in the two pictures projected on the retinas. (See also parallax.)

Synthetic listening.
(See analytic listening.)

t1 (initial time-delay gap or ITDG).
In concert hall acoustics, the time interval, measured in milliseconds, between the arrival at a seat in the hall of the direct sound from a source on stage to the arrival of the first significant reflection. It corresponds with the subjective impression of "intimacy."

Abbreviation for Time Delay Spectrometry.

A computer based platform for measuring audio devices and acoustic environments, manufactured by Techron and more recently Goldline under license from the Jet Propulsion Laboratory, Pasadena, California. See also Time Delay Spectrometry.

The speed of occurrence of the beats for a given metric structure. In a musical score, the tempo is specified in terms of the number of metric units per minute, for example, quarter-note = 60, in which the time value of each quarter-note is 1 second. The inverse of tempo, the time between beats, is called the beat period.

An adjective meaning "pertaining to time."

Temporal acuity.
The degree to which the auditory system can resolve, or separately distinguish, events separated by extremely brief time periods.

Temporal coherence boundary.
Defines the threshold for hearing a repeating two-tone sequence as composed of a single auditory stream across a range of frequency differences between the tones and rates of tone presentation when the listener is trying to hear a single stream. Above the boundary, the sequence is always heard as two streams. Below it, the sequence may be heard as a single stream. This boundary is contrasted with the fission boundary, which defines the threshold for hearing the same kind of repeating sequences when the listener is trying to hear two separate streams. Above the fission boundary, the sequence may be heard as two streams, but below it the sequence is always heard as a single stream.

Temporal lobe.
A region of the lateral part of cortex (just center of and slightly behind the ears) concerned with audition and containing primary auditory cortex (i.e., the first cortical area to which auditory signals are relayed, also known under the name of Heschl's gyri).

Temporal structure.
(See rhythm pattern.)

Tensor tympani.
Like the stapedius, a small muscle in the middle ear. Contraction of the muscle increases the stiffness of, and thus lessens the amount of energy conducted by, the ossicular chain. Though to a significantly lesser extent than the stapedius, the tensor tympani is involved in acoustic reflex, which is the automatic, protective response of the intratympanic muscles to intense sound stimulation. ,

3-D display.
In TDS measurements, the 3-D display shows the change in magnitude/frequency response versus time for a number of individual TDS sweeps. Each sweep is offset in time by a constant amount and on the screen form a three-dimensional surface display sometimes called a "waterfall" plot. The three dimensions are time, energy and frequency.

A division of Lucasfilm, San Rafael, California. Also, a set of specifications for the enhancement of sound playback in the residential environment.

Also referred to as sound quality or sound color. The classic negative definition of timbre is: the perceptual attribute of sound that allows a listener to distinguish among sounds that are otherwise equivalent with respect to pitch, loudness, and subjective duration. Contemporary research has begun to decompose the attribute into several perceptual dimensions of a temporal, spectral, or spectro-temporal nature.

Time Delay Spectrometry (TDS).
A method, conceived by Richard Heyser, that permits a spectrum that has been delayed to be measured with the signal delay removed. TDS measures in the frequency domain, then transforms the results mathematically for interpretation in the time, energy or frequency domains. The principal advantages of TDS measurements are superior noise and distortion rejection properties, fast data gathering capability, and the ability to make acoustical measurements under actual use situations. TDS measurements include the frequency response, phase response, and time response data associated with other techniques, plus energy-time curves, polar energy-time curves, and energy-time-frequency curves (3-D displays).

A sensation of noise, frequently of ringing, in the ears. Tinnitus aurium refers a subjective sensation of noises in the ears. Objective tinnitus refers to abnormal or pathological sounds originating within the body, in the region of the ear, which are audible to others than the subject. ,

Tonal system.
A set of musical rules that characterize Western music since the Baroque (17th century), Classical, and Romantic styles. This system is still quite prominent in the large majority of traditional and popular musics of the Western world. Other musical systems in use in the West do not conform to these rules, and are consequently called non-tonal or atonal.

The principal note or chord of a key in the Western tonal system.

Instabilities present in the oscillation pattern of a physical object that is set into vibration before the object settles into a stable oscillation. Also called attack transients (see amplitude envelope). Similar oscillatory instabilities ("legato transients") can be observed when the object changes state suddenly as occurs when a musical instrument changes pitch (by changing fingering on a woodwind instrument, pushing on a valve or piston in a brass instrument, pressing down on a string with a finger, or lifting one up on a string instrument). Transients are often characterized by a noisy or inharmonic spectrum. (See also harmonicity, noise, spectrum.)

(See chord.)

In the home entertainment context, pertaining to the auditory, visual and haptic sensory modalities.

Tuning system, musical.
(See scale.)

Tympanic membrane (eardrum).
A thin, translucent, elliptically-shaped and slightly concave membrane at the end of the meatus. The eardrum is made up of four layers. The outermost layer is continuous with the skin of the meatus, and the innermost layer is continuous with the mucous membrane of the middle ear. Of the two inner layers, the outer layer is composed of radial fibers, while the inner layer is composed of non-radial fibers. The tympanic membrane attaches to the malleus (hammer) of the middle ear. ,

Virtual auditory environment.
A perceived auditory environment which has been manipulated so that it does not correspond to the immediate physical environment. A trivial example is the use of headphones, which typically foster the sense of sound originating within the head, while the physical situation contains two sound sources located on either side of the head.

Visual capture.
The phenomenon is which visual perception dominates when visual cues and other sensory cues--auditory, proprioceptive, haptic, etc.--are in direct conflict. In audio design, the effect allows a loudspeaker to be placed at some distance away from a video display without the audience perceiving the disparity in location between the visual event generated on the screen, and the sonic event generated in the distant speaker. There are limits to vision's tendency to "overpower" the other senses: In the case of audio design, the limits can be usefully defined in terms of angular disparity, beyond which the audience "hears" the sonic event as being spatially distinct from, and thus conflicting with, the locus of the visual event.

In concert hall acoustics, warmth is defined as liveness of the bass, or fullness of the tone between 75 and 350 Hz, relative to that of the mid-frequency tones (350 to 1,400 Hz). Musicians sometimes describe as "dark" a hall that has too strong a bass, or whose high frequencies are greatly attenuated.

Weber's law.
Discovered by Ernest Heinrich Weber in 1834. States that the smallest detectable change (jnd) in intensity is a constant fraction of the level of stimulation. Georg Fechner turned Weber's law into a psychophysical logarithm of the magnitude of stimulation (I), or S = k log I. A great deal of psychophysical research has attempted to establish the Weber-Fechner law for sensory dimensions other than intensity, e.g., frequency and duration in audition. While the empirical data conform fairly well to the law over a certain range of values for each dimension, they can differ substantially at extremes of the range of perceptible values.

Whole-tone scale.
(See scale.)