From an Invited Lecture "Sala Acustica" Given to Top Audio, The Hi End Audio Show In Milano Italy, October, 1991 by Arthur M. Noxon, PE, MSME, MS
An old saying is known in many countries "The chain is only as strong as its weakest link." Often, when we talk about the parts in our audio system, they are referred to as components and interconnects in the audio chain. Each component processes the signal and the interconnects transfer the signal from one component to another. This is hopefully all for the better, resulting in quality sound arriving at our ears.
Modern audio equipment such as that found in any hi end show or good dealer demo room is very accurate. Because this electronic equipment comprises most but not all parts of the audio chain, it cannot perform any better than the weakest link of the whole audio chain. We take a few minutes of time now to illustrate just how the final interconnect - the Room Acoustic - is not only one of the links in the audio chain, it is the last and the weakest link in the audio chain. What we do with this critical audio link will also be discussed.
Listening In the Desert
The story all begins because we are people who listen to sound. Imagine if we play a stereo in the quiet desert, the only sound we hear would be the direct sound from the speakers. No reflections, no noise, nothing to complicate what we hear. This would be an anechoic space, one without echo or reflection.
Let us imagine further that we next position ourselves in the vicinity of a large stone wall, located far behind, crossing from back left to right. Sound from the speaker will first pass by us, traverse a long distance and then reflect off the wall. However, this wall is too far away and we do not hear its reflection. Because of the distance, the sound takes a long time to return to us and also it grows very weak - too weak to be heard.
Next we imagine to relocate our stereo system closer to the wall. The reflection arrives more quickly than before and it is louder, so this time we hear an echo. Once again we relocate even closer to the wall and the time delay for the echo becomes smaller while the strength of the echo grows stronger.
We can move even closer to the wall and at about 10 meters the distinct quality of a separate echo begins to disappear even though the reflection is stronger than ever before. And by 8 meters distance the echo effect has completely disappeared. The wall reflection is without doubt quite loud but it is no longer clearly heard as an echo. This reflection follows the direct signal so close in time that our ear-brain system fuses the two separate sounds into one single sound and we now have realized the "sound fusion" effect. Those reflections that shortly follow the direct signal will blend into and become part of the direct signal itself. It is a natural feature of the hearing process.
Listening In The Room
Next, we relocate our thought experiment into a more conventional setting, your hifi listening room. The room has four walls, a floor and a ceiling. Six reflecting surfaces means there are six times the number of initial reflections than when we listened with our back to the stone wall. After these first six, early reflections pass by there develops a sequence of reflections that rapidly become very complicated. This is because each reflection rebounds across the room only to meet another wall and be reflected again, then again and again.
If the room is big, the reflections are sufficiently time delayed so that we hear the echo of the room. This echo is often not one simple echo but many echoes arriving in a confused manner, randomly scattered over time. If the room is small, the reflections are not time delayed sufficiently for us to hear the room as a separate echo. We do hear small room reflections quite loudly, we just do not recognize the reflections as a separate sound. It is only because we do not consciously recognize the reflections in the small room as a distinct acoustic signal that we tend to forget that they compose the majority of the sound we hear. The speaker is the last component in the audio chain and the room acoustic is the last interconnect. Distortions in any of the interconnects, including the last one will degrade the quality of the audio signal.
The Acoustic Interconnect
Long time delay reflections, echoes are very bad for good listening. But what about the short time delay reflections, those that belong to the listening room? In some ways they are actually good for listening but in other ways they become a hindrance. Because of sound fusion, the early reflections add to the direct sound and make it seem louder than it actually is. For conversational speaking this is a benefit. However, in audio we don't actually need the help of room reflections to make loud sound, we have the amplifier and speakers to deliver sound power.
Most people do not own listening rooms large enough to have an echo. We also do not own rooms that are completely without reflections. Hifi listening rooms have many reflections, most of which fall within the 1/20 second sound fusion period of time. So we always hear the room right along with the speaker. This is why the audio experts always say that the room is part of the audio chain. The Room Acoustic interconnect is not only the last link in the audio chain. With the high quality of today's audio electronic components and interconnects, the Room Acoustic has certainly become the weakest link, and because of sound fusion in small sized listening rooms, the Room Acoustic is the most forgotten link in the audio chain.
Sometimes it is hard to remember that the room is an interconnect because we are so used to interconnects as being cables with plugs that we buy at the hifi shop and take home in a box. The sound we hear is due to the combination of all the purchased electronic components and electronic interconnects plus the existing distortion and confusion from the last interconnect, the room acoustic. Curiously, people in audio often upgrade their electronic interconnects long before they even think of improving their acoustic interconnect.
What can we expect from our room? It is simply no more than one of the rooms that came with the house. It may even be a nice room but it was never built to be a distortion free wave guide for hi end audio signals, it was built for eating, sleeping or visiting. Hi end audio needs something more than a simple, contractor built room to be in control of the last interconnect in an otherwise quality audio chain.
Although some reflections in the small listening room are supportive to understanding spoken words, too many reflections, for too long a period of time will confuse. Such is the sound we hear when discussing furniture arrangements inside an empty house or apartment, it is an echo problem. In hi end audio, the echo can be a problem but in typical carpeted, furnished rooms the problem is usually not an echo. Most small rooms have too many early reflections within the sound fusion period which degrades musical clarity and sound stage imaging. First, we look into the process of acoustical imaging in audio.
To begin, let's return to the desert and listen to just one speaker. We relax; stare ahead and begin to "see" the sound coming from the speaker as we expect to occur. Next, we turn up the sound from the second speaker. By the time they are equally loud, the acoustic image floats between the two speakers. This is exactly what stereo is really all about, not two sounds from two separate speakers but the appearance of one sound in the space between two speakers. The stereo experience is intended to be an acoustic hologram, a mirage of sound in the desert.
In the next experiment we again use one speaker and add a wall located just to the side of the speaker. Now we receive the direct signal from the speaker and very quickly following is a reflection off the wall. The image of the speaker shifts again away from the speaker and floats somewhere between the speaker and the wall. The wall reflection produces a sound not exactly like a second speaker yet similar effects are observed. The wall reflection is weaker and slightly time delayed compared to the signal from the speaker. This is quite different from the previous stereo speaker experiment in which the two signals arrived at the same time and with the same intensity.
This entire imaging process has been greatly studied and is called the "Haas effect", after a scientist of that name. Signals as low as 10 dB below the direct signal and as time delayed as much as 10 milliseconds will produce image position changes. Clearly, with sound traveling 1,130 ft/second (334 m/sec), the wall reflections in the typical sized listening room fall well within the Haas effect time window.
When we get the same signal from the two separate locations and within a very short period of time, our brain is tricked into thinking that sound came from a spot between those two locations, that can even be in front of or behind them. These signals create a "virtual" sound source location. If we get 3 such signals then the image is localized amongst the 3 directions. But if we get many signals of different strengths and delays and from different directions - we lose track of where the sound seems to come from. We think the sound comes from everywhere which is the same as nowhere. Too many early reflections cause confusion in stereo imaging. The sound stage is no longer crystal clear, it becomes fogged over, detail is lost, image is blurred and sound depth becomes flat.
We recognize this problem to exist whenever the speakers do not "disappear". In a good room, the speakers actually seem to be silent most of the time, all while and throughout the front of the room a vivid audio stage show is taking place.
Small room acoustic reflections appear in two, and very different forms. Not only form but their effects are also different. Imagine the piano keyboard and middle C - 256 Hz. All notes to the left of center are in the BASS cleft and those to the right are TREBLE. Room reflections occur for all of the frequency range. Those reflections that cause problems with imaging belong for the most part to the treble cleft, above middle C. Reflection in the bass range creates problems not so much for imaging but effect the clarity, musical honesty and realism. Unlike treble, reflections in the bass range do not bounce around the room like balls. Their wavelengths are so long that the reflected waves fold back one onto another. The resulting compositions are generally known as room resonances or room modes.
Room resonances will cause one note to sound loud and the next to sound quiet. They also cause attack transients to pick up a coloration in tone. Speakers project the treble forward, directly into the room. If you step behind the speaker the treble is no longer heard coming from the speaker. Not so with the bass range. The wavelengths are larger than the speaker and the sound expands evenly in all directions. You hear bass as loudly behind the speaker as in front of the speaker. The treble is beamed forward but the bass cannot be beamed. We actually have to put 10 times more bass power into the room than treble to get the same direct sound level to the listener. This is partially due to acoustic efficiency of speakers in the bass range but primarily due to the direct bass wave having to be delivered to the whole room. The treble is more efficiently produced and directed primarily towards the listener.
There are two types of resonance reflection problems. The sound that the speakers make first expands outward and contacts the walls, floor and ceiling in the front of the room. These very early reflections fold back upon one another to create "head end ringing". The quivering of the air in the front of the room occurs very quickly within 1/10 second and so it colors the attack transients. It is only during the attack transients of music can we identify phase and time alignment. This most important quality in modern hi end audio equipment is heavily degraded by head end ringing in the mid bass range.
The second type of resonance takes much longer to develop, typically 1/4 second. It engages the full length of the room and tends to belong to the lower notes, the deep bass. These are the full 3 dimensional room modes that are usually talked about. They cause some notes to be excessively loud and others too quiet. Room resonances cause the otherwise smooth frequency response curve of even the very best audio equipment to become very irregular at the listening position.
Room Acoustic Upgrade
Reflections do need control in the audio listening room. Treble reflections are the most commonly understood. Carpet, drapery and furniture work reasonably well with treble. The typical products such as acoustical foam and fiberglass wall panels are only for upper treble acoustics. It takes about 4 inches or 10 cm of such material to effect the full treble range. One must be very careful about over damping the brightness of the listening room by excessive misuse of low cost treble absorptive materials. Some of the more recent audio quality, treble range acoustic products are not only full treble bandwidth but have built-in sound scattering panels to keep the listening area controlled yet still sounding bright.
The lower half of the musical scale, the bass range, needs BASS TRAPS. For many years bass traps were only found built into the walls of recording studios. But over the last decade acoustic products available in audio have been expanded to include small, efficient, free standing bass traps. Sound absorption depends on two factors, surface area and its absorption coefficient or efficiency. Modern bass traps are cylinder shaped in order to provide a large surface area as well as high efficiency in a small package.
One very fortunate aspect of bass in small rooms is that the corners of the room act like sound collectors, megaphones in reverse. Because bass goes backwards as well as forwards, it should be no surprise that bass traps are regularly found both behind the speakers and behind the listener. Bass is always extra loud in the corners of a room. That is why it is such a good idea to locate bass traps in the corners of the room. You can always recognize a properly set up a listening room by the presence of corner loaded bass traps in every corner of the room.