The output of a loudspeaker is difficult to measure because it has to be done with a
microphone. Measurement microphones are usually omnidirectional which means that they
pick up, not only the output from the loudspeaker, but also any background noise and
sound reflected from objects around the loudspeaker.
This means that echoes from the walls of the room where the measurements are done
corrupt the direct output from the 'speaker. One solution to this problem is to build an
anechoic chamber-- a very expensive option. Another technique is to make the
measurements outdoors with the test speaker and microphone at the top of a tower. This
works well, but only on sunny days when there is no background noise.
A third alternative becomes possible if we can capture the output from the loudspeaker
as a digital signal. Once the acoustic signal from the 'speaker has been digitised it
can be analysed mathematically. Suppose that we feed an impulse into the loudspeaker.
The Fourier transform of the impulse is the frequency response, a sum that can be done
easily in a computer. So, if we capture the impulse data we can find the frequency
response of the loudspeaker. The trick is to choose which part of the impulse to
transform. By deliberately truncating the 'tail' of the impulse we can effectively cut
off the reflections since these arrive at the measurement microphone later than the
direct sound. The reflections are removed by a window in time.
This windowing technique is very powerful and is used in many commercial loudspeaker
measurement packages. Its chief disadvantage is that it is difficult to calibrate the
microphone to show the absolute sound pressure level measured. This is because of the
mathematical technique used (FFT) combined with the normal methods for mic' calibration
(use of piston-phones and so on). Even so the method produces good relative
measurements.
If you need to make a set of measurements for comparison you must use identical settings
for each. Otherwise the relative levels that you record will not be comparable.
Measurement Considerations
There are a few fundamental rules that apply to this form of measurement.
1. The time to the first reflection determines the window length. This is fixed by the
size of the measuring room.
2. The window length determines the low frequency cutoff. The low frequency cutoff of
the measurement is always greater than the reciprocal of the window length.
3. Because the data that is captured and processed has a constant number of points per
Hz there are very few data points in the lowest octaves of the measurement. Half the
data points will be in the top octave.
4. The width of the impulse determines the bandwidth of the measurement. The wider the
bandwidth required the narrower the pulse must be.
5. The energy in the pulse is determined by its width and height. A wide tall pulse will
contain more energy than a short narrow one.
What do you need?
- PC min. 1GHz
- min. 512Mb memory
- 60mB free Harddisk capacity
- Windows 98, Windows 2000, Windows Vista ( depends of software )
- Fullduplex Soundcard
- Measurement Microphon
- extern Microphon Pre Amplifier
- recommended: Power Amplifier (min. 15Watt @ 4ohms)
Single channel measurement setup for acoustical
measurements
fig. 1
Dual channel measurement setup for acoustical measurement
fig. 2
To protect the soundcard input from high voltage that is generated by the power
amplifier, it is recommended to use a
voltage probe circuit, as shown in fig. 2.
Values of resistors R1 and R2 have to be chosen for arbitrary attenuation (i.e. R1=8200
and R2=910 ohms gives probe
with -20.7dB (0.0923) attenuation if the soundcard has usual input impedance - 10kOhms).
In a single channel mode and semi dual channel mode
this probe is not connected.
Semi Dual channel measurement setup for acoustical
measurement
fig. 3
Setup
To make an on axis response for the loudspeaker you will need to set your speaker up on
a stand as farfrom all reflecting surfaces as possible. You should also try to find the
quietest place to do the measurements as noise added to the response will degrade the
results. The measurement microphone should be placed about 1 m in front of the speaker
on your chosen measurement axis. When you have set up the mic and speaker measure the
distances to the nearest reflecting surface with a tape measure. Normally this will be
either the floor or the ceiling.
Low frequency cutoff = 343 /( (2 * (x2 + h2)0.5)
Where 343 m.s-1 is the speed of sound. For a room with a ceiling height of 2.4 m and a
measuring distance of 1 m this gives:
Low frequency cutoff = 343 /( (2 * ((0.52 + 1.22)0.5) = 132 Hz
So, for this size room you know that any data shown below 132 Hz is
rubbish! Bear in mind that you should put an extra margin on this because there
are very few data points collected in the lower octaves of the measurement.
Connect the equipment.
Dealing with Noise
Where possible you should try to use a quiet room for your measurements, maybe using the
room outside normal working hours. However, you may find that the background noise in
your test environment still makes it difficult to get good results. If this happens
there are two things that may help.
- Use a steep high pass filter on the microphone to remove low frequency noise.
- You can tailor this filter to the practical low frequency limit of your
measuring room.
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