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Concept
Loudspeaker measurement
Summary
Loudspeaker measurement is the practice of determining the behaviour of loudspeakers by measuring various aspects of performance. This measurement is especially important because loudspeakers, being transducers, have a higher level of distortion than other audio system components used in playback or sound reinforcement.
One way to test a loudspeaker requires an anechoic chamber, with an acoustically transparent floor-grid. The measuring microphone is normally mounted on an unobtrusive boom (to avoid reflections) and positioned 1 metre in front of the drive units on the axis with the high-frequency driver. While this can produce repeatable results, such a 'free-space' measurement is not representative of performance in a room, especially a small room. For valid results at low frequencies, a very large anechoic chamber is needed, with large absorbent wedges on all sides. Most anechoic chambers are not designed for accurate measurement down to 20 Hz and most are not capable of measuring below 80 Hz.
A tetrahedral chamber is capable of measuring the low frequency limit of the driver without the large footprint required by an anechoic chamber. This compact measurement system for loudspeaker drivers is defined in IEC 60268-21:2018, IEC 60268-22:2020 and AES73id-2019.
An alternative is to simply lay the speaker on its back pointing at the sky on open grass. Ground reflection will still interfere but will be greatly reduced in the mid-range because most speakers are directional, and only radiate very low frequencies backward. Putting absorbent material around the speaker will reduce mid-range ripple by absorbing rear radiation. At low frequencies, the ground reflection is always in-phase, so that the measured response will have increased bass, but this is what generally happens in a room anyway, where the rear wall and the floor both provide a similar effect. There is a good case, therefore, using such half-space measurements, and aiming for a flat half-space response.
Official source
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