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.
Anechoic measurement
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.
Tetrahedral chamber
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.
Half-space measurement An alternative is to simply lie 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, for using such half-space measurements and aiming for a flat half-space response. Speakers that are equalised to give a flat free-space response will always sound very bass-heavy indoors, which is why monitor speakers tend to incorporate half-space and quarter-space (for corner use) settings, which bring in attenuation below about 400 Hz. Digging a hole and burying the speaker flush with the ground allows far more accurate half-space measurement, creating the loudspeaker equivalent of the boundary effect microphone (all reflections precisely in-phase) but any rear port must remain unblocked, and any rear-mounted amplifier must be allowed cooling air. Diffraction from the edges of the enclosure is reduced, creating a repeatable and accurate, but not very representative, response curve.
Room measurements At low frequencies, most rooms have resonances at a series of frequencies where a room dimension corresponds to a multiple of half wavelengths. Sound travels at about 1,100 feet per second (340 m/s), so a room 20 feet (6.1 m) long will have resonances from 27.5 Hz upwards. These resonant modes cause large peaks and dips in the sound level of a constant signal as the frequency of that signal varies from low to high. Additionally, reflections, dispersion, absorption, etc., all strongly alter the perceived sound, though this is not necessarily consciously noticeable for either music or speech, at frequencies above those dominated by room modes. These alterations depend on speaker locations with respect to reflecting, dispersing, or absorbing surfaces (including changes in speaker orientation) and on the listening position. In unfortunate situations, a slight movement of any of these, or of the listener, can cause considerable differences. Complex effects, such as stereo (or multiple channel) aural integration into a unified perceived "sound stage" can be lost easily. There is limited understanding of how the ear and brain process sound to produce such perceptions, and so no measurement, or combination of measurements, can assure successful perceptions of, for instance, the "sound stage" effect. Thus, there is no assured procedure that will maximise speaker performance in any listening space (with the exception of the sonically unpleasant anechoic chamber). Some parameters, such as reverberation time (in any case, really applicable only to larger volumes), and overall room "frequency response" can be somewhat adjusted by addition or subtraction of reflecting, diffusing, or absorbing elements, but, though this can be remarkably effective (with the right additions or subtractions and placements), it remains something of an art and a matter of experience. In some cases, no such combination of modifications has been found to be very successful.
Microphone positioning All multi-driver speakers (unless they are coaxial) are difficult to measure correctly if the measuring microphone is placed close to the loudspeaker and slightly above or below the optimum axis because the different path length from two drivers producing the same frequency leads to phase cancellation. It is useful to remember that, as a rule of thumb, 1 kHz has a wavelength of 1 ft (0.30 m) in air, and 10 kHz a wavelength of only 1-inch (25 mm). Published results are often only valid for very precise positioning of the microphone to within a centimetre or two. Measurements made at 2 or 3 m, in the actual listening position between two speakers, can reveal something of what is actually going on in a listening room. Horrendous though the resulting curve generally appears to be (in comparison to other equipment), it provides a basis for experimentation with absorbent panels. Driving both speakers is recommended, as this stimulates low-frequency room 'modes' in a representative fashion. This means the microphone must be positioned precisely equidistant from the two speakers if 'comb-filter' effects (alternate peaks and dips in the measured room response at that point) are to be avoided. Positioning is best done by moving the mic from side to side for maximum response on a 1 kHz tone, then a 3 kHz tone, then a 10 kHz tone. While the very best modern speakers can produce a frequency response flat to ±1 dB from 40 Hz to 20 kHz in anechoic conditions, measurements at 2 m in a real listening room are generally considered good if they are within ±12 dB.
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