Loudspeaker Measurement - Microphone Positioning

Microphone Positioning

All multi-driver speakers (unless they are coaxial) are difficult to measure correctly if the 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 real 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, and efforts to produce anything like a flat response below 100 Hz are likely to provide endless scope for experimentation, and exercise of patience. It is a major challenge to achieving audio quality. Complex and expensive DSP equipment and state of the art (and so not yet finalized) algorithms are being used to attempt to address these issues, but are not yet routinely practicable.