Microphone Intermodulation Distortion Measurement
Microphone distortion is very difficult to measure because typically the loudspeaker used to measure the microphone will have greater amplitude response irregularities and distortion than the microphone. By weighting the output signal from the generator with the reciprocal response of the loudspeaker’s fundamental, it is possible to produce a constant sound pressure level versus frequency at the microphone position. If separate test tones are fed to two separate loudspeakers, the loudspeakers’ harmonic distortion will have no influence on the measured intermodulation frequency components. Consequently, only the distortion of the microphones will be measured (Fig. 2). Ref. Brüel&Kjær Application Note “Audio Distortion Measurements”
The purpose of this sequence is to measure the Intermodulation Distortion (IM) of a microphone. Unlike traditional IM distortion measurements performed on loudspeakers, this technique requires the test stimulus to be played through two separate source loudspeakers, one for the playback of the Master (fixed) tone and one for the playback of the Slave (sweep) tone. If a single source loudspeaker were employed, it would generate its own IM products which would be impossible to separate from that of the microphone under test. The sequence’s default stimulus parameters are a 100 Hz tone (Master) and a stepped sine sweep from 10 kHz to 224 Hz (Slave). Stimulus levels should be set to maintain a 4:1 ratio of Master to Slave. Sequence defaults are 94 dBSPL (Slave) and 106 dBSPL (Master).
The test sequence plays the Master Tone through one source speaker (typically a subwoofer) and the Slave through another (typically a full-range speaker or dual-concentric design) with the two speakers placed as physically close to one another as possible. The stimulus playback is captured by the DUT mic and the Recorded Time Waveform is passed through a HarmonicTrak analysis step with its Distortion Tab configured for IM analysis. The final Display Step shows the DUT’s response to the Slave sweep, IM2, IM3 and Total IMD.