Effects of various microphones on the calibration of artificial mouths

Artificial mouths used in telephonometric measurements are calibrated to produce a frequency-independent sound pressure of a known level at a given calibration point. Common practice in the telephone industry has been to measure this sound pressure with a Type L Laboratory Standard Microphone, using its free-field response curve. For a fixed artificial mouth calibration, the sound pressure indicated by various Type L microphones has been determined over the 100–10000-Hz frequency range. The results show an expected difference at high frequencies between “free-field” microphones and “pressure” microphones with a free-field correction applied. They also show an overall shift in level that depends on the microphone geometry. This latter result can be attributed to differences in the effective acoustic centers of the microphones. Using a Type M microphone and moving the calibration point farther from the lip ring of the artificial mouth substantially reduces the difference between the indicated and true free-field sound pressures. IEEE Standard 269–1971, “Method for Measuring Transmission Performance of Telephone Sets,” is being revised to implement these changes.