Microphone Calibrations

There are two types of calibration, the constant-pressure or pressure calibration, and the constant-field or field calibration. The difference between the two is this: The microphone itself by reason of its presence in the sound field causes a distortion of the oncoming sound waves, although this effect is small for some types. Accordingly, a calibration made where the pressure is uniform over the diaphragm and measured at the diaphragm will not agree (especially at the higher frequencies) with a calibration made where the sound is picked up in an unobstructed space some distance from the source.

A thermophone sometimes is used in pressure calibrating microphones. It consists essentially of an enclosed chamber which can be tightly sealed against the face of the microphone to be calibrated. There are two very thin gold-leaf thermal elements near the bottom of the chamber. These are kept heated by a constant current, upon which an alternating current of the frequency at which the calibration is desired is superimposed. Gold leaf has low thermal capacity, and accordingly the impressed alternating current produces relatively large temperature variations. These in turn cause expansion and contraction of the surrounding gas, which constitute sound waves of deter-minable pressure. Calculations for determining this pressure can be made from the constants and operating data.¹¹