Absolute Pressure Calibration of Microphones

Abstract

A tourmaline crystal disk was used both as a microphone (direct piezoelectric effect) and as a sound source (converse piezoelectric effect). Application of a principle of reciprocity to the acoustic measurements gave an absolute determination of the piezoelectric modulus d33 + 2d31 of tourmaline under hydrostatic pressure. A condenser microphone was calibrated by the tourmaline disk. The same principle was applied to data obtained by using a condenser microphone as both source and microphone to secure an absolute calibration of another condenser microphone. It was proved experimentally that the tourmaline disk and the condenser microphones satisfied the principle of reciprocity. The absolute acoustic determination of the piezoelectric modulus gave d33 + 2d31 = 2.22 × 10−17 coulomb/dyne. The “reciprocity” calibrations agreed with the results of electrostatic actuator, pistonphone, and “smoke particle” calibrations, but disagreed with thermophone calibrations of the condenser microphones.