Difference method for measuring the vibration sensitivity of microphones

In some applications, microphones are subjected to sufficient vibration that their vibration response is of interest (e.g., feedback from earphone to microphone in wireless telephones, and hearing aids). A procedure for measuring this microphone property should take into account the airborne sound produced by the apparatus. In the present method, two microphones of the same type are mounted facing each other with diaphragms parallel, and form the plane, circular ends of a cylindrical enclosure. The dimensions of the air-filled volume between the two microphones are chosen to ensure that both microphones experience the same sound pressure. When the enclosure is vibrated in a direction normal to the plane of the microphone diaphragms (i.e., along the long axis of the cylinder), the vibration responses of the two microphones are essentially out of phase. The difference signal between the two microphone outputs will then provide a measure of the mean vibration response of the microphones from which the common airborne response of the microphones has been subtracted. The extent to which the response to sound within the apparatus interferes with the measurement of the vibration response of individual microphones may then be estimated. Results will be presented for a miniature electrical microphone.