Microphone diaphragm thermal noise floor measurement using a laser vibrometer

The response of a microphone to thermal noise can be extremely difficult to predict and can adversely affect the performance of a given design. The direct measurement of the thermal noise floor of a microphone diaphragm could greatly improve our understanding of the parameters that affect the noise performance. A laser vibrometer can serve as a powerful tool, enabling precise and non-intrusive measurements of the diaphragm thermal noise. The experimental setup for measuring the diaphragm thermal noise using a laser vibrometer is described. Having both the noise floor, and the response to sound for a given microphone, the sound pressure-referred noise floor can be estimated. Measured and predicted results are presented for a variety of microphones. These measured results describe how design parameters, such as the size of the diaphragm, the dimensions of the backside volume, and overall geometry, impact the thermal noise floor measurements.