Influence of air masses on microphone vibration sensitivity

Abstract

A study is presented of the primary design parameters that influence the vibration sensitivity of a microphone. The sensitivity to vibration is generally determined by the mass of the pressure-sensing diaphragm along with the mass of air that moves with it. The sound-sensing performance is improved as the pressure-sensing diaphragm is made thinner, but for a thin enough diaphragm, the moving air mass is not negligible relative to that of the diaphragm itself. In the present study, we develop a simple duct-acoustic model to account for the effect of the co-vibrating air. It is shown that an idealized massless, thin microphone diaphragm will still produce unwanted vibration signal due to acceleration of the air masses within the microphone. For a small microphone, the predicted pressure related acceleration sensitivity is found to be a simple function of the mass per unit area of the air inside of the microphone package. The acceleration sensitivity predicted using a finite element model of a one micrometer thick clamped flexible silicon diaphragm agrees with that predicted by the simple duct model. Measured and predicted acceleration sensitivities are compared for several MEMS and sub-miniature electret microphones of different back volume lengths . It is found that the primary design parameter determining vibration sensitivity for these microphones is the effective length of the column of air inside the microphone’s packaging. Microphones that have longer air-filled volumes had greater pressure related acceleration sensitivity.