Larry Marcus
principle transducer and telecom engineer
Many wireless phones tout their water resistance these days. BUT, for microphones, there are several potential problems the designer must be aware of before using environmental shielding barriers. These barriers or membranes that give so-called “better than IP67 protection” (see the IEC 60529 dust and water ingress protection standard) have both acoustic mass and acoustic resistance which will affect the microphone performance. The main effect for omnidirectional mic installation is that the sensitivity will be lowered a small amount and the upper end of the response can be changed due to shifts in acoustic resonances. Much more drastic effects can be seen in arrays of omnidirectional or gradient/differential mics. In these mics, the mass and resistance and the associated variation of these properties can severely shift the polar/directional characteristics and the sensitivity. These shifts in performance are due to the subtraction of the pressures electrically in the case of omni arrays or acoustical subtraction in the case of a gradient mic. Subtracting large numbers to get small numbers as in these cases yields small numbers that still carry the large variation effects. So, besides matching omnidirectionals for sensitivity and response for arrays (not even mentioning the other confounding issue, self-noise) or selecting a good gradient/directional mic, careful attention must be paid to the surrounding acoustical elements, INCLUDING THE BARRIER ELEMENTS, in any design.
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