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4 - ACOUSTIC COUPLING

The transduction of energy from the sound wave to the movement of the microphone diaphragm is produced by either pressure or pressure-gradient acoustic coupling, or by a combination of both.

4.1 - PRESSURE ACOUSTIC COUPLING

In the case of a pressure microphone (i.e. using pressure acoustic coupling), the sound wave can only impinge on the front side of the diaphragm, the other side being enclosed but maintained at atmospheric pressure through the capillary air vent shown in the side of each capsule in Figures 5 and 6. The diaphragm is subjected to a force generated by the difference between the instantaneous pressure created by the sound wave at the front of the diaphragm, and the relatively constant value of atmospheric pressure behind the diaphragm. The sound pressure will obviously vary as the sound wave moves past the microphone – the movement of the diaphragm responding to the instantaneous pressure exerted on the diaphragm. Figures 5 and 6 show two pressure microphone capsules (moving coil and condenser) in the variable pressure field generated by a sound wave.


Figure 5 - Moving Coil Microphone Pressure Capsule in a Travelling Sound Wave


Figure 6 - Condenser Microphone Pressure Capsule in a Travelling Sound Wave
The ribbon microphone as such is rarely used as a purely pressure microphone, however we shall see that in the high frequency range the ribbon microphone gradually becomes a pressure transducer with respect to the response on axis, both in front and behind the transducer – the response on the side remaining pressure-gradient.