A pressure microphone is one that responds to changes in sound pressure. A common example of a pressure microphone is the one with a diaphragm, the back side of which is terminated in a closed cavity (see
Fig. 5.1). A tiny hole through the wall of the cavity keeps the average pressure inside the cavity at atmospheric pressure. However, rapid changes in pressure, such as those produced by a sound wave, cause the diaphragm to move backward and forward.
If a pressure microphone is placed in a small cavity in which the pressure is varied, as shown in
Fig. 5.2, the output voltage will be the same regardless of what position the microphone occupies in the cavity. On the other hand, if a pressure microphone is placed at successive points 1, 2, 3, and 4 of
Fig. 5.3a, it will respond differently at each of these
points for reasons that can be seen from
Fig. 5.3b. The pressure drops
p
1,
p
2,
p
3, and
p
4 are different from each other by an amount Δ
p, if the spacings Δ
x are alike.
If a pressure microphone is placed in a plane sound wave of constant intensity
I (watts flowing through a unit area in the plane of the wave front), the force acting to move the diaphragm will be independent of frequency because
prms=Iρ0c−−−−√
[see
Eq. (1.12)].