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Fig. 7. Mechanical deflection across the membrane of a Brüel & Kjaer type
4138 pressure-field microphone. (A) Envelopes of deflection (gain) for driving
frequencies of 25 and 70 kHz. (B) Area scans of microphone membrane
deflections for driving frequencies of 25 and 70 kHz. The deflections are
shown each time for four different phases along the oscillation cycle.
Deflections are additionally shown as profiles, looking at the membrane from
its side. Red indicates positive gain (or outward membrane deflections), and
green indicates negative gain (or inward membrane deflections).