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).