Frequency limitations of coupler-based calibrations of the pressure sensitivity of measurement microphones

Calibration of the pressure sensitivity of measurement microphones in a coupler requires by definition that the incident pressure be uniform over the surface of the microphone diaphragm. This requirement is readily satisfied at frequencies up to the cut in frequencies of the radial modes and, if the microphone is not azimuthally symmetric, the spin modes. Above the lowest natural frequency of either of these modes, the uniformity of the pressure distribution will be destroyed. This effect is alluded to in current standards, ANSI S1.15-2005/Part 2 and IEC 61094-5:2001, but the suggested countermeasure “to use more than one coupler with different dimensions” or to compare “calibrations performed in a variety of other jigs and couplers” does not resolve the problem. Specification of the maximum excitation frequency to preserve pressure uniformity in a coupler is presented for several laboratory standard and working standard microphones.