Acoustic Transfer Impedance of Plane-Wave Couplers

The precision of the primary pressure reciprocity calibration technique at high frequencies is limited by the accuracy in calculating the acoustic transfer impedance. The expression given in the current International Standard is only dealing with plane waves in the coupler and is not valid at higher frequencies. In this paper the acoustic transfer impedance is derived from the simultaneous solution of the wave equations governing the acoustic field in the coupler and the motion of the transmitter and receiver microphone diaphragms. Numerical results for common one-inch and half-inch couplers are presented. At low frequencies the results coincide with the plane-wave theory. The correction factors accounting for the radial wave motion are introduced and compared with the values given in literature.