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Sound Pressure in Insert Earphone Couplers and Real Ears

It is known that sound pressure, measured in couplers via a probe-tube microphone, often shows a pressure vs frequency response that drops sharply at a single frequency. In this study sound pressure was theoretically determined at various locations within a hard-walled cylindrical cavity, driven by a constant-volume velocity source with circular symmetry. At each location in the volume, a transfer impedance was defined as the ratio of pressure to inlet-volume velocity. In the region around the inlet, the transfer impedance passes through zero as it changes from negative to positive reactance with increasing frequency. Two hard-walled cavity examples were examined in detail (1) the main cavity of a 2-cm³ HA-2 coupler, and (2) a cavity having dimensions approximately equal to the occluded ear canal between an earmold tip and the eardrum. Contours of constant minimum sound pressure vs frequency are given for these two cylindrical volumes with experimental verification. Implications for probe microphone calibration and measurement of sound pressure in ears are discussed.