Making Pressure Measurements in Insert Earphone Couplers and Real Ears

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Abstract
When pressure measurements in couplers were made via a probe tube, it was often observed that the pressure frequency response would drop sharply at a single frequency. The frequency (f 0) of this minimum depended on cavity dimensions and relative position and size of probe tube and sound inlet tube. An equation from Ingård [J. Acoust. Soc. Amer. 20, 665 (1948)] served to explain this phenomenon. Pressure was theoretically determined at various locations within a hard‐walled cylindrical cavity when driven by a constant volume velocity inlet of circular symmetry. At each location, a transfer impedance was thus defined as the ratio of pressure to input volume velocity. In the region around the inlet port, each transfer impedance passes through zero from negative to positive reactance as frequency increases. Two hard‐walled cavities were examined in detail: (1) the main cavity of a 2‐cm3 HA‐2 coupler and (2) a cavity approximating the dimensions of the external ear canal (between earmold tip and eardrum). Contours of constant f 0 are given for these two cavities with experimental verification. The phenomenon can be modeled as a series inertance/compliance. [Supported by Knowles Electronics, Inc.]

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