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Proposed Technique for Earphone Calibration
The current American National Standard method for coupler calibration of earphones is intended only for traditional supreaural earphones or insert earphones. No standard for the measurement of circumaural earphones exists. The proposed method uses a modified Zwislocki acoustic coupler mounted in an anthropometrically and acoustically average manikin, produced by Knowles Electronics, in order to simulate the acoustic load that an average human ear would place on the earphone element. The procedure permits the testing of open and scaled circumaural and supraaural earphones. A conversion function was generated from published data on the random-incidence (nondirectional) sound-pressure response of the manikin ear at "eardrum" position. This conversion transforms the measured eardrum pressure response that a headphone loaded by an ear would generate into the equivalent field response. A computerized signal-collection and -analysis system subtracted this conversion function from stored headphone-response data. Responses were averaged across samples of left and right earphone elements and across repeated measurements that sampled variability due to headset placement, seal around the ear, and headband tension. Analog plots of headphone frequency response were nearly indistinguishable from computer-regenerated plots of that same frequency response. Comparisons of raw data and corrected data show the result to be within ±0.1 dB of the intended conversion.
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