Low noise microphone for cochlear emissions

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Abstract
A specialized microphone for use in measuring cochlear emissions contains four modified Knowles microphones mounted in mechanical opposition for minimum vibration sensitivity in their summed output. That output is electrically equalized to provide a flat frequency response from 100 Hz to 12 kHz when measured with a disposable foam eartip containing 10 mm of 3.8-mm-i.d. probe tubing. To facilitate measurement of stimulated emissions, and especially cochlear distortion products, the construction includes two 1.35-mm-i.d. earphone coupling tubes mounted to pass through the 3.8-mm-i.d. probe tubing, permitting the delivery of stimuli from two independent ER-2 insert earphones, each providing a flat eardrum-pressure frequency response to 12 kHz. The complete microphone exhibits a typical noise spectrum level of − 20 dB SPL at 1 kHz decreasing to − 26 dB at 5 kHz, 8 to 10 dB below that of a single Knowles EA-1954 microphone and 5 to 10 dB below the apparent noise level of good young ears based on the estimate of Killion [J. Acoust. Soc. Am. 59, 424–433 (1976)].

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