Two sets of provisional reference threshold sound pressure levels have been provided by the manufacturer for the calibration of the ER-3A insert earphone. One set is for use with the B&K DB-0138 coupler (an HA-2 2-cc coupler that comes with a 5-mm-diam by 2-mm-long stainless steel coupling nipple preceding the 18 mm of 3-mm-diam earmold-simulation portion specified in S3.7-1973), and the other set is for use with the HA-1 coupler (the standard “In The Ear hearing aid” 2-cc coupler) with a foam eartip sealed directly to the top surface of the coupler. Several recent studies have demonstrated that the ER-3A may be directly substituted, without recalibrating, for a TDH-39/MX-41AR earphone in some cases, but most available data have not been reduced to a form suitable for establishing a revised estimate of the reference threshold levels. This paper reports the results of such a data analysis.
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Wilber et al. (1987) studied this question.