The effect of reference microphone placement on sound pressure levels at an ear level hearing aid microphone

The present study was designed to investigate the effects of reference microphone location on probe tube microphone measures of hearing aid response. The reference microphone of a clinical probe tube microphone system was located either at a position on the cheek (position A) or in close proximity to the microphone of an ear level hearing aid (position B). With sound pressure level (SPL) held constant at the reference microphone, the SPL at the position of the hearing aid microphone was measured at 14 test frequencies using a 1/8-inch condenser microphone. Measures were obtained on ten male and ten female subjects. Results indicated large frequency-dependent deviations in SPL at the hearing aid microphone, compared to that measured at the reference microphone, when the reference microphone was at position A. In the 1200 to 2000 Hz range, the SPL at the hearing aid microphone was as much as 9.5 dB higher than at the reference microphone. There were no large frequency-dependent variations with the reference microphone in position B, but the SPL at the hearing aid microphone location was approximately 3 dB higher than at the reference microphone. Results suggest that estimates of hearing aid output can be affected markedly by the reference microphone location. Clinical implications of the impact of reference microphone location on probe microphone measures of hearing aid gain and saturation sound pressure levels are discussed.