Real-ear measures in evaluation of frequency response and volume control characteristics of telephone amplifiers.

The spectral frequency response, frequency response range, and volume control linearity of five telephone amplifiers were examined using real-ear measures. All measurements were performed in KEMAR's (Knowles Electronics Manikin for Acoustic Research) ear canal using a composite speech-shaped waveform as the stimulus. Spectral frequency response and response range of each device was obtained at four volume control settings and compared to those of a standard telephone receiver. Only two of the amplifiers replicate the spectral frequency response of the standard receiver and show an increase in the amount of gain provided with increasing volume control rotation. The remaining three amplifiers show a more restricted spectral frequency response and response range when compared to those of the standard receiver. The volume control characteristics of the amplifiers were somewhat more uniform. Overall results indicate that the spectral frequency response and response range of telephone amplifiers can be objectively evaluated using real-ear measures, and these measures are essential in determining the usefulness of certain telephone amplifying devices.

Individualized correction factors in the preselection of hearing aids.

This study investigated three issues involving corrections for individual ear acoustics in hearing aid prescriptions: (a) the extent to which inconsistencies in the sound-field reference position can affect comparative corrections for the real-ear unaided response (REUR); (b) the extent to which individual variability in the real-ear-to-coupler level difference (RECD) supports the use of individual measurements as opposed to an average-ear estimate; and (c) the adequacy of using KEMAR estimates of the effects of the location of the hearing aid microphone. In Experiment 1, KEMAR REURs using over-the-ear and under-the-ear reference positions were compared with KEMAR REURs using a center-of-head reference position. Maximum differences of 4-9 dB were found in the 1500- to 5000-Hz range, depending on test conditions. In Experiment 2, the ear canal response of an insert earphone was compared to the 2-cc coupler response of the same earphone to calculate the RECD. Individual RECDs for a population of hearing aid candidates were compared to the RECD for KEMAR. For 8 of the 15 subjects (9 of 18 ears), the RECD was more than 4 dB different from KEMAR at two or more third-octave frequencies between 500 and 4000 Hz. In Experiment 3, the effect of the location of the hearing aid microphone for in-the-ear (ITE) and in-the-canal (ITC) locations was compared with the over-the-ear (OTE) location for 18 ears and for KEMAR. The effects varied across individual ears, but all ears and KEMAR showed positive gain in the high frequencies for the ITE and ITC locations. The relevance of these results to hearing aid prescription practices is discussed.