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.