Second Biennial Hearing Aid Research and Development Conference

September 22-24, 1997
National Institutes of Health
Bethesda, Maryland

The Impact of Restoring Normal Loudness Growth on Speech Understanding as a Function of Signal-to-noise Ratio and Input Level

The goal of traditional hearing aid fittings has been to make sound audible but not uncomfortable. Recently, investigators have suggested that technology is available in the form of compression signal processing that will allow individuals to perceive loudness normally as a function of frequency and input intensity level. There are limited data as to whether achieving normal loudness growth with amplification results in superior performance on speech tasks. Understanding speech at a variety of input levels and signal-to-noise ratios is the typical goal of both the audiologist and individual with hearing loss.

Fifty adult subjects with hearing loss between 15 and 40 dB at 500 Hz and 40 and 80 dB at 4000 Hz were fit with the Siemens MUSIC completely-in-the-canal hearing aids. This is a wide dynamic range, two-channel, curvilinear compression instrument. The initial fitting was based on the DSL[i/o] fitting strategy using loudness judgments obtained from the Contour Test at 500, 1000, 2000, 3000, and 4000 Hz. The original parameters of the fit (overall gain, slope, etc.) were optimized by performing the Contour Test for warble tones and speech in the soundfield. The hearing aids were manipulated until normal loudness growth was optimized across frequencies as evidenced by loudness contours that matched loudness contours previously obtained on normal hearing individuals.

After successfully restoring loudness growth to normal, subjects completed the Speech in Noise Test (SPIN) at three input signal levels (55, 65, and 80 dB SPL). This task consists of repeating the last word of a sentence in a background of multitalker babble. Two signal-to-noise ratios (+3, and +8) were used at each presentation level. Subjects listened through the optimized hearing aid response (normal loudness growth), through a linear response programmed in the second memory of the hearing aid and matched to the optimized response for the 65 dB SPL input level, and through the unaided ear. The data presented will illustrate the differences between the nonlinear (normal loudness growth), linear, and unadied responses as a function of presentation level and/or signal-to-noise ratio.

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