Effects of expansion on consonant recognition and consonant audibility

Abstract

Background: Hearing aid expansion is intended to reduce the gain for low-level noise. However, expansion can also degrade low-intensity speech. Although it has been suggested that the poorer performance with expansion is due to reduced audibility, this has not been measured directly. Furthermore, previous studies used relatively high expansion kneepoints.

Purpose: This study compared the effect of a 30 dB SPL and 50 dB SPL expansion kneepoint on consonant audibility and recognition.

Research design: Eight consonant-vowel syllables were presented at 50, 60, and 71 dB SPL. Recordings near the tympanic membrane were made of each speech token and used to calculate the Aided Audibility Index (AAI).

Study sample: Thirteen subjects with mild to moderate sensorineural hearing loss.

Results: Expansion with a high kneepoint resulted in reduced consonant recognition. The AAI correlated significantly with consonant recognition across all conditions and subjects.

Conclusion: If consonant recognition is the priority, audibility calculations could be used to determine an optimal expansion kneepoint for a given individual.

Figure 1

Mean test ear air conduction thresholds. Error bars represent ± 1 standard deviation.

Figure 2

Plot of RMS SPL of /θi/ for subject 11 by hearing aid condition. Thin lines represent 50 dB SPL CV input level and thick lines represent 71 dB SPL CV input level. Portions above threshold are audible.

Figure 3

Mean AAI by condition and input level. Error bars represent ± 1 standard deviation. Lin = Linear; Exp30 = Expansion with 30 dB SPL kneepoint; Exp50 = Expansion with 50 dB SPL kneepoint.

Figure 4

Mean CV percent correct by condition and input level. Error bars represent ± 1 standard deviation. Lin = Linear; Exp30 = Expansion with 30 dB SPL kneepoint; Exp50 = Expansion with 50 dB SPL kneepoint.

Figure 5

Scatterplot of CV percent correct versus AAI for each condition, speech input level, and subject. Solid line is the fitted linear regression equation for all the conditions. Lin = Linear; Exp30 = Expansion with 30 dB SPL kneepoint; Exp50 = Expansion with 50 dB SPL kneepoint.

Figure 6

Waveform plots of /si/ for subject 10. The left column is for an input level of 50 dB SPL, and the right column is for an input level of 71 dB SPL. The consonant portion and vowel portion are labeled on the bottom two graphs. Notice that the scaling is different for the two input levels.