Effect of low-frequency gain on speech intelligibility and sound quality in a competing voices situation
In hearing aids, low-frequency (LF) gain in the octave bands centered at 125, 250, and 500 Hz is often reduced either because of open fittings or to minimize upward spread of masking. However, pitch cues extracted from LF resolved harmonics are considered important for speech intelligibility (SI) in competing voices situations. Furthermore, the sound quality is typically described as more natural with higher LF gain. Here, the effects of modifying LF gain on SI and sound quality for a female target voice masked by either a same-sex or different-sex masker voice were assessed.
Ten young normal-hearing (NH) listeners and 11 older hearing-impaired (HI) listeners with sensorineural and moderate LF hearing loss participated. Two conditions with an increase in LF gain of either 4 dB or 8 dB relative to the prescribed-gain condition were tested. In the prescribed-gain condition, the HI listeners were fitted individually on both ears using a linear CAMEQ rationale, while no gain was applied for the NH listeners. A self-scored closed-set Danish matrix-type SI test (Dantale II) was used. The gain was applied directly to the stimuli and the speech stimuli were presented via headphones. The percentage of correctly identified words was evaluated at four fixed signal-to-noise ratios (SNRs). All NH listeners were measured at the same absolute SNR while all HI listeners were measured at constant SNRs relative to their individual speech reception thresholds.
For NH listeners, no significant effect of increased LF gain on SI was found. At 0 dB SNR in the prescribed-gain condition, the HI listeners scored significantly lower with the same-sex versus different-sex masker, whereas the scores were similar for the two maskers at SNRs different from 0 dB. The HI listeners did not benefit either from higher LF gains in terms of SI. However, an interaction between LF gain and masker type (same sex versus different sex) was found. This is because a 4-dB increase in LF gain was beneficial in the different-sex masker condition but detrimental in the same-sex masker condition.
A subjective quality rating of the different LF gain conditions was also performed by the same HI listeners using the speech and masker material from the SI test at 0 dB SNR. The quality ratings were significantly higher with increased LF gain compared to the prescribed-gain condition.
Overall, the results suggest that a higher LF gain does not affect SI but may be beneficial in terms of sound quality.