Hearing aid Amplification at Soft Input Levels

Hearing loss is associated with decreased audibility for soft sounds, and impaired loudness perception. Hearing aids are used to help improve these deficits. The general aim of hearing aid fitting is to fit the dynamic range of environmental sounds into the reduced dynamic range of hearing. There is however little consensus among hearing aid researchers regarding how much gain is appropriate, in particular for sounds at low input levels.

The overall aim in this project is to determine the degree to which hearing aids should amplify soft sounds to audibility without compromising listening comfort. An important hearing aid parameter for determining gain for soft sounds is the compression threshold (CT). Lowering the CT increases the gain at low input levels.

In this project, the influence of different factors on the preferred CT were investigated in a series of laboratory listening experiments and a field trial. The influence of compression release time on the preferred CT was investigated in a laboratory listening test with 12 hearing-impaired participants.

When a short release time was used, the participants predominately preferred a moderate CT, but when a long release time was used, there was equal preference for moderate and low CTs. The implication is that the release time influenced the preferred gain for soft input levels. This finding was followed up in a field trial.

Twenty hearing aid users (10 new and 10 experienced) compared two hearing aid programs (low and moderate CT) in their daily lives in two trial periods. The two CT programs were combined with either fast-acting or slow-acting compression in each trial period. Overall, the participants most often preferred the moderate CT, except in situations with quiet or distant speech when combined with slow-acting compression.

In listening situations that the participants themselves nominated as important, the majority did not report a preference. Compared to the new hearing aid users, the experienced users more often preferred the low CT and thereby more gain at low input levels. Overall, the results were not strongly in favour of the use of either a low or moderate CT.

The findings provide evidence that experienced hearing aid users prefer more gain at low input levels than new hearing aid users. The compression speed also influences the preferred CT. These findings have implications for how hearing aids should be fit to new and experienced users.