The compressor in a hearing aid is the feature that adjusts gain according to the current sound environment and the client’s hearing loss. The target gain settings in the compressor are based on the Widex fitting rationale, focusing on three different sound levels: soft sounds, normal speech levels, and loud sounds.
A strategy for soft, normal and loud sounds
The Widex fitting strategy for soft sounds involves keeping them above the client’s hearing threshold. This entails a very low compression threshold with the EDRC (Enhanced Dynamic Range Compression) known from Widex hearing aids since Senso Diva.
The general fitting strategy for soft sounds is a normalisation strategy aiming at matching the loudness perception of a normal-hearing person as closely as possible.
For normal speech levels, Widex aims at providing optimal speech intelligibility. This is achieved on the basis of an equalisation scheme where each frequency region is amplified sufficiently to make the speech cues audible to the user.
Gain is also optimised according to the ambient sound environment by the implementation of the Speech Enhancer, which optimises gain in regions with speech using the Speech Intelligibility Index (SII).
The general strategy for loud sounds is that they should be perceived as loud but not uncomfortably loud by the user. A normalisation strategy where a sound perceived as loud by a normal-hearing person will also sound loud to the hearing aid user is employed.
The speed with which gain is regulated in the compressors (the attack and release times) is an important factor for the sound quality.
Research indicates that a slow-acting compression system results in the best sound quality and that users generally prefer very low compression rates for most input levels.
By using predominantly slow regulation, Widex offers a compressor that adjusts gain according to the general sound level, but with the sound quality of a linear system.
Moreover, the compressor also preserves the spectral and temporal contrasts in speech signals.