Listening to Music Through Hearing Aids: Potential Lessons for Cochlear Implants

Abstract

Some of the problems experienced by users of hearing aids (HAs) when listening to music are relevant to cochlear implants (CIs). One problem is related to the high peak levels (up to 120 dB SPL) that occur in live music. Some HAs and CIs overload at such levels, because of the limited dynamic range of the microphones and analogue-to-digital converters (ADCs), leading to perceived distortion. Potential solutions are to use 24-bit ADCs or to include an adjustable gain between the microphones and the ADCs. A related problem is how to squeeze the wide dynamic range of music into the limited dynamic range of the user, which can be only 6-20 dB for CI users. In HAs, this is usually done via multi-channel amplitude compression (automatic gain control, AGC). In CIs, a single-channel front-end AGC is applied to the broadband input signal or a control signal derived from a running average of the broadband signal level is used to control the mapping of the channel envelope magnitude to an electrical signal. This introduces several problems: (1) an intense narrowband signal (e.g. a strong bass sound) reduces the level for all frequency components, making some parts of the music harder to hear; (2) the AGC introduces cross-modulation effects that can make a steady sound (e.g. sustained strings or a sung note) appear to fluctuate in level. Potential solutions are to use several frequency channels to create slowly varying gain-control signals and to use slow-acting (or dual time-constant) AGC rather than fast-acting AGC.

Keywords: automatic gain control; cochlear implants; compression speed; dynamic range; hearing aids; music.

Figure 1.

Preferences for slow versus fast compression for individual subjects and for the mean across subjects, as obtained by Moore and Sęk (2016b) using a simulated HA. Bars falling above the horizontal line at the centre of the panel indicate preferences for slow compression over fast compression. Redrawn from Moore and Sęk (2016b).

Figure 2.

Simplified schematic diagram of the signal processing preformed in several CIs.

Figure 3.

Schematic input-output functions for the advanced bionics CIs for input dynamic ranges (IDRs) of 50, 65, and 80 dB.

Figure 4.

Schematic diagram of the “voice guard” system used in the oticon medical CI.

Figure 5.

Schematic diagram of the adaptive mapping from the envelope magnitude in a given channel to the electrical output for that channel, as used in the Oticon medical voice guard system.