Hearing aid delay in open-fit devices - coloration-pitch discrimination in normal-hearing and hearing-impaired

The delay in digital hearing aids is known to cause distortions due to the mixing of the delayed hearing aid sound with the undelayed direct sound. The most apparent delay-based distortion is a pitch sensation referred to as coloration-pitch. This study investigated coloration-pitch discrimination at the lower end of typical hearing aid delays and aimed at establishing thresholds where a large change in coloration-pitch is just perceptible.

Simulations were verified against a reference hearing aid for numerous conditions of delay, insertion gain and incident direction. The simulations were used in a psycho-acoustics experiment in which coloration-pitch discrimination thresholds were measured as a function of hearing aid delay and gain. Participants discriminated between coloration-pitches produced by delay changes of 12%. Insertion gains compensating for mild to moderate hearing losses were used.

Nine normal-hearing and twelve hearing-impaired participants with mild to moderate hearing loss participated.

The simulations accurately reflected the acoustics of the open-fit hearing aid. Discrimination thresholds were between 0.3 ms and 1 ms depending on insertion gain and hearing status.

High sensitivity for changes in coloration-pitch was found for delays >1 ms. For shorter delays participants were increasingly insensitive to changes in coloration-pitch.