Focus on hearing aids
12 March 2003
Brian Moore, Professor of Auditory Perception at the University of Cambridge, explains how new developments in digital hearing aid research could help to overcome some of the problems associated with traditional analogue aids.
The most common cause of hearing loss in developed countries is damage to the inner ear – the cochlea. This type of hearing loss results in several changes in the way that sounds are perceived:
- There is a loss of sensitivity, so weak sounds cannot be detected.
- Intense sounds may appear as loud to the hearing-impaired person as they do to a normally hearing person; the loudness 'catches up' with the normal value at high intensities. This is called loudness recruitment.
- There is a loss of frequency selectivity – the ability to separate the different frequencies present in sound. This makes it harder to 'hear out' the individual sounds in a mixture of sounds and contributes to the difficulty in understanding speech in noise.
- Sounds may appear distorted, and tones may sound noise-like.
Traditional hearing aids partially compensate for loss of sensitivity by amplifying sounds. However, they do not compensate for loudness recruitment, reduced frequency selectivity and perceived distortion. Strong sounds may be over-amplified and cause discomfort because of the effects of loudness recruitment. Hearing aids with compression can partially compensate for the effect of loudness recruitment by amplifying weak sounds more than strong sounds. Some aids apply compression independently in two or more frequency bands. This can compensate more effectively for hearing loss that varies with frequency (as is usually the case).
Many manufacturers now sell digital hearing aids. These incorporate a miniature computer to process sounds. Digital aids provide many possibilities for processing sounds in order to compensate more effectively for the hearing impairment. However, at present there is a lack of knowledge about what forms of processing are most effective – i.e. about the software that should be used with the aids. Deafness Research UK has done much to support this work in recent years. Current research in my laboratory is based on the use of both experimental digital hearing aids and commercial digital hearing aids.
This work includes:
- The development and evaluation of more effective forms of compression to compensate for loudness recruitment.
- The development and evaluation of methods for compensating for reduced frequency selectivity, with the goal of improving the perception of speech in noise.
- Studies of why some sounds appear distorted, and of methods of processing the sounds to reduce perceived distortion and improve speech perception in such cases.
