Improvements in hearing aids
Contact Name: Brian Moore
Contact Email: firstname.lastname@example.org
Brian Moore received his B.A. in Natural Sciences in 1968 and his Ph.D. in Psychoacoustics in 1971, both from the University of Cambridge, England. He is currently Professor of Auditory Perception in the University of Cambridge. He has also been a Visiting Professor at Brooklyn College, the City University of New York, and the University of California at Berkeley and was a van Houten Fellow at the Institute for Perception Research, Eindhoven, the Netherlands. His research interests are: the perception of sound; mechanisms of normal hearing and hearing impairments; relationship of auditory abilities to speech perception; design of signal processing hearing aids for sensorineural hearing loss; methods for fitting hearing aids to the individual; design and specification of high-fidelity sound-reproducing equipment; perception of music and of musical instruments. He is a Fellow of the Royal Society of London, a Fellow of the Academy of Medical Sciences, a Fellow of the Acoustical Society of America, a Fellow of the Association for Psychological Science, and an Honorary Fellow of the Belgian Society of Audiology and the British Society of Hearing Aid Audiologists. He is a member of the Experimental Psychology Society (UK), the British Society of Audiology, The American Speech-Language Hearing Association, The American Auditory Society, the Audio Engineering Society, the Association for Research in Otolaryngology and the American Academy of Audiology. He is President of the Association of Independent Hearing Healthcare Professionals (UK). He has written or edited 17 books and over 590 scientific papers and book chapters. He has twice received the Littler Prize of the British Society of Audiology. In 2003 he was awarded the Acoustical Society of America Silver Medal in physiological and psychological acoustics. In 2004 he received the first “International Award in Hearing” from the American Academy of Audiology. In 2008 he received the “Award of Merit” from the Association for Research in Otolaryngology and the Hugh Knowles Prize for Distinguished Achievement from Northwestern University. In 2010 he gave the Distinguished Heyser memorial Lecture to the Audio Engineering Society. He is wine steward of Wolfson College, Cambridge.
Despite the advances in signal processing in hearing aids over the past 20-30 years, hearing aids are still far from restoring “normal” hearing. This partly reflects limitations of impaired auditory systems, such as reduced frequency selectivity and reduced sensitivity to temporal fine structure, but also reflects limitations in the hearing aids themselves. Some very basic limitations are:
(1) The gains achieved on real ears are often substantially different from those programmed into the manufacturer’s software, even when averaged over many test ears. In other words, something is systematically wrong in the calibration of the fitting systems. A very common problem is a failure to meet target gains for frequencies above about 3 kHz.
(2) The compression ratios obtained on real ears are often substantially different from (usually below) those programmed into the manufacturer’s software. As a result, soft sounds remain inaudible and strong sounds are too loud.
3) Despite claims of wide bandwidth, most hearing aids are unable to meet the fitting targets of methods like NAL-NL2 or CAM2 for frequencies above about 4 kHz.
4) The output of many hearing aids often falls off markedly for frequencies below a few hundred Hz. This does not create severe problems when listening to speech, but produces severe degradations of sound quality for music.
More subtle problems arise as side effects of the signal processing in hearing aids. Processing such as multi-channel amplitude compression, noise reduction, and adaptive directionality changes the amplitude modulation patterns of the signal and this can have adverse effects on speech intelligibility and sound quality. For listening to music, many hearing-impaired people prefer a linear amplifier with high-quality headphones to their hearing aids. There is increasing evidence that the intelligibility of speech in background sounds is strongly affected by the amplitude fluctuations in the background sounds, even for “steady” noise (Stone et al, 2012). Improved models for predicting the intelligibility of speech in fluctuating background sounds are needed to assess the deleterious effects of the processing in hearing aids, and to select parameters of the processing that minimise these deleterious effects.
Systems for acoustic feedback reduction/cancellation can also have serious deleterious effects when listening to music. The designers of these systems need to evaluate them with music signals, not just speech.
Supported by the MRC.
Stone, M.A., Füllgrabe, C. & Moore, B.C.J. 2012. Notionally steady background noise acts primarily as a modulation masker of speech. J Acoust Soc Am, 132, 317-326.