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   Rehabilitation Engineering Research Center
   on Hearing Enhancement

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Dr. Ross on Hearing Loss

Listening with a "Third Ear": FM Systems

by Mark Ross, Ph.D.
May 2004

No one doubts that in the last ten years hearing aids have become smaller and “smarter.” Many are now capable of processing speech and noise in ways that we could hardly even dream of in years past. Features like multiple bands, wide dynamic range compression, automatic feedback suppression systems, and directional microphones are now routine. Still, in spite of these and other developments, none are as capable of increasing the speech to noise ratio (the intensity level of the speech relative to the noise) as much as a close-talking microphone (where a microphone placed just a few inches from a talker’s mouth). And of all the factors that affect speech perception capabilities, none are as important as the signal to noise (S/N) ratio.

It’s obvious that if a speech signal cannot be heard over the background noise, it will not be understood. While perhaps not so obvious, it’s also clear that the more a speech signal surpasses the noisy background (up to about 20 or 30 dB for a hard of hearing person), the better someone will understand a verbal message. No hearing aid feature yet developed is able to strip only the noise (which could be other persons’ speech) at times when speech and noise arrive simultaneously at the microphone. Directional microphones are one hearing aid feature that can and do improve the S/N, but they accomplish this by suppressing sounds from any other direction than directly in front. Obviously, they are a highly desirable feature included in many modern hearing aids. To take the best advantage of their capabilities, however, people wearing directional microphone hearing aids do have to consciously position themselves favorably in respect to the location of the speech and noise (desired signal in front, noise to the rear if possible). The problem with directional microphones, no matter how advanced they may be, is that they remain fixed to the head. I’m not being facetious. It is physically impossible for hearing aid users to locate themselves in an optimum location for receiving a desired signal. Hearing aid users can hardly place an ear six inches from the mouth of a lecturer, a tour guide, a passenger in a vehicle when they are driving a car, or one’s conversational partner at a reception or a restaurant!

It is in these type of situations that FM systems, and at the present time only FM systems, can significantly help a hard of hearing person hear better. An FM listening system is basically an FM (frequency modulated) radio. A microphone picks up the desired signal, and a transmitter “broadcasts” the signal to an FM receiver (the “radio”) worn by the recipient. Unlike hearing aids, the microphone of the FM system can be placed close to the source of a sound, no matter where it is. It is this simple fact, i.e. the “portability” and flexibility inherent in personal FM systems, that is the heart of their potential effectiveness.

Initially, FM systems were designed specifically for use by hearing-impaired children in schools. The teacher wore a large lavaliere microphone-transmitter around her neck (later replaced by a lapel microphone and a belt transmitter). The original FM receivers, worn on the body, directed sound signals to the children’s ears via miniature earphones. Later, various ways of coupling their own hearing aids to FM receivers were devised. These systems had a profound impact on the educational placement and performance of children with hearing loss. They helped to overcome the deleterious effects of poor classroom acoustics, then found in most classrooms in schools throughout the country. For the first time, ever, children could be assured of a highly positive signal to noise ratio while permitting both teachers and students full physical mobility throughout the classroom. In my judgment, it was the availability of FM systems that made mainstream placement of children with hearing loss a feasible educational placement alternative.

Few adults, however, chose to use these original FM systems. They were cosmetically unacceptable to many people with hearing loss and just a bit too unwieldy for everyday use. Adults who did employ them, however, reported that these systems provided a significant boost in their ability to receive spoken messages in a number of situations (e.g. lectures of various kinds, tours, noisy restaurants). Still, in spite of their proven ability to improve speech perception in many listening conditions, only a relatively few adults were sufficiently motivated (and secure in themselves) to use them. Indeed, many hearing-impaired adults were not even informed of their existence, since most hearing aid dispensers simply “assumed” that their clients would not be interested in such a cosmetically unappealing device and never raised the possibility with them.

This is where the new generation of personal FM systems comes in. These systems are smaller and much more convenient to use than the older ones. The microphone/transmitters are about the length of an eyeglass case, but narrower. They can be hand-held in situations where the source of sound is relatively close or physically placed next to more distant sound sources. They represent, in fact, an easily portable, “third” ear. The signals broadcast by the transmitter are detected by an FM “receiver” boot plugged into the bottom of behind-the-ear (BTE) hearing aids. Many of the newer generation of BTE hearing aids, not just from the manufacturer of a specific FM system, will accept such a boot. One company (AVR Sonovation) makes BTE hearing aids with internal FM receivers. Thus clients cannot only be informed of the potential advantages of a personal FM system, but because trials would be so easy to arrange, an actual trial should be easy enough to provide for those people who are potentially good candidates.

Let us consider some of the types of situations in which having such a “third” ear can be employed. I shall review here mainly those that I myself experienced and comment on a few other potential applications, particularly in various kinds of employment situations.

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Copyright 2011 by the RERC on Hearing Enhancement -- All Rights Reserved
Last modified: 07/01/2013

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