Introduction

This page contains some information about assistive listening devices.

Most of the material in this Web page is up to date as of July 1996.

Other sources of information about assistive listening devices:

• Assistive Listening Devices FAQ
• The role of assistive listening devices in the classroom

Infra-red systems (and leakage problems with other systems)

Infra-red systems are used a lot in movie theaters and other indoor places but had not been usable in sunlight because the sunlight would ruin the transmission. You do have to be sure not to block the line of sight between the receiver and the transmitter. The fact that infra-red operates by line-of-sight means that it's safer to use for confidential conversations than FM or audio loops are. (One has to be inside the room to pick up an infra-red signal--or at least within sight of the transmitter---but a person can be in another room and pick up an FM or audio loop signal if the transmission is strong enough.) Thus it's a problem to have two rooms looped with audio loops right next to each other since there'll be interference, and it's also a problem to have an audio loop in a nearby room if you have people needing to use a neck loop or telephone---they'll pick up the audio loop instead when they turn their t-switch on.

FM systems

FM systems are the most versatile since they are portable, they can be used outside, and the tunable ones can be tuned to different frequencies if there's interference on a particular frequency. The Chapparal db50 is one tunable FM system, but there may be others as well. Chorus makes a device which is like an universal listening system---you can switch parts to make it pick up an audio loop, an infra-red system, or FM. I don't know if you can change FM frequencies on it.

(Contributed by Dana Mulvany at 10 Jan 1994.)

The Microvox FM System

Microvox (from Phonak) is an FM system. There is a transmitter, worn by a parent or a teacher, with a lapel microphone, close to the mouth... the transmitter sends the speaker's voice, via FM radio waves to the receiver, which is worn by the hearing aid user.

The receiver is coupled to the user's personal hearing aids via an audio input boot and wire. The speaker's voice is transmitted to the hearing aids at a loudness level that is above the other noise in the environment, i.e. air conditioner, other children shuffling feet and papers, etc. The user, depending upon the degree of hearing loss and upon how well they do with amplification in general, is then given an improved listening environment. The user can select to have his microphones on his hearing aids stay on most of the time, or off except when used.

The Microvox is a nice system in that it is very small. Just today, I was working on my budget for the next two years, and I am considering upgrading our FM systems at the school where I work to Microvox units. In the not too distant future, Phonak will come out with what will be called MicroLink... the "FM receiver cyrstal will be completely contained in the audio input boot, which will attach to the hearing aid (must be a Phonak hearing aid)...This is a very exciting development that I only learned about yesterday. I am more and more frequently recommending personal FM systems such as the Microvox to families... Good signal to noise ratio that is obtained through FM systems is not only for school any more...

(Contributed by Katherine C. Morehouse at 28 Sep 1995.)

FCC Considering New Band for FM Systems

The Federal Communications Commission is taking action to make the 215 to 216 megahertz band available to a new low-power radio service, including auditory assistance devices, such as FM systems and auditory trainers. Comments were requested by July 18.

Currently, the 72 to 76 megahertz band is inundated with interference from cellular telephones, pagers, and other devices. A FM systems manufacturer petitioned the FCC for the use of 215 to 217 megahertz, which had been used at one time for aircraft beacons. The FCC put out a notice of proposed rulemaking, essentially a request for comments.

Self Help for Hard of Hearing People, Inc., the American Academy of Audiology, and others support the allocation of this new band because:

1. It will make additional frequencies available for FM systems.
2. It will provide interference-free channels, because of power limitations on all transmitters in the band. (This has been a major problem in schools and places of public accommodation because of interference from cellular telephones, pagers, and other devices.)
3. It will provide opportunities to provide improved FM systems because of antenna efficiency in the higher bands.

The FCC is now reviewing comments, and is expected to render a decision in the near future.

(Contributed by Cynthia L. Compton, Assistive Devices Center, Department of Audiology and Speech, Gallaudet University; was originally distributed in the 1995 Aug 08 issue of TFA Technical Topics.)