Large Area Assistive Listening Systems: Review and Recommendations

2.2.3 Receivers

The receivers for FM assistive listening systems are basically FM radios "tuned" to the transmitting frequency. Most manufacturer supply a number of receivers that vary in complexity and secondary features, but all are designed to accord with the characteristics of their own transmitters. Companies that make 8, 10, 16, 37 or 40 wide or narrow band channels available in the transmitter also provide receivers that can detect any one or all of these channels. Channel selection is generally accomplished by a slide switch, push button, or wheel rotation. For some FM receivers, it is necessary to remove the back and "tune" a rotary wheel while listening to a test signal from the transmitter. This is not as "user friendly" as those receivers which permit pre-set channel changes by discrete switch adjustments.

Depending upon the specific transmitting frequency, it may be possible to interchange transmitters and receivers from different companies. Whenever, however, a receiver from one manufacturer is being employed in conjunction with the transmitter of a different manufacturer, complete compatibility may be questionable because of subtle differences in the RF and acoustic properties of the receivers. Engineers design receivers to conform to the electronic characteristics of their own transmitters.

Power to the FM receivers are supplied by disposable or rechargeable batteries. Battery life for the rechargeable batteries range from 6 to 10 hours (or up to 35 hours according to one report), while the life span of the disposable batteries (either 9 volt, AAA, or AA) vary from 18 to 70 hours depending upon volume setting and type of coupling. Convenient pocket recharging-carrying cases are available in which the receivers can be recharged while being stored. Unlike many personal FM receivers, those used with ALS generally do not include a warning light signal when the battery is weak.

The receivers come with a number of coupling options. While the ADA requires a set number of receivers in different venues, it does not stipulate the precise type or percentages of the different kinds of coupling alternatives. For people whose hearing aids contain "T" coils, the most convenient and desirable option is for them to plug a neckloop or silhouette inductor into the FM receiver, or to place electromagnetic headphones right over in-the-ear hearing aids. They are then able to take advantage of the "prescribed" characteristics of their own aids when listening through an ALS. Many BTE hearing aids can accept a direct audio input (DAI) from an FM (or IR) receiver through a wire connector. This will also permit users to benefit from the prescribed electroacoustic characteristics of their own hearing aids (keeping in mind, however, the possibility that either inductive or direct audio input coupling may not preserve the microphone response of a hearing aid).

People whose hearing aids do not include "T" coils can either remove their hearing aids and use earbuds or earphones, or place earphones right over the hearing aids (this will not work for people wearing behind-the-ear hearing aids). We cannot now predict whether such acoustical coupling will produce audible feedback or what acoustical changes this produces in the hearing aid's response. This topic has not been investigated with the current generation of miniature hearing aids. Consumers would have to try using an earphone with their own hearing aid in order to determine if acoustic feedback occurs. While it may be difficult at first for a venue to ensure the proper "mix" and number of coupling alternatives, with time and experience venues should soon learn what type of coupling arrangements their patrons prefer.

Some manufacturers of FM systems depict a user with a monaural ear-bud in their promotional material. Unfortunately, this depiction sends an implied message that monaural use of an ALS is the routine and desirable listening condition. This is inaccurate when it comes to people with normal hearing and even more inaccurate for people with hearing loss using an ALS. They need all the acoustical help they can get and, unless contraindicated by audiological findings, two-ear listening should be the routine in all ALS situations.