learning to hear with assistive listening deviceA ssistive Listening Devices (or ALD’s) are used to minimize the effects of noise, echoes, and distortion when a person who is hard of hearing must listen to speakers at a distance or in noisy places. ALD’s aim to deliver “clear speech”. Many ALD’s can also be used to listen to television and radio.

All ALD’s operate on essentially the same principle. A microphone is placed close to the mouth of the speaker. The sounds picked up by this microphone are converted to a form that can be transmitted without noise, interference, or acoustical distortion. This transmission is then picked up by a suitable receiver that converts it back to sound.

Four Major Types of Transmission Methods

Four major types of transmission methods are used by various assistive listening devices: direct connection, inductive (or loop), FM, and infrared. Each of these systems has advantages and disadvantages, and can be used in some situations but not in others.

Hard-wired devices, sometimes called “direct devices”, connect the listener to the microphone and the amplifier with wires. In these devices, the sound travels as electrical current in the wires. They allow the listener to place the microphone close to the mouth of the speaker in face-to-face situations. If there are more speakers, the microphone must be handed from one to the other, and the wires restrict mobility. Privacy is guaranteed, as you are the only listener. Hard-wired devices work well in the car and can also be used to listen to radio or TV by placing the microphone close to the speaker. They are also the least expensive ALD’s.

Inductive, or loop, systems use the “T” (telecoil) switch of the hearing aid (see the definitions in the glossary of Chapter 6 of the Ear Smarts handbook, “The Telephone”). The room is equipped with a wire loop, which emits a magnetic field that carries the sound to the hearing aid’s telecoil. The telecoil converts the magnetic field back into sound. While some loops are home-built, it is best to purchase a commercial system and have it installed professionally.

These systems are sometimes used in churches or group meeting places, and they require the user to have a T-switch. Be prepared to move around to different parts of the room, as the strength of the loop may not be the same everywhere. Personal neck loops, used with FM and infrared receivers, will be discussed later.

Mini FM Personal TransmitterFM systems are essentially one-way walkie-talkies that provide great freedom of movement. The sound travels through the air by radio waves. The transmitters are miniature radio stations, and the receivers are similar to radio receivers. These components can vary in size – from the size of a small transistor radio to a small, wireless “boot” receiver that fits onto a behind-the-ear hearing aid. They can operate on several channels and up to a couple of hundred feet distance.

FM systems are ideal for one-on-one communication when you don’t want the interference of wires (walking, hiking, field trips, guided tours, for example). They are used extensively in classrooms and in other group listening situations such as lectures, concerts, and churches. One transmitter can serve any number of receivers. For people who are hard of hearing who need to hear in small meetings, small and powerful conference microphones, which pick up the speakers’ voices within a circle of up to 20 feet and transmit them via FM, can be placed in the middle of the meeting table. The listener wears an FM receiver. FM systems are subject to interference, and they themselves can interfere with sensitive electronic equipment (for example, in hospitals). FM systems do not guarantee privacy as any receiver on the same FM frequency can pick up the transmission, even through walls.

Infrared Hearing SystemInfrared (IR) systems use invisible light to transmit sound. They require an unobstructed path between the source (think of the transmitter as a flashlight) and the receiver (an electronic eye). This places some restriction on the mobility of the user. However, privacy is easy to achieve as walls, curtains, and windows usually stop the transmission. IR systems can be used outdoors, but not in direct sunlight. Generally, electrical systems do not affect IR devices, but some fluorescent lights may create problems.

Infrared systems are often used for listening to the TV or radio. They are also used in meeting rooms, movie theatres, concert halls and churches. Conference microphones using IR are also available and provide more privacy than FM because FM can travel through walls, but infrared cannot.

How does the sound get to the user’s ear?

With all ALD’s, a crucial issue is the way the sound is finally delivered to the hard of hearing listener’s ear.

Hearing AidIf the user wears a hearing aid, it is best to “couple” the ALD to it. If the hearing aid has a T-switch, one can use a neck-loop or a silhouette that fits behind the ear. A neck loop is an unobtrusive plastic coated wire, which plugs into the FM/IR receiver and creates magnetic energy (an electro-magnetic field), which transmits the sound to the hearing aid telecoil (T-switch). Functioning in the same way, a silhouette is a flat piece of plastic shaped like a behind-the-ear hearing aid, which sits over the ear between the hearing aid and the head. The T-switch allows the user to turn off the hearing aid microphone to eliminate background sound. A “TM” switch allows the user to hear both the speaker and environmental sound.

Some hearing aids can be fitted with direct audio input. In this case, a small wire, usually attached to a “boot”, runs from the hearing aid to the FM/IR receiver. There are also tiny FM receivers that attach directly to the bottoms of some behind-the-ear hearing aids, eliminating wires.

Head sets or ear plugs can also be used with hard-wired, FM, or IR systems, if the user is not wearing a hearing aid. Before purchase, always try them out for comfort and the level and quality of sound they can deliver.

Tips for selecting an ALD System

Selecting an assistive listening (ALD) system can be confusing. In preparation, try to answer the following questions:

  • Is privacy important? If yes, you must go to an infrared or direct system.
  • Will the system be used by one person, or several?
  • Will the system be used in known environments? If yes, is it possible to check on electromagnetic interference (EMI), possible interference from sunlight, and distances and movement between speakers and listeners?
  • Will the equipment be used in certain restricted environments? (You cannot use an FM system on airplanes, or an FM in hospitals.)
  • Is it possible to maintain line-of-sight alignment – in a theatre, listening to TV, lectures, interviews, face-to-face discussions or a card table? If not, you must go to an FM or loop system rather than an IR system.
  • Will you have the ability to adjust the equipment if necessary? Will you be able to cope with the controls on the equipment to adjust for changes in listening conditions?
  • Will you be assertive enough to ensure that the system will be used? For example, with an FM or IR system, the speaker must be asked to wear the transmitter and must be shown how to use it.
  • Will the system operate under adverse conditions of humidity, dust, and mechanical abuse?
  • Will the system be maintained, cleaned, batteries charged, and cords kept in good repair?
  • What is the best coupling method for you? Direct audio input, T-switch with a neck loop/silhouette, or earphones/earbuds?
  • How critical is the cost to you? Is it necessary to sacrifice features and quality to save money?
  • Do you appreciate that systems cannot work under all conditions and that expectations must be realistic?

Once you have the answers, you are prepared to look at the different devices offered and to evaluate to what extent they satisfy your individual requirements.

Pay attention to yourself as well as the technology while you are selecting a system. Your acceptance and knowledge will be crucial. You may find the technology unfamiliar; the sound quality may differ from what you first expect; you may be self-conscious about using the equipment; your expectations may run too high.

Always try out the ALD’s under realistic conditions. Most vendors will sell on approval, so this is feasible. Appropriate training is also necessary, and the help of an experienced user who is hard of hearing is invaluable.

Thanks for this article go to:

Charles A. Laszlo, CM, OBC, PhD, PEng, CCE
Professor Emeritus
Department of Electrical and Computer  Engineering, University of British Columbia


About The Photo (top left):

In this picture Jade, the student, has a “boot” or mini FM wireless receiver attached to her Behind-the-Ear hearing aid (a boot can only used with a BTE hearing aid). Catherine, the teacher, is wearing a very small FM transmitter (this small transmitter could also be attached to a pocket and it has a stand so that it could be placed on a lectern or table.) There are no wires involved. Using this device, Jade can hear very clearly, much more clearly than with her hearing aid alone, from anywhere in the classroom – and often even from the hallway! Personal FM systems are great for students, meetings, travel, restaurants or any situation where the speaker is too far away from the listener or where the speaker’s voice is muffled by background noise. In this picture Jade is using a Phonak Smart Link SX personal FM system. Ask your hearing aid provider for more information.


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