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Audio Frequency Induction Loops

An Assistive Listening Tool

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Updated August 15, 2010

This is a brief discussion of Audio Frequency Induction Loop technology, how it is used for assistive listening, and the advantages of it when compared to other assistive listening systems.

The telecoil in hearing aids was introduced by hearing aid manufacturers many years ago and was known as the "telephone" or "telecoil" position on the hearing aid switch. It was intended to make it easier for the hearing aid user to hear over a telephone, by picking up the sound via the magnetic field generated by the diaphragm coil in the receiver of the telephone. Telephone handsets are now required by law to have this capability.

This article will address common questions we have encountered over many years of audio induction loop research and design.

  • What is an Audio Frequency Induction Loop?
  • How does an Audio Frequency Induction Loop work?
  • Why use an Audio Frequency Induction Loop?
  • Where are Audio Frequency Induction Loops used?
  • Are there situations where an Audio Frequency Induction Loop is not suitable?
  • What are the alternatives to an Audio Frequency Induction Loop System?
  • Are Infrared (IR) or FM systems viable assistive listening alternatives?
  • Are all hearing aids compatible with Audio Frequency Induction Loops?
  • What about digital hearing aids?
  • Do Audio Frequency Induction Loops interfere with heart pacemakers?

What is an Audio Frequency Induction Loop?

An audio frequency induction loop is a way of transmitting sound through a wire loop to the telecoil in a hearing aid or a suitable receiver. They are used most frequently to improve the speech intelligibility of hearing aid users by eliminating background noise.

How does an Audio Frequency Induction Loop work?

It is well known principle that when an alternating current is passed through a wire, a magnetic field is generated around the wire. If a second wire is brought within this magnetic field, a corresponding alternating current is created within the second wire. In technical language, it is said that a current is "induced" in the second wire. Hence the term "induction."

In the most basic form, an audio induction loop system consists of a loop of wire around the perimeter of an area that is connected to an induction loop amplifier. An input signal is provided to the induction loop amplifier, and the induction loop amplifier drives an audio current (note current not voltage) through the loop in the form of a strong alternating current.

The loop itself consists of a single core insulated wire, one turn of which is placed around the perimeter of the room. As the alternating current from the amplifier flows through the loop, it creates a magnetic field within the looped area and "induces" the telecoil in a hearing aid or in a specifically design induction loop receiver within the looped area.

When a hearing aid user switches their hearing aid to the "T" position on the hearing aid, the telecoil in the hearing aid picks up the fluctuations in the magnetic field and converts them back into alternating currents. The alternating currents are amplified and converted by the hearing aid into sound.

The magnetic field within the looped area is strong enough to allow freedom of movement within the looped area and still receive the sound at a comfortable listening level. The performance of induction loop systems is specified by the IEC118-4 international standards.

The input to the induction loop amplifier can be a sound source such as a television or stereo, a public address or sound reinforcement system, a dedicated microphone, or any sound source that users inside the looped area wish to hear more clearly.

Not all loop layouts are a simple single wire surrounding a room, but this explanation illustrates the basic principles.

Why use an Audio Induction Frequency Loop?

People who suffer from hearing loss require more than just increasing the volume of sound into their ears. The loss of hearing is generally associated with the brain's neurological processing of information. For people with normal hearing, a signal to noise ratio of 6dB is required for a reasonable level of speech intelligibility. This represents quite a noisy background, and includes sounds such as reverberation, air conditioning, ventilation systems or background noise such as those associated with a crowd of people.

When a person loses about 80% of their hearing, they generally need a signal to noise ratio of 15 to 20dB for a reasonable level of speech intelligibility. This can be difficult to achieve unless the desired signal is taken straight from the basic source and transmitted directly through the loop system to avoid any reverberation or additional ambient noise. Delivering a pure, clean signal directly to the hearing aid maximizes the benefits of digital hearing aids and delivers the best possible sound possible to the hearing aid user.

Audio Frequency Induction Loops have the following advantages:

  • Using the built-in T coil in the hearing aid means hearing aid user always has their "receiver" with them
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