Overview of the peripheral auditory system
Hearing Loss
Interpretation of an audiograms
Assistive listening devicess
Cochlear implants
FM system

FM system

An FM system is an assistive listening device that helps improve speech understanding in less ideal listening environments. Less ideal environments refer to rooms or space in which the speaker is away from the listener, or in which there is much noise or reverberation.

Due to hearing loss, hearing-impaired children understand speech more difficult in the environments mentioned above. Educational settings are usually very noisy, and neither hearing aids nor cochlear implants can overcome the listening difficulty. Therefore, in addition to hearing aids or cochlear implants, children with hearing loss need an FM system help them hear better and learn better.

An FM system transmits the sound signal via radio waves, and is composed of a microphone, a transmitter and a receiver (the microphone is built in the transmitter in some models). International Telecommunication Union (ITU) assigns a specific frequency band for the use of FM system, and this band can be further divided into multiple channels for different speakers communicating with listeners in the same space without signal interference. Many transmitters and receivers on the market have the feature of multiple channels.

There are different ways to couple the receiver of an FM system to hearing aids and cochlear implants. Below is a brief introduction.

(1) Coupling to hearing aids
The most common way is to couple the receiver to a hearing aid with an audio shoe (see below).

Through the audio shoe, the signal received by the receiver can be transmitted to the hearing aid. The hearing aid must have the feature of direct audio input (DAI) in order to be connected to an FM system. Otherwise, the hearing aid is not compatible with an FM system. Some hearing aids are coupled to design-integrated receivers, saving the trouble of coupling and uncoupling the audio shoe and receiver. However, the drawback is the high price.

(2) Coupling to cochlear implants
There are several ways to couple a receiver to the speech processor depending on the implant model. A more common way is using an interface as does on hearing aids (see pictures below).

Newer implants (for example, Freedom of Cochlear, Harmony of Advanced Bionics) use either a specialized cable or earhook.

☆FM cable for Freedom (left) ☆iConnect earhook for Harmony (right)

Like hearing aids, there are design-integrated receivers for cochlear implants as well, but they are expensive and only work with specific speech processors (see pictures below).