On March 14, 2002, an international reader from Australia wrote:

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Michael Der wrote:

Hi XXXXX,

As promised, here is my comment on the benefits of FM systems. 

First off, I like to raise the issue of noise in the typical classroom setting.  Background noise in a classroom will vary depending on the types of activities that children are engaged in, the age and number of students, and the acoustical treatment of the room.  Children need a 10 dB louder signal than adults in order to full comprehend the speech that they hear.  The reason for this is due to adult’s ability to call on their experience to fill in gaps in speech.  Children don’t have the same linguistic experience, so if they do not hear every word clearly, they may not be able to fully comprehend what they are hearing. 

Below is a list of at risk populations to communication problems in the classroom setting.

-         young children with normal hearing

-         non-native English speakers

-         children with language disorders/listening difficulties

-         children with minimal sensorineural hearing loss (16-25 dBA loss)

-         children with minimal high frequency hearing loss

-         children with recurring ear infections

-         children with unilateral hearing loss

-         children with mild to profound sensorineural hearing loss

The classroom acoustics are an important variable in ensuring that optimal listening conditions exist for the students.  However, if the room acoustics are poor then a child’s ability to listen effectively will be significantly compromised.  Ideally, anything that on e can do to increase the signal to noise ratio of speech to a child’s ear will be of benefit to a child in the classroom.  One solution to achieve this is by using a wireless FM system.  The use of FM systems can significantly increase the signal to noise ratio of speech reaching your child’s ears.  The instructor/teacher speaks directly into a microphone located several inches from his mouth.  The signal is then transmitted to the receiver attached to a child’s ear.  What this does is effectively eliminating the distance between the speaker and the listener, thus greatly increasing the signal to noise ratio. 

Below are some typical signal to noise ratios:

-         one to one conversation: +10

-         television: +5

-         small groups: +3

-         place of worship: 0

-         car: 0

-         concert/movie: +3

-         restaurant: -5

-         large group: -10

If a child demonstrates a hearing loss, the more severe that loss, the greater the need for signal to noise ratio improvement.

The impact of a speech signal is significantly impacted by the distance from the speaker.  Intensity decreases with distance at a rate of 6 dB for each doubling of distance.  What this translates to is that the signal to noise can decrease from +6 dB at 12 feet, to –3 dB at 24 feet.  In terms of a child’s ability to discriminate speech i.e. hearing and understand what was said, the following data holds true:

-         82% successful discrimination at 6 feet from the speaker

-         55% at 12 feet

-         36% at 24 feet

FM systems improve signal to noise ratio on the average 15 to 20  dB over a personal hearing aid (Hawkins 1984).  FM systems provide more of the less intense high frequency information, critical for speech recognition.  FM systems extends the distance of effective communication.

There is a tremendous amount of literature that supports the benefits of FM systems for classrooms.  Much of this information is readily available over the internet.  I recommend you look on the internet for support material as well.  If you approach the manufacturer of the FM devices or hearing aids, I am certain they will be more than happy to provide you will additional information.   Below is a short list of references.  

Good luck,

Michael Der

Audiologist 

Madell, J.R., & Sandrock, C. (1997).  Selecting an FM System:  When a hearing instrument is not enough.  The Hearing Review, 3-16.

Ross, M. (Eds.). (1992).  FM Auditory training systems:  Characteristics selection and use.  Maryland:  York Press.

Bartschi, A. (1998).  Wireless solutions for hearing instruments.  High Performance Hearing Solutions, 3, 48-52.

Berg, Frederick S. Acoustics & Sound Systems in Schools. Singular Publishing Group, Inc. San Diego, 1993.

Bess, Fred H., Gravel, Judith S., Tharpe, Anne Marie.  Amplification for children with auditory deficits.  Bill Wilkerson Center Press; Nashville, 1996.

Crandell, Carl C., Smaldino, Joseph J., Flexer, Carol.  Sound-field FM amplification.  Singular Publishing Group, Inc. San Diego, 1995.

Edwards, C. Today’s lesson:  Noise in the classroom. Vibes.  The Canadian Hearing Society’s Quarterly Magazine.  19-20, July 1997.

Flexer, Carol.  Classroom management of children with minimal hearing loss.  The Hearing Journal. 48 (9): 10-58, Sept. 1995.

Hetu, R. Truchon-Gagnon, C., Bilodeau, Sylvie A. Problems of Noise in School Settings: A review of literature and the results of an exploratory study.  JSLPA. 14 (3): 31-39, Sept. 1990.

Lewis, Donna E., Assistive devices for classroom listening. AJA.  58-68, March 1994.

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