1. FM Systems for School Aged Children
Linda Thibodeau, Ph.D.
Advanced Hearing Research Center
Callier Center for Communication Disorders
University of Texas at Dallas
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2. Supportive information Website of Linda Thibodeau
May be obtained at the
Website of Linda Thibodeau
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3. Welcome to this lecture on FM Systems!
I hope you will find this information interesting and rewarding as you learn of the significant differences that can be achieved with
FM Systems!
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4. Copyright by Linda M. Thibodeau 2005
OVERVIEW
1. Rationale for use of FM Systems
2. FM Systems
3. FM Evaluation Procedures
4. Use of FM Systems
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5. Let’s start with the Rationale for using FM Systems.
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6. 1. Rationale for use of FM Systems
A. Signal-to-Noise Ratio
B. Typical Classroom Noise
C. Audio Demonstration of
Classroom Noise
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7. Signal-to-Noise Ratio
A major problem for all persons with hearing loss is…. Listening in Noise !
The problem is described by the
signal-to-noise ratio or SNR.
The amount of “Signal,”
what you WANT to hear
. . .compared to. . .
“Noise”,
ANY sound you don’t want to hear.
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8. In a large group, distance and noise can be a problem.
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9. This results in a poor signal-to-noise ratio.
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i.e. the intensity of the noise at the listener’s ear is greater than the signal of the speaker.
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A poor S/N ratio is -10 dB
the noise is 10 dB more intense than the signal
80 dB
70 dB
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A good S/N ratio is +20 dB
the signal is 20 dB more intense than the noise
70 dB
90 dB
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So...ideally we want to have a positive S/N ratio and a constant signal level regardless of distance between the speaker and the listener.
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We can accomplish this by placing a microphone on the speaker and delivering the sound directly to the listener.
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15. B. Typical Classroom Noise
Optimal noise level
for a class with students with hearing loss would be
30-35 dBA,
yet the typical level is about
60 dBA.
Average conversational speech is about 60 dBA, which means the SNR is often 0 dB!
(ASHA, 2005)
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16. (the teacher’s voice is 15 dB more intense than the background noise).
The optimal SNR for teaching students with hearing loss would be +15 dB,
(the teacher’s voice is 15 dB more intense than the background noise).
However, the typical SNR in classrooms is only about
+5 to – 7 dB!
(ASHA, 2005)
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17. Effects of Noise on Speech Recognition
Finitzo-Heiber and Tillman (1978) measured the effects of noise and reverberation on speech recognition scores with 8 to 12 year old
children with
Normal Hearing and Hearing loss .
Copyright by Linda M. Thibodeau

18. 95% - Only missed 5% of words
In a soundbooth:
No Noise, Little Reverberation
Children with Normal Hearing scored
95% - Only missed 5% of words
Children who were Hard of Hearing scored = 83% - Only missed 17% of words
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In a situation similar to a typical classroom:
Normal Hard of
Hearing Hearing
SNR of +12 dB: % %
SNR of 0 dB: % %
These results suggest that children with hearing loss may only be hearing about one fourth of speech in a typical
noisy classroom.
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20. C. Audio Demonstration of Classroom Noise
Click on each loudspeaker to hear:
Typical Classroom Noise
A teacher reads a story in a classroom with a typical SNR
Same teacher reads a story in
an optimal SNR provided by
using an FM System
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21. second section of the lecture.
Now let’s look at the operation and design of FM Systems in our
second section of the lecture.
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22. Copyright by Linda M. Thibodeau 2005
2. FM Systems
A. Introduction to FM Technology
B. Transmitters/Microphones
C. Receivers/Sound Transducers
D. Cochlear Implants and FM Systems
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A. Introduction to FM Technology
FM Systems use a frequency-modulated
signal to transmit the speech through the
air similar to an FM Radio station.
The system involves a microphone and
transmitter on the speaker and a receiver
for the listener with some type of
sound transducer.
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All FM systems have these parts-
Microphone
Transmitter
Receiver
Sound Transducer
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FM Transmission Process
1) Speaker talks into
microphone which
generates rapid
fluctuations of
voltage
2) Voltage fluctuations are passed to a transmitting antenna
3) Voltage fluctua-
tions produce fluctuations of electric and magnetic fields around the antenna at
a specific frequency
5) Electromagnetic
waves produce
fluctuations in voltage at a
receiving antenna
4) Electric and
Magnetic field
fluctuations travel
as electromagnetic
waves
6) The voltage fluctuations are amplified
8) The voltage
fluctuations in that frequency range are
sent to the
acoustic transducer
9) The transducer
converts the voltage
fluctuations into
sound waves to
go to the ear
7) A filter selects the frequency of interest
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Transmitting Frequencies
Typically each transmitter is assigned a frequency,
also called a channel. These may be coded by numbers,
colors, or letters. The channel component may be
called an oscillator.
Examples:
Number Color Letter Trans. Freq.
#1 Red/Gray A
#2 Brown/Gray B
There may be as many as 40 different channels used
in a school.
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27. Now let’s look more closely at the Transmitter/Microphone features….
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28. B. FM Transmitters/Microphones
Descriptive Categories
1) Microphone Type
2) Microphone Location
3) Channel Options
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29. FM Transmitters/Microphones
Microphone Type:
The microphone characteristics can vary.
Omni-directional Microphone-
picks up sound from all around
Directional Microphone-
picks up sound primarily from the top
Multi-Directional Microphone-
picks up sound from all around OR focused area
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30. FM Transmitters/Microphones
Microphone Location:
All Transmitters are worn on the body, but the microphone placement may vary.
On the Body
On the Head
On the Lapel
On the Cheek
(also called Boom mic)
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31. FM Transmitters/Microphones
Channel Options on the transmitters:
Single, but fixed-Oscillator channel set at the factory and cannot be changed Multi-Frequency
Manual frequency selection-Oscillator
can be removed and changed to another frequency
Wireless frequency selection-Channel
is changed digitally
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32. Wireless Channel Changeability
1) Direct Frequency Synchronization-
Pressing a button on the transmitter sends a signal to the receiver to synchronize the channel
Ex. Phonak MLxS with Campus S
2) Automatic Frequency Synchronization-
Walking near a plate on the wall that contains transmitter changes the channel to a preset number
Ex. Phonak MLxS with Wall Pilot
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Automatic Frequency Synchronization-Students wearing Phonak MLxS walking past Wall Pilot as they enter classroom to get synchronized to the correct frequency for that class.
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34. OTHER OPTIONS - Transmitters
Programmable-Allows the channels to be set digitally via a connection to a computer.
Audio Input Jack-An input jack for the audio signal from another source such as a tape player, VCR, or computer.
No FM/Low Battery Lights-Alerts the users that the batteries or FM switches should be checked.
Talk Over-Allows the microphone to be active
even when the transmitter is connected to another audio source such as VCR
Mute Switch-Allows the microphone to be
deactivated so conversations can be private.
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Now let’s look more closely at the Receiver/Sound Transducer features….
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36. C. FM Receivers/Sound Transducers
Descriptive Categories
1) Type-Relationship to Personal Amplification/Cochlear Implant
2) Receiver Location
3) Channel Options
4) Sound Transducers
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37. FM Receivers/Sound Transducers
1) Type-Relationship to Personal Amplification/Cochlear Implant
There are two main types with respect to interacting with Personal Devices:
Basic-Does not interface with Hearing aid or Cochlear
Personal-Does interface with Hearing aid or Cochlear Implant
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38. FM Receivers/Sound Transducers
2) Receiver Location:
Some FM Receivers interface with a personal hearing aid worn at the ear? (i.e.one that the student wears throughout the day)
Students and parents generally like this option!!!
Some are worn:
On the Body On the Ear
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39. FM Receivers/Sound Transducers
3) Channel options on FM Receivers?
Single, but fixed-Oscillator channel set at the factory and cannot be changed
Multi-Frequency
Manual frequency selection-Oscillator
can be removed and changed to another frequency to match transmitter
Wireless frequency selection-Channel
is changed digitally by pressing a
button on the transmitter that is in close
proximity
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40. FM Receivers/Sound Transducers
Two Basic Categories:
Basic System-Child takes off personal hearing aid each day and puts on the FM Receiver that picks up the teacher’s voice AND acts like a hearing aid.
Personal System-Child keeps the personal hearing aid on all day and the FM Receiver is interfaced through one of three arrangements.
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41. FM Receivers/Sound Transducers
Basic System Options
Button Earphone with Body-worn FM system
Ear Level FM Receiver with limited power
Soundfield Speaker to set on desktop or
mounted on the wall
FM Amplifier with Behind the Ear hearing aid
Personal System Options
Neckloop with Personal Aid on T Switch
Direct Audio Input with Personal Aid
Silhouette with Personal Aid on T Switch
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Let’s look at some illustrations of these different types of sound transducers!
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43. Button Earphone with Body-worn FM system
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44. Basic Ear Level FM Receiver with limited Power
For Persons with Normal hearing, Mild loss, Auditory Processing Disorder
EduLink
by Phonak
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45. Soundfield Speaker to set on desktop or mounted on the wall
Desktop Speaker
Wall Mounted
Speakers
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46. FM Amplifier with Behind the Ear hearing aid
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47. Neckloop with Personal Aid on T Switch
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48. Direct Audio Input with Personal Aid
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49. Silhouette with Personal Aid on T Switch
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50. OTHER OPTIONS - FM RECEIVERS
Programmable/Digital-Allows the features of the receiver to be set via a connection to the computer which results in more precise control of the FM signal.
Audio Input Jack-Allows a direct electrical connection to a VCR, computer, or tape/CD player.
No FM/Low Battery Lights-Warning lights to indicate the switch settings and/or batteries need checking.
Microphones-When the FM receiver is not interfaced with a personal hearing aid that has a microphone, a microphone will be needed on the FM receiver to pick up the sounds nearby the student but at a lower level than the teacher’s voice.
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51. Now let’s consider interfacing FM System with Cochlear Implants….
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52. D. Cochlear Implants and FM Systems
A child may receive a cochlear implant when they receive no benefit from a hearing aid. After a surgical procedure, they receive sound stimulation through electrical impulses applied directly to the auditory nerve. FM systems can be used with cochlear implants to provide that optimal SNR.
The same FM Transmitters and some of the same FM Receivers can be used.
Electrical Coupling of the FM Receiver to the Cochlear Implant can occur in two arrangements:
Patch cords connect FM Receiver to Cochlear Implant
Direct Plug-in of FM Receiver
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53. Patch cords connect FM Receiver to Cochlear Implant
Ear Level Processor Body Worn Processor
FM Receiver
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54. Direct Plug-in of FM Receiver
Cochlear Implant
Speech Processor
FM Receiver
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It is very important to evaluate FM Systems to ensure proper settings for maximum benefit. Let’s see how that can be accomplished in our third section of the lecture….
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56. 3. FM Evaluation Procedures
A. Electroacoustic
B. Real Ear
C. Behavioral
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57. A. ELECTROACOUSTIC PROCEDURES
ASHA GUIDELINES FOR FITTING AND
MONITORING FM SYSTEMS (1999)
This document provides Recommended Measures to be performed to compare response through amplification settings with and without the FM system.
ELECTROACOUSTIC PROCEDURES
Performed by an Audiologist
Child need not be present, only need child’s
Hearing Aid (HA) and FM System
Very objective, efficient way to set systems
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First step is testing the child’s HA alone. It is attached to a microphone and a sound is delivered, and the output is measured and graphed.
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59. Next the FM Receiver is attached and the HA/FM system is evaluated.
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The output curves are compared to verify that Curve #2 (FM + HA) shows that the FM signal will be higher than the Hearing Aid alone signal.
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B. Real Ear Procedures
Settings can also be verified by placing a small tube microphone in the child’s ear canal and measuring the sound level while
1) first wearing the HA alone, then
2) while wearing the HA + FM system 1) 2)
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62. C. Behavioral Testing with FM Systems
Threshold Testing typically not recommended due to lack of significant information
Speech Recognition Testing is more meaningful
Without FM System
With FM System, with examiner wearing FM transmitter
Test each arrangement in Quiet and in Background Noise if time permits
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Test Arrangement
Examiner Side of Test Booth Student inside Test Booth
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64. Behavioral FM Verification Case Illustration
Third Grader who resisted wearing FM system.
Testing done to show him benefits.
Hearing Aid Alone
Quiet %
In typical classroom noise 60%
Hearing Aid with FM
In typical classroom noise 90%
Quiet %
With the FM System, the speech recognition score improved from 60% in noise to 90%!
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65. We’re now at the fourth, and final section of the lecture….
Let’s consider some ABC’s of using FM Systems in the Classroom!
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4. Use of FM Systems
A. Microphone Technique
B. Care and Use
C. Settings for Various Teaching Arrangements
D. Troubleshooting
E. Common Problems
F. Manufacturers
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A. Mic Technique
1. proper distance 6-8 in from mouth
(unless using boom mic, then 2 in)
2. don't stand near noise
3. turn off when leaving the classroom
4. antennae must hang free, not wrapped around transmitter
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B. Care & Use
1. clean units with damp cloth
2. if put in prolonged storage - need even temperature
3. annual electroacoustic checks
4. daily listening checks
5. keep in chargers when not in use
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69. C. Settings for Various Teaching Arrangements
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70. Settings on FM Receivers
FM Receivers usually have switches that can be set to send certain signals to the listener:
FM Only-Sends only the teacher’s voice
Environmental (ENV) Only-Sends
only the voices picked up by the
microphone on the FM Receiver
FM+ENV-Sends the teacher’s voice and the environmental sounds around the student, ideally the teacher’s voice is about 10 dB more intense than the ENV signal
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1) Teacher lecture
Set Receiver to pick up FM Only Signal
I think they like
my tux!
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72. 2) Class discussion led by the teacher
Set Receiver to pickup FM + Environmental (ENV) Signals
I’m not sure
they’re listening.
How To Select
Hearing Aids....
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73. 3) Independent work at desk
Set Receiver to pickup up ENV Signals
I’d rather be
shopping!
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4) Group work
Set Receiver to pickup ENV Signals
I’d rather be
fishing.
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D. Troubleshooting -
Works best when two people check each component separately and the add components progressively according to the following steps:
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1. check FM receiver in microphone mode (as a body aid)
2. check FM receiver & transmitter in FM mode
3. check FM receiver & transmitter in FM & Microphone mode
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4. check FM Receiver & transmitter & personal aid (boot with loop silhouette)
5. check personal aid alone.
6. annually check each component, daily check entire system (#4)
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E. Common problems
1. setting volume controls - must be determined by audiologist
2. matching oscillators between transmitter and receiver
3. setting the mic switch
4. neckloops often noisy & inconsistent signal
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5. changes in frequency response re: to hearing aid alone when using direct input or neckloop
6. access to audiologist who can monitor
7. dead batteries
8. broken cords
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F. Manufacturers
Web pages are VERY helpful, but do not hesitate to contact manufacturer to ask questions:
1. Phonic Ear
2. Telex
3. Comtek
4. AVR Sonovation
5. Phonak
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SUMMARY
The SNR can be significantly improved by using an FM system.
Because there are so many options, the audiologist is responsible for selection and fitting of FM systems and the necessary orientation and counseling.
Any connection of hearing aid or cochlear implant with an FM system should be tested in controlled conditions to determine benefit.
FM systems must be monitored on a regular basis by trained personnel under an audiologist’s supervision.
Copyright by Linda M. Thibodeau

82. Last Minute Advice regarding using FM Systems….
If you have a lot of tension and you get a headache, do what it says on the
aspirin bottle: "Take two aspirin"
and  
"Keep away from children."
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REFERENCES
American Speech, Language, and Hearing Association (2005). Acoustics in Educational Settings: Technical Report, ASHA Supplement 25, In Press.
American Speech, Language, and Hearing Association (1999). Guidelines for Fitting and Monitoring FM Systems. ASHA Desk Reference, 2,
Finitzo-Hieber, T., & Tillman, T.W. (1978). Room acoustics effects on monosyllabic word discrimination ability for normal and hearing-impaired children. Journal of Speech and Hearing Research, 21,
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Acknowledgements
Erin Schafer, M.S.
University of Texas at Dallas
Paul Dybala, M.S.
Gary Overson, M.A.
Jack Scott, M.A.
Marcia Crouch, M.S.
Plano Regional Program for the Deaf
Amy Popp, M.S.
Cochlear Corporation
Sallie Frye, M.S.
Frye Electronics
Helmut Ermann
Phonak Hearing Aids
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