FMs and ALDs
20
Assistive Devices Objectives Describe effects of FM system on speech perception Recommend appropriate assistive listening devices for a given child.
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Transcript of FMs and ALDs
Assistive Devices
Objectives Describe effects of FM system on speech
perception Recommend appropriate assistive listening
devices for a given child.
Assistive Devices - FM
ANSI Standard 12.60 - Classroom acoustics
Noise no greater than 30 dB-A
Reverberation time (RT60) no greater than 0.6/0.7 s
Assistive Devices - FM
60 dB room noise 4’ away: 66 dB
+6 SNR 8’ away: 60 dB
0 SNR 16’ away: 54 dB
-6 SNR
Assistive Devices - FM
Mic 1’ to mouth 60 dB room noise
1’ away: 78 dB +18 SNR
1’ away: 78 dB +18 SNR
1’ away: 78 dB +18 SNR
Assistive Devices - FM Types
Microphones Lavalier
Boom
Handheld
Receivers Self contained
Direct audio input to CI or HA
Induction Neckloop T-Coil
FM Microphone Styles
FM Receivers
Soundfield FM
Desktop FM
Conundrum of the FM v. HA/CI Microphone
FM only – only teacher’s voice is audible FM+Mic – teacher plus environment audible
For severe/profound loss, also own voice is audible
Adaptive FM
FM system automatically adjusts level of hearing aid/CI microphone
Subjectively preferred
Better performance on speech recognition
(Thibodeau 2010)
Troubleshooting FM
Transmitter and receiver must be on same channel
Both need to be charged Cables need to be connected
Bluetooth adapters
Music
Music or
Other ALDs
Closed/open captioning
Vibrating alarm clocks Flashing alerts
phone, doorbell Amplified telephone Phone relay
Voice Carry-Over TTY relay IM relay (AT&T and AOL Instant Messenger)
Factors affecting classroom understanding
Distance
Directionality
Reverberation
Noise
Signal to noise ratio
Affected by Noise and DistanceDistance
Directionality
Reverberation
Noise
Critical distance Where indirect
sound level equals direct sound level
Distance
Directionality
Reverberation
Noise
Indirect energy
Direct
energy
Critical distance
Within 1/3 of critical distance Direct energy 10
dB above indirect
Distance
Directionality
Reverberation
Noise
Within 1/3 of critical distance
Distance
Directionality
Reverberation
Noise
Critical distance
More than 3x the critical distance
Beyond 3x critical distance Direct energy 10
dB below indirect
Distance
Directionality
Reverberation
Noise
Critical distance
Directional microphones are only effective within critical distance!!
