FMs and ALDs

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  • 1. Assistive Devices
    Objectives
    Describe effects of FM system on speech perception
    Recommend appropriate assistive listening devices for a given child.
  • 2. 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
  • 3. 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
  • 4. 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
  • 5. Assistive Devices - FM Types
    Microphones
    Lavalier
    Boom
    Handheld
    Receivers
    Self contained
    Direct audio input
    to CI or HA
    Induction
    Neckloop
    T-Coil
  • 6. FM Microphone Styles
  • 7. FM Receivers
  • 8. Soundfield FM
  • 9. Desktop FM
  • 10. 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
  • 11. Adaptive FM
    FM system automatically adjusts level of hearing aid/CI microphone
    Subjectively preferred
    Better performance on speech recognition
    (Thibodeau 2010)
  • 12. Troubleshooting FM
    Transmitter and receiver must be on same channel
    Both need to be charged
    Cables need to be connected
  • 13. Bluetooth adapters
    Music
    Music or
  • 14. 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)
  • 15. Factors affecting classroom understanding
  • 16. Signal to noise ratio
    Affected by Noise and Distance
  • 17. Critical distance
    Where indirect sound level equals direct sound level
    Indirect energy
    Direct energy
  • 18. Within 1/3 of critical distance
    Direct energy 10 dB above indirect
    Within 1/3 of critical distance
    Critical distance
  • 19. Beyond 3x critical distance
    Direct energy 10 dB below indirect
    More than 3x the critical distance
    Critical distance
  • 20. Directional microphones are only effective within critical distance!!
    Critical distance