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Getting the Most out of FM amplification

Getting the Most out of FM amplification. Presentation to Clarke School Mainstreaming Workshop, Springfield MA, Oct 2007 Preparation supported, in part, by NIDRR grant #H1343E98 to Gallaudet University. Part 1. Enemies of hearing. Speech loud. Speech quiet. Distance.

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Getting the Most out of FM amplification

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  1. Getting the Most out ofFM amplification Presentation to Clarke School Mainstreaming Workshop, Springfield MA, Oct 2007 Preparation supported, in part, by NIDRR grant #H1343E98 to Gallaudet University.

  2. Part 1. Enemies of hearing

  3. Speech loud Speech quiet Distance Speech loses 6 dB for every doubling of distance 72 66 60 54 48 Speech 

  4. S P E E C H NOISE Noise • Obscures speech sounds that are weaker than itself • Distracts the listener

  5. Speech loud and well above noise Speech quiet and lost in noise Distance and noise Speech loses 6 dB for every doubling of distance 72 66 60 55 48 Speech  Noise  55 55 55 55 55

  6. Direct sound Reverberation Enclosed space Room acoustics - the reverberation effect Free field

  7. Reveberation only (5%) Noise plus reverberation(23%) Noise Only (10%) Noise and reverberation(Data from Pegan, 2007, AuD project)

  8. Enemies of hearing • Distance • Noise • Reverberation • Plus sensorineural hearing loss • Interactions among the above • And don’t forget- rapid speech- unclear speech- accented speech- difficult material- unfamiliar words

  9. Part 2. The FM solution

  10. 80 dB Listening via the local hearingaid microphone 60 dB Noise 55 dB Noise 55 dB Noise 55 dB

  11. 80 dB Listening via a remote microphoneand direct audio input 80 dB Noise 55 dB Noise 55 dB Noise 55 dB

  12. Modulator/ transmitter Demodulator/ receiver Listening via a remote microphoneand FM wireless link Basically, we have providedan invisible wire between microphone and hearing aid

  13. Part 3. Cautions

  14. Caution #1. FM is not magic. • The FM link does not add anything to the signal. • It only takes away the negative effects of distance, noise, and reverberation. • The student hears no better via the FM microphone than he does via his hearing aid microphone when the talker is close by.

  15. BUT what happens when we turnon the hearing aid microphone? It depends on therelative gains via the two microphones

  16. Caution #2. When Aid+FM is used (the default) - - incorrect balance of the two sensitivities can reintroduce the noise and reverberation that the FM was supposed to remove.

  17. Equal outputFM gain is 20 dB less than Aid gain S/N = 60/55 Aid gain = +40 Output S/N= 100/95 = 5 dB Aid+FM Output= the higher of the two Speech = 100 dB Noise = 95 dB S/N = 100/95 = 5 dB Benefit of FM = 0 dB S/N = 80/55 FM gain = +20 Output S/N = 100/75 = 25 dB

  18. 2. Equal gainFM gain equals Aid gain (transparency) √ S/N = 60/55 Aid gain = +40 Output S/N= 100/95 = 5 dB Aid+FM Output= the higher of the two Speech = 120 dB Noise = 95 dB S/N = 120/95 = 25 dB Benefit of FM = 20 dB S/N = 80/55 FM gain = +40 Output S/N = 120/95 = 25 dB

  19. Adjusting for transparency (equal gain) • Confirm aid fitting(Quiet  audible: Loud  comfortable) • Connect FM; set to Aid+FM • Mute FM mic or place in quiet spot • Measure aid output for 65 dB aid input • Place FM mic in test box; remove aid • Measure aid output for 65 dB FM input • Adjust FM gain until the two outputs match

  20. Problems in adjusting for transparency • Compression in FM transmitter causes FM gain to change with FM input.

  21. Compressionof combined signal Compressionof FM signal Relative gain via FM and AID changes with changing FM input

  22. Problems in adjusting for transparency • Compression in FM transmitter causes FM gain to change with FM input. • Higher output for FM signal may be uncomfortable if not controlled by compression in aid (i.e., improper fitting). • Therefore initial FM-gain setting must be validated behaviorally

  23. Caution #3. FM benefit applies only to the talkerwith the microphone - But comments, questions, and answers from other students are part of the total learning experience

  24. With single microphone ((((((((((

  25. With two microphones

  26. Caution #4. Psycho-social issues • Intrusiveness • Embarrassment • Self-consciousness • Difference • Hassle-factor

  27. Psycho-social issues addressed through: • Student counseling/preparation • Class counseling/preparation • Teacher/staff counseling/preparation • Instruction/explanation/demonstration • Behavioral data (single-word dictation with and without FM microphone)

  28. Caution #5. Monitoring and maintenance • Equipment fails • Formal plan needed for prompt detection of problems • Speedy repair • Loaner while waiting • Periodic audiological verification and validation

  29. Summary • Enemies of hearing: distance, noise, reverberation • Virtually eliminated by FM microphone at talker’s mouth • For best benefit: • FM microphone close to mouth • Balance of FM and Aid gain (audiologist) • Access to contributions of other students • Counseling/preparation/instruction/demon-stration/behavioral data • Formal program for monitoring and maintenance • Periodic verification and validation (audiologist)

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