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.
Dec 21, 2015
Getting the Most out ofFM amplification
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.
Part 1. Enemies of hearingPart 1. Enemies of hearing
Distance
72 66 60 54 48Speech
Speech quietSpeech loud
Speech loses 6 dB for every doubling of distance
• Obscures speech sounds that are weaker than itself
• Distracts the listener
Noise
SSPPEEEECCHH
NOISENOISE
Distance and noise
72 66 60
55 5555 55 55
55 48Speech
Noise
Speech quiet andlost in noise
Speech loses 6 dB for every doubling of distance
Speech loud andwell above noise
Free fieldEnclosed space
Room acoustics
- the reverberation effect
Room acoustics
- the reverberation effect
Direct sound
Reverberation
Noise and reverberation(Data from Pegan, 2007, AuD project)
Reveberationonly (5%)
NoiseOnly
(10%)
Noise plusreverberation
(23%)
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
Part 2. The FM solution
Listening via the local hearingaid microphone
60 dB
80 dB
Noise 55 dBNoise 55 dBNoise 55 dB
Listening via a remote microphoneand direct audio input
80 dB
80 dB
Noise 55 dBNoise 55 dBNoise 55 dB
Listening via a remote microphoneand FM wireless link
Modulator/transmitter
Demodulator/receiver
Basically, we have providedan invisible wire between
microphone and hearing aid
Basically, we have providedan invisible wire between
microphone and hearing aid
Part 3. Cautions
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.
BUT what happens when we turnon the hearing aid microphone?
It depends on therelative gains viathe two microphones
It depends on therelative gains viathe two microphones
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.
1. Equal outputFM gain is 20 dB less than Aid gain
S/N = 80/55
S/N = 60/55Aid gain = +40
FM gain = +20
Aid+FM Output= the higher of the two Speech = 100 dBNoise = 95 dBS/N = 100/95 = 5 dBBenefit of FM = 0 dB
Output S/N = 100/75 = 25 dB
Output S/N= 100/95 = 5 dB
2. Equal gainFM gain equals Aid gain (transparency)
S/N = 80/55
S/N = 60/55Aid gain = +40
FM gain = +40
Aid+FM Output= the higher of the two Speech = 120 dBNoise = 95 dBS/N = 120/95 = 25 dBBenefit of FM = 20 dB
Output S/N = 120/95 = 25 dB
Output S/N= 100/95 = 5 dB √
Adjusting for transparency (equal gain)
1.Confirm aid fitting(Quiet audible: Loud comfortable)
2.Connect FM; set to Aid+FM
3.Mute FM mic or place in quiet spot
4.Measure aid output for 65 dB aid input
5.Place FM mic in test box; remove aid
6.Measure aid output for 65 dB FM input
7.Adjust FM gain until the two outputs match
Problems in adjusting for transparency
1.Compression in FM transmitter causes FM gain to change with FM input.
Compressionof FM signal
Compressionof combined signal
Relative gain via FM and AIDchanges with changing FM input
Problems in adjusting for transparency
1.Compression in FM transmitter causes FM gain to change with FM input.
2.Higher output for FM signal may be uncomfortable if not controlled by compression in aid (i.e., improper fitting).
3.Therefore initial FM-gain setting must be validated behaviorally
Caution #3. FM benefit applies only to the talker
with the microphone
- But comments, questions, and answers from other students are part of the total learning experience
Teacher Self Otherstudent
Signal-to-Noise
Ratio25 dB 25 dB 5 dB
With single microphone
((((((
((((
Teacher Self Otherstudent
Signal-to-Noise
Ratio25 dB 25 dB 25 dB
With two microphones
Caution #4. Psycho-social issues
- Intrusiveness- Embarrassment- Self-consciousness - Difference- Hassle-factor
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)
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
Summary - Enemies of hearing:
distance, noise, reverberation- Virtually eliminated by FM microphone at
talker’s mouth- For best benefit:1.FM microphone close to mouth 2.Balance of FM and Aid gain (audiologist)3.Access to contributions of other students4.Counseling/preparation/instruction/demon-
stration/behavioral data5.Formal program for monitoring and
maintenance6.Periodic verification and validation (audiologist)