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)