digital hearing aids and fm radio systems fm advantage procedures

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Digital Hearing aids andDigital Hearing aids and fm radio systems fm radio systems

fm Advantage proceduresfm Advantage procedures

David Evans,MD, Connevans Limited.{click here to jump to practical section} V 2003.2

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IntroductionIntroduction

Digital hearing aids are here to stay! Different procedures for setting up &

testing digital hearing aids with fm systems is necessary – fmAdvantage

The fmAdvantage protocols can be used with any testbox but using testboxes with DSP test signals is preferable

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Do digital hearing aid Do digital hearing aid users benefit from using users benefit from using an fm system in class?an fm system in class?

YES !

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Practical advantagePractical advantage We are about to look at graphs of

hearing aid output when used with an fm system …. however …. do not forget the obvious, remember the benefit of a wireless microphone

Whatever the distance and whichever direction the tutor faces the fm system is providing consistency of sound – no DSP aid alone can match that

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An audio demonstration An audio demonstration

Lets listen to a hearing aid alone recorded through a coupler .... … and then with an radio aid to improve the quality of sound

Play demo Which sound would you

prefer?

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Meanwhile we had better Meanwhile we had better return to the plot?return to the plot?

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fmAdvantagefmAdvantage

Motivated by the wish to ensure better fmAdvantage in classrooms

Motivated by the wish to maximise the benefits of fm use by utilising the way the DSP hearing aids work to obtain a beneficial s/n ratio

Lets consider WDRC aids

Wide Dynamic Range Compression

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WDRC aidsWDRC aids

How does a WDRC aid cope with the different sound levels from its own microphone and from an fm radio system?

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FM signal enters before any DSP, in parallel with the microphone input

The loudest input will drive the compressor

Design of DSP hearing aidsDesign of DSP hearing aids

H.A. MIC

F.M.

INPUT

A/D DSP D/A REC.> > >INPUT

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DSP WDRC hearing aids DSP WDRC hearing aids …… Many have no volume control The aid is programmed to

automatically adjust it’s gain to present a comfortable sound range

Different gain is programmed for different frequency slices or bands

Different compression for different input levels

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Child’s voice & classroom sound

Background Noise

Listening levelChild’s voice & classroom sound

Background Noise

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Child’s voice & classroom sound

Background Noise

Listening levelChild’s voice & classroom sound

Background Noise

fm – teacher’s voice

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Listening levelChild’s voice &

classroom sound

Background Noise

fm – teacher’s voice

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Listening level

Child’s voice & classroom sound

Background Noise

fm – teacher’s voice

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Listening level

Child’s voice & classroom sound

Background Noise

fm – teacher’s voice

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Listening level

Child’s voice & classroom sound

Background Noise

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Listening level

Child’s voice & classroom sound

Background Noise

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Listening levelChild’s voice &

classroom sound

Background Noise

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Child’s voice & classroom sound

Background Noise

Listening levelChild’s voice & classroom sound

Background Noise

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DSP WDRC aid listening …DSP WDRC aid listening …Relative levels

Child’s voice & classroom sound

Background Noise

Listening levelChild’s voice & classroom sound

Background Noise

Repeat demo

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The resultThe result

Better hearing aid use

as a result of Reduced background noise Improved sound consistency Improved clarity Providing a good listening experience

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The futureThe future The future will surely see TOD's with

hipro boxes and NOAH software on their laptops

Is there no limit to their expertise?

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For the moment TOD’s will need good information from the clinic – how have the hearing aid programmes been set?

Also needed is a stetoclip tester with a variable attenuator to listen & confirm what happens in which situation

It is important to understand and demonstrate to a user how their hearing aid & fm system works in different situations

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Listening testsListening tests Must be carried out in each different

teaching situation Noisy rooms … playground … quiet

rooms … music rooms … lunchtime? What hearing aid programme is

intended for which situation?

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New test procedures New test procedures Who will it involve? Teachers of the Deaf, Educational

Audiologists, hospital Audiologists and sometimes clinicians

Are adult and paediatric requirements different?

Yes, children’s hearing aids require regular checks of their performance over time

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Historically ………..Historically ………..

Historically radio aid systems were balanced for equal output

65db and 75db inputs were used to balance the radio aid in a testbox

And then we turned the volume up a bit anyhow!

The fmAdvantage procedure should let you set it correctly

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Linear hearing aid with fm system

Teacher’s voice

Child’s neighbour’s voice

At the Child’s ear

Teacher

Classroom noise

Child

fm and noise advantage lost

Traditional equal output Traditional equal output balancing approach (65/75)balancing approach (65/75)

75dB SPL

60dB SPL

65dB SPL 105dB SPL

110dB SPL

Can the child hear the teacher clearly?

60dB SPL

Classroom noiseClassroom noise

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From now onwardsFrom now onwards

Move from ‘Equal output’ approach to ‘fm Advantage’ approach - we want the fm system to have an advantage

But as we do not want to overload linear hearing aids they will require a different protocol to non linear

But what is the fmAdvantage approach?

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What is the fmAdvantage What is the fmAdvantage approach for WDRC aids?approach for WDRC aids?

65db input level for the hearing aid 65db also for the fm system By balancing with a 10dB lower level

into the transmitter than traditionally the receiver output is set 10dB higher

The result is a 10dB+ fmAdvantage

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Two questionsTwo questionsQ: Q: Is this for all hearing aids?Is this for all hearing aids?

A: A: No – there is a new proposal for No – there is a new proposal for linear aids toolinear aids too

Q:Q: Has this been tried?Has this been tried? A: A: Earlier wave paediatric sites and Earlier wave paediatric sites and

Connevans users have used the Connevans users have used the fm advantage since last year. fm advantage since last year. PC Werth now advise the same PC Werth now advise the same fmAdvantage protocolfmAdvantage protocol

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Noise reduction aidsNoise reduction aids

No DSP NR aid actually recognises speech

Their logic is … Speech is not constant Thus constant is not speech Thus constant is noise and the aid

will reduce the gain of that frequency slice

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When placed in a testboxWhen placed in a testbox

What happens is ….. Hearing aid hears constant tone Constant tone = noise Hearing aid turns itself off Oh dear!

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Hearing aid test tonesHearing aid test tones

Puretone Composite tone Now a new DSP composite

tone The DSP tone is best described

as a ‘chirpy sound’ which randomly comes and goes before the aid can respond

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How may a DSP aid react?How may a DSP aid react?

The following video clip demonstrates how a DSP aid with noise reduction might react in a test box.

The first curve shows the aid response with a DSP composite test signal.

The second curve shows the aid turning itself down in the presence of a constant composite test signal.

36Next slide (click on picture to repeat)

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IssuesIssues

How about the American ASHA guidelines? We believe the fmAdvantage procedures to be better

Measuring distortion cannot be done with a DSP aid in a testbox

A realistic listening test is essential

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The fm Advantage The fm Advantage protocols and proceduresprotocols and procedures

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fm Advantage fm Advantage

Digital or analogue ?

Whether the hearing aid is digital or analogue is not important, the relevant factor is whether the hearing aid is operating in linear or non linear mode

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LinearLinear 'Linear' is when an SPL change at

the input is equally reflected at the output.

i.e. a 5dB change at the input gives a matching 5dB change at the output.

Hearing aids with either output limiting compression, linear peak clipping or soft peak clipping are regarded as being linear for the purposes of these procedures.

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Linear test levelsLinear test levels

65dB SPL for the hearing aid 70dB SPL for the fm radio system

hopefully less people will then need to turn up the volume!

70dB is chosen to minimise the risk of distortion in a linear aid

80dB SPL – the reality part of the test, will we get an fm advantage?

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Non linearNon linear

'Non linear' is when an SPL change at the input is not reflected at the output.

i.e. 9dB change of input does not give a 9dB change of output

Hearing aids with WDRC or full range compression are 'Non Linear'

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Non linear test levelsNon linear test levels

65dB SPL for the hearing aid

65dB SPL for the fm radio system

80dB SPL – the reality part of the test, will we get an fm advantage?

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How many fmAdvantage How many fmAdvantage procedures?procedures?

4 in all

2 for hearing aid direct input

2 for hearing aid telecoil input

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Direct inputDirect input

1 - Linear analogue aids and digital aids operating in linear mode

2 - Non linear analogue aids and digital aids operating in non linear mode

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‘‘T’ Telecoil inputT’ Telecoil input

3 - Linear analogue aids and digital aids operating in linear mode using telecoil ‘T’ pickup for use with an inductive neck loop

4 - Non linear analogue aids and digital aids operating in non linear mode using telecoil ‘T’ pickup for use with an inductive neck loop

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The fm Advantage The fm Advantage procedures …procedures …

A practical demonstration

{jump to test steps}

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Please do not panic!Please do not panic!

We are not doing all 4 today Please refer to the printed copy for all

the procedures …. or …. The fm Advantage procedures are on

the Connevans website The fm Advantage procedures are also

on the Connevans CD You can also register for automatic

future email updates fmadvantage@connevans.com

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Today’s demonstrationToday’s demonstration

Procedure 2 - Non linear analogue aids and digital aids operating in non linear mode

Using a WDRC (digital) aid

and a DSP stimulus

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ObjectiveObjective

To ensure that the sound from the radio system transmitter has an advantage over the general room noise picked up by the hearing aid and that the overall system functions as intended

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The practical advice bit The practical advice bit before you start ………before you start ……… Understand the different

programmes programmed on a particular hearing aid

Ask which hearing aid programme is intended for fm use?

Carry out realistic listening tests – not just in a quiet room

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General assumptionsGeneral assumptions

The hearing aid(s) have been separately assessed as working normally

The hearing aid(s) are recognised as being correctly fitted for the user

The fm Advantage setting up procedures are carried out with the hearing aid left at the normal user settings

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General assumptions General assumptions - cont- cont

A suitably quiet area is available for working in

The test box is set to display output The test box has been re-levelled A lapel microphone must be used for

the procedure DSP stimulus is preferred –

otherwise use what you have(!) and work by comparison

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Step 1Step 1

Set the hearing aid test box to display output SPL

Choose the most suitable stimulus - DSP composite preferable

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Step 2Step 2

With a 65dB SPL test box signal obtain a test box output response, curve 1, for the hearing aid at normal user volume

View test box layout

View example curve 1

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Step 3Step 3

If you don’t have a multicurve testbox take note of the hearing aid output at the curve peak –

– as we are using a multicurve testbox today we do not need to worry about this step

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Step 4Step 4

Taking care to not alter the hearing aid user settings, remove the hearing aid and coupler from the chamber

Always place the metal coupler on something soft so it does not pick up ‘vibrations’

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Close the chamber lid

Step 5Step 5

Place the radio system microphone in the test chamber

Turn on the transmitter

View test box layout

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Step 6Step 6

Connect the radio system receiver to the hearing aid and turn on

Mute or muffle the hearing aid microphone

View test box layout

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Step 7Step 7

With a multicurve test box select a new curve, curve 2

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Step 8Step 8

Leave the test box signal set at 65dB (at the frequency of the curve peak if using a puretone)

Turn on the test stimulus

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Step 9Step 9 Adjust the radio system receiver

output volume control to match curve 1

Always adjust the volume upwards, if necessary turn it down and then slowly back up again

An overbalance of 1dB will increase background noise 4dB with a 4:1 compression aid

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Step 10Step 10

Store the test box response for the hearing aid and radio system combined – curve 2

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Step 11Step 11

Compare curve 2 with the original response (curve 1) of the aid alone

Adjust the radio system receiver tone and output controls as required to give a best overall match. Repeat steps 9 & 10 until you achieve your best match.

View example curves 1 & 2

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Step 12Step 12

With a multicurve testbox select a new curve, curve 3

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Step 13Step 13

Set an 80dB SPL test box signal

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Step 14Step 14 Obtain a test box response for the

hearing aid and radio system combined – curve 3

With DSP aids the practical advantage will be more than shown on the graphs as it is the loudest signal – the teacher’s voice - which determines the amount of compression

View example curves

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Step 15Step 15

The result for curve 3 is very difficult to predict, it depends how the compression characteristics have been configured – sorry!

With the 80dB stimulus is the overall system functioning as intended?

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Step 16Step 16

The level of distortion for the overall system should be checked with a listening test

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Step 17Step 17

It is advisable to keep an annotated printout for quality monitoring purposes

Return to step 1 to repeat procedure

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The listening test is essential to ensure that the system is

functioning correctly

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Two last thoughtsTwo last thoughts

Has enough time been timetabled for the setting up and testing of new digital hearing aids?

Do I have time to ensure a working fm system or do I have time to help repeat or clarify a lesson?

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Any questions ?Any questions ?

david@connevans.com

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Thank you for your timeThank you for your time