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1 Digital Hearing aids and fm radio systems fm Advantage procedures David Evans, MD, Connevans Limited.{click here to jump to practical section} V 2003.2.{click.

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Presentation on theme: "1 Digital Hearing aids and fm radio systems fm Advantage procedures David Evans, MD, Connevans Limited.{click here to jump to practical section} V 2003.2.{click."— Presentation transcript:

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2 1 Digital Hearing aids and fm radio systems fm Advantage procedures David Evans, MD, Connevans Limited.{click here to jump to practical section} V {click here to jump to practical section}

3 2 Introduction 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

4 3 Do digital hearing aid users benefit from using an fm system in class? YES !

5 4 Practical 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

6 5 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?

7 6 Meanwhile we had better return to the plot?

8 7 fmAdvantage 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

9 8 WDRC aids How does a WDRC aid cope with the different sound levels from its own microphone and from an fm radio system?

10 9 FM signal enters before any DSP, in parallel with the microphone input The loudest input will drive the compressor Design of DSP hearing aids H.A. MIC F.M. INPUT A/DDSPD/AREC. >> > INPUT

11 10 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

12 11 DSP WDRC aid listening … Relative levels Child’s voice & classroom sound Background Noise Listening level Child’s voice & classroom sound Background Noise

13 12 DSP WDRC aid listening … Relative levels Child’s voice & classroom sound Background Noise Listening level Child’s voice & classroom sound Background Noise fm – teacher’s voice

14 13 DSP WDRC aid listening … Relative levels Listening level Child’s voice & classroom sound Background Noise fm – teacher’s voice

15 14 DSP WDRC aid listening … Relative levels Listening level Child’s voice & classroom sound Background Noise fm – teacher’s voice

16 15 DSP WDRC aid listening … Relative levels Listening level Child’s voice & classroom sound Background Noise fm – teacher’s voice

17 16 DSP WDRC aid listening … Relative levels Listening level Child’s voice & classroom sound Background Noise

18 17 DSP WDRC aid listening … Relative levels Listening level Child’s voice & classroom sound Background Noise

19 18 DSP WDRC aid listening … Relative levels Listening level Child’s voice & classroom sound Background Noise

20 19 DSP WDRC aid listening … Relative levels Child’s voice & classroom sound Background Noise Listening level Child’s voice & classroom sound Background Noise

21 20 DSP WDRC aid listening … Relative levels Child’s voice & classroom sound Background Noise Listening level Child’s voice & classroom sound Background Noise Repeat demo

22 21 The result Better hearing aid use as a result of Reduced background noise Improved sound consistency Improved clarity Providing a good listening experience

23 22 The future The future will surely see TOD's with hipro boxes and NOAH software on their laptops Is there no limit to their expertise?

24 23 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

25 24 Listening 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?

26 25 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

27 26 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

28 27 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 balancing approach (65/75) 75dB SPL 60dB SPL 65dB SPL 105dB SPL 110dB SPL Can the child hear the teacher clearly? 60dB SPL Classroom noise

29 28 From 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?

30 29 What is the fmAdvantage 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

31 30 Non-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 preserved FM Advantage balancing approach (65/65) 75dB SPL 60dB SPL 65dB SPL 95dB SPL Classroom noise 110dB SPL 60dB SPL Classroom noise 100dB SPL

32 31 Two questions Q: Is this for all hearing aids? A: No – there is a new proposal for linear aids too Q: Has this been tried? A: Earlier wave paediatric sites and Connevans users have used the fm advantage since last year. PC Werth now advise the same fmAdvantage protocol

33 32 Noise 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

34 33 When placed in a testbox What happens is ….. Hearing aid hears constant tone Constant tone = noise Hearing aid turns itself off Oh dear!

35 34 Hearing 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

36 35 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.

37 36 Next slide (click on picture to repeat)

38 37 Issues 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

39 38 The fm Advantage protocols and procedures

40 39 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

41 40 Linear '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.

42 41 Linear 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?

43 42 Non 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'

44 43 Non 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?

45 44 How many fmAdvantage procedures? 4 in all 2 for hearing aid direct input 2 for hearing aid telecoil input

46 45 Direct 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

47 46 ‘T’ 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

48 47 The fm Advantage procedures … A practical demonstration {jump to test steps}

49 48 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 updates

50 49 Today’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

51 50 Objective 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

52 51 The practical advice bit 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

53 52 General 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

54 53 General assumptions - 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

55 54 Step 1 Set the hearing aid test box to display output SPL Choose the most suitable stimulus - DSP composite preferable

56 55 Step 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

57 56 Step 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

58 57 Step 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’

59 58 Close the chamber lid Step 5 Place the radio system microphone in the test chamber Turn on the transmitter View test box layout

60 59 Step 6 Connect the radio system receiver to the hearing aid and turn on Mute or muffle the hearing aid microphone View test box layout

61 60 Step 7 With a multicurve test box select a new curve, curve 2

62 61 Step 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

63 62 Step 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

64 63 Step 10 Store the test box response for the hearing aid and radio system combined – curve 2

65 64 Step 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

66 65 Step 12 With a multicurve testbox select a new curve, curve 3

67 66 Step 13 Set an 80dB SPL test box signal

68 67 Step 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

69 68 Step 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?

70 69 Step 16 The level of distortion for the overall system should be checked with a listening test

71 70 Step 17 It is advisable to keep an annotated printout for quality monitoring purposes Return to step 1 to repeat procedure

72 71 The listening test is essential to ensure that the system is functioning correctly

73 72 Two 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?

74 73 Any questions ?

75 74 Thank you for your time

76 75 Hearing aid in test box

77 76 Hearing aid out of chamber

78 77 Hearing aid & radio system

79 78 Curve 1 – hearing aid alone

80 79 Curve 2 – aid & radio system

81 80 Curves 1 & 2

82 81 Curves 1, 2 & 3

83 82 Stetoclip with variable attenuator

84 83 Distortion: a mid frequency phenomenon DigifocusGemini Selectra


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