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
1
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
2
Do digital hearing aid
users benefit from using
an fm system in class?
 YES !
3
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
4
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?
5
Meanwhile we had better
return to the plot?
6
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
7
WDRC aids
 How does a WDRC aid cope
with the different sound levels
from its own microphone and
from an fm radio system?
8
Design of DSP hearing aids
FM signal enters before any DSP, in parallel
with the microphone input
INPUT
H.A.
MIC
A/D
INPUT
> DSP >
D/A
> REC.
F.M.
The loudest input will drive the compressor
9
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
10
DSP WDRC aid listening …
Relative levels
Child’s voice &
classroom sound
Listening level
Background Noise
11
DSP WDRC aid listening …
Relative levels
fm – teacher’s voice
Child’s voice &
classroom sound
Listening level
Background Noise
12
DSP WDRC aid listening …
Relative levels
fm – teacher’s voice
Listening level
Child’s voice &
classroom sound
Background Noise
13
DSP WDRC aid listening …
Relative levels
fm – teacher’s voice
Listening level
Child’s voice &
classroom sound
Background Noise
14
DSP WDRC aid listening …
Relative levels
fm – teacher’s voice
Listening level
Child’s voice &
classroom sound
Background Noise
15
DSP WDRC aid listening …
Relative levels
Listening level
Child’s voice &
classroom sound
Background Noise
16
DSP WDRC aid listening …
Relative levels
Listening level
Child’s voice &
classroom sound
Background Noise
17
DSP WDRC aid listening …
Relative levels
Listening level
Child’s voice &
classroom sound
Background Noise
18
DSP WDRC aid listening …
Relative levels
Child’s voice &
classroom sound
Listening level
Background Noise
19
DSP WDRC aid listening …
Relative levels
Listening level
Child’s voice &
classroom sound
Background Noise
Repeat demo
20
The result
 Better hearing aid use
as a result of
 Reduced background noise
 Improved sound consistency
 Improved clarity
 Providing a good listening experience
21
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?
22
 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
23
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?
24
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
25
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
26
Traditional equal output
balancing approach (65/75)
Linear hearing aid with fm system
Teacher’s
voice
75dB SPL
At the
Child’s ear
Child’s
neighbour’
s
voice
65dB SPL
110dB SPL
fm and noise
advantage lost
105dB SPL
60dB SPL
Classroom60dB
noiseSPL
Classroom
noise
Teacher
Classroom noise
Child
Can the child hear the
teacher clearly?
27
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?
28
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
29
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
31
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
32
When placed in a testbox





What happens is …..
Hearing aid hears constant tone
Constant tone = noise
Hearing aid turns itself off
Oh dear!
33
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
34
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.
35
Next slide (click on picture to repeat)
36
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
37
The fm Advantage
protocols and procedures
38
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
39
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.
40
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?
41
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'
42
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?
43
How many fmAdvantage
procedures?
 4 in all
 2 for hearing aid direct input
 2 for hearing aid telecoil input
44
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
45
‘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
46
The fm Advantage
procedures …
A practical demonstration
{jump to test steps}
47
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
48
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
49
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
50
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
51
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
52
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
53
Step 1
 Set the hearing aid test box to
display output SPL
 Choose the most suitable stimulus
- DSP composite preferable
54
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
55
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
56
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’
57
Step 5
 Place the radio system
microphone in the test chamber
 Turn on the transmitter
View test box layout
 Close the chamber lid
58
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
59
Step 7
 With a multicurve test box select
a new curve, curve 2
60
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
61
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
62
Step 10
 Store the test box response
for the hearing aid and radio
system combined – curve 2
63
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
64
Step 12
 With a multicurve testbox select a
new curve, curve 3
65
Step 13
 Set an 80dB SPL test box signal
66
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
67
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?
68
Step 16
The level of distortion for the overall
system should be checked with a
listening test
69
Step 17
It is advisable to keep an annotated
printout for quality monitoring
purposes
Return to step 1 to repeat
procedure
70
The listening test is essential
to ensure that the system is
functioning correctly
71
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?
72
Any questions ?
david@connevans.com
73
Thank you for your time
74