An Informational
presentation for the
audiological professional
from Connevans Limited
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Digital hearing aids can cause
‘funny noises’ when used with
personal fm systems
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A question for you
Do you have more reported occurrences
of noise interference from fm users using
A – DSP hearing aids
B – Analogue hearing aids
C – Cochlear implants
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Let us predict your answer
 We guess that reported occurrences
of fm noise interference from
analogue hearing aids is rare
 We guess that with digital hearing
aids and cochlear implant processors
reported fm noise interference is
more often … yes ?
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Electromagnetic interference
 A number of digital hearing aid users
have reported occasional loud noises
when using their fm system
 Occurrences were unpredictable
 Mostly noticed when the transmitter
was turned off
 The sound was a ‘whooshing’ noise
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Electromagnetic interference
 We have identified the cause to be
ElectroMagnetic Interference (EMI)
generated by DSP hearing aids
 As the d/i lead is also
the aerial, any radio
interference from a
hearing aid travels
directly into the receiver
FM
Receiver
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EMI testing of DSP aids
The tests were carried out in a RF (Radio
Frequency) screened chamber
The hearing aid was placed in the test chamber
with the direct input shoe and lead connected
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Diagram of
Test setup
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Background noise from test system up to 10MHz,
as displayed by the spectrum analyser with the
hearing aid switched off
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Harmonic structure of DSP aid interference
up to 10MHz
Note the DSP clock at 991KHz and its many harmonics
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Wideband harmonic structure of DSP aid
interference up to 500MHz using 400mm d/i lead
fm systems work here
USA UK USA
Note the peak at 120MHz near fm radio aid systems
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The peak moves to lower frequencies as the d/i lead length increases
Zoomed in structure of interference near 174MHz
(UK fm systems) – animated sequence
e.g. channel 51 receiver
listens here
The peaks are EMI noise from the hearing aid – they move
with time and will eventually cross the fm receiver frequency,
which is why noise is only sometimes heard.
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 The fundamental processor clock frequency in the
DSP aid running at around 1MHz is not stable
 The frequency drifts with time, battery level, temperature
& processor activity
 At the 72nd harmonic (72MHz-USA fm) any
frequency drift is 72 times larger
 At the 174th harmonic (174MHz-UK fm) any
frequency drift is 174 times larger
 This means that the EMI drifts across all fm
receiver frequencies at some time
 Let’s look again at the animation …
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Zoomed in structure of interference near 174MHz
(UK fm systems) – animated sequence
e.g. channel 51 receiver
listens here
The peaks are EMI noise from the hearing aid – they move
with time and will eventually cross the fm receiver frequency,
which is why noise is only sometimes heard.
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Objectives of this
informational presentation
 To raise awareness of the issue
 To encourage hearing aid
manufacturers to improve their
designs
16
Solutions
 Some DSP hearing aids produce
much less interference than others
so these will become preferred for
use with fm systems
 Keep the fm transmitter on when the
receiver is in use
 Use a transmitter microphone mute
facility when sound is temporarily not
needed
 Switch off receiver when not needed
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One last question
 Are FM systems worth the effort?
 A positive
yes
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Thank you for your time
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