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

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

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

Diagram of Test setup

Background noise from test system up to 10MHz, as displayed by the spectrum analyser with the hearing aid switched off

Harmonic structure of DSP aid interference up to 10MHz Note the DSP clock at 991KHz and its many harmonics

Wideband harmonic structure of DSP aid interference up to 500MHz using 400mm d/i lead Note the peak at approx 120MHz near fm systems The peak moves to lower frequencies as the d/i lead length increases USA UK USA fm systems work 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. Zoomed in structure of interference near 174MHz (UK fm systems) – animated sequence

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 72 nd harmonic (72MHz-USA fm) any frequency drift is 72 times larger At the 174 th 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

Objectives To raise awareness of the issue To encourage hearing aid manufacturers to improve their designs

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