NOTE

The information in this Web page is up to date as of March 1996. Since then there were several developments in the subject.

Introduction

With the discussion taking place regarding the introduction of digital mobile telephones (cell-phones) in the USA, this document has been posted to bring awareness of the situation in the UK, where digital mobile telephones have been in operation since 1992. The number of digital mobile phone users has grown rapidly since then, and they are now common throughout Western Europe.

This document is based on a factsheet produced by the Royal National Institute for Deaf People, UK. It has been updated, and the UK specific parts noted.

You may want to see also Hearing Aids and EMI (Electromagnetic Interference).

NEW MOBILE TELEPHONES

Recently a number of new mobile telephones (also called cell-phones) have come onto the market. They use digital techniques which can cause interference to hearing aids. The interference is a buzzing sound, which can be anything from negligible to extremely painful.
The amount of interference depends on the design of the hearing aid, the power of the mobile telephone and the distance between the hearing aid and telephone.

How big a problem is it?

Some hearing aids will pick up interference from these new mobile telephones when they are being used by other people nearby. Although someone may not find themselves near a mobile phone user very often, it could be very annoying or uncomfortable when it does happen, particularly where it is difficult to move away (in a restaurant, for example). At present NO hearing aid is able to be used with a handheld digital mobile telephone.

If considering purchasing a mobile telephone:

All handheld digital mobile telephones cause severe interference to all hearing aids when the handset is placed close to the hearing aid. Analog mobile telephones do not cause this interference. However, there are NO hearing aid compatible mobile telephones in the UK (neither analog nor digital) so the hearing aid can not be used on the 'T' position. This means that behind-the-ear hearing aids will not pick up the sound well from any mobile telephone.

Only if you have an in-the-ear or canal-aid, should you consider using an analog mobile telephone. Before purchasing a mobile telephone, to be used with a hearing aid, it is important that test calls are made with the telephone while actually using the hearing aid, to ensure that the telephone does not cause interference.

At what distance can interference occur?

Some models of hearing aid are more susceptible to the interference than others. With a susceptible hearing aid, it is possible that passing cars with mobile telephones in use, or sitting close to a person using a hand-held mobile telephone, could cause noticeable interference. There should be no noticeable interference to even the most susceptible hearing aid, from even the most powerful mobile telephone, when the distance is greater than 3 metres.

Do all hearing aids pick up the interference?

Yes. At present, no hearing aid has been found that does not pick up this interference when the mobile telephone transmitter is placed close to the hearing aid. This means that no handheld digital mobile telephone can be used with any hearing aid. However, not all hearing aids will pick up interference from a distance of one metre.

Can I use the T-switch on my hearing aid?

Interference can occur on both the M (microphone) and T (telecoil) positions.

What is being done about it?

The IEC (International Electrotechnical Commission) is urgently looking into this problem, with a view to producing a standard method for measuring the level of interference picked up by hearing aids. This will enable the comparison of hearing aids, and enable manufacturers to test their designs.

Which mobile telephones are digital?

The mobile telephone operators are using a variety of labels to describe their new digital mobile telephones. The systems vary in power, radio frequency and type of use, but all operate on essentially the same digital principle. These are some of the trademarks and acronyms (for the UK), but others may be in use also:

Mercury One-2-One, Orange, EuroDigital, MetroDigital, TDMA, GSM, PCN, DCS-1800, CT-2, DECT, Cellnet-GSM, Vodafone-GSM.

What about cordless telephones?

Soon cordless digital telephones will be available. These also work on the same principle, but at a low power. It is possible that they may cause interference when held close to a hearing aid, so again, they should be tried with the hearing aid before purchasing.

Can my hearing aid be modified to not pick up the interference?

No. Hearing aids are too intricate and there is no space available for modifications. Hearing aid manufacturers are investigating how to design new hearing aids that are more immune to the interference.

Technical Background

Conventional mobile telephones use analog technology, with just one conversation on each radio frequency at a time. As there are only a limited number of radio frequencies available, this means that the demand is greater than supply and this keeps the cost high. The digital systems, however, squeeze up to 8 conversations onto each radio frequency, without the different conversations being aware of each other. This makes the radio 'air space' much cheaper. This is done by TDMA - Time Division Multiple Access. In the case of GSM (Global System for Mobile communications), the radio frequency is divided up into 8 time slots of approx. 0.5 ms each, repeating approx. every 5 ms. When using the mobile telephone, every 5 ms of speech is digitally coded and sent out as a 0.5 ms burst of radio signal. These bursts, at a rate of 214 per second, are picked up by the wiring in the hearing aid, detected by any non-linearity in the circuit, and then amplified. The interference consists of a 214 Hz buzz plus harmonics. In some hearing aids, even turning down the volume control has no effect.

It is the unusual type of radio signal that causes the interference. Before this type of signal existed, radio frequency interference was not a problem for hearing aid designers. Several suggestions have been made on how to improve the design of hearing aids to make them less susceptible. These ideas are being investigated by hearing aid manufacturers. Unfortunately, it seems that there will be no cheap way to modify existing hearing aids to improve their immunity. Hearing aids do not last forever, and it is hoped that new hearing aids will be more immune, and therefore that this will become less of a problem.

Several organizations are investigating the matter, and hearing aid manufacturers are working towards designing hearing aids that pick up less of the interference. That's why it is important at this stage to have a standard way of measuring the immunity of the hearing aids. This standard is being developed with a sense of urgency. With the standard, it will be possible to compare hearing aids, so that the hearing aid purchaser can obtain a hearing aid with good immunity.

© Colin M. Bootle & RNID Science and Technology Unit
original: September 1993 updated: July 1994

(Contributed by Colin M. Bootle at 14 Jul 1995.)