How does a hearing aid work?

14 October 2012

Interview with

Søren Laugesen, Oticon Research Centre, Eriksholm, Denmark

Alan -   And I think it's a good point to come in and start talking about how we restore hearing for hearing-impaired people, we'll come back to Owen in a second. And in my day-job, I'm a hearing aid researcher and so, I decided to talk about hearing aids and here is someone who has just been fitted with hearing aids, talking about why they wanted them.

Thelma -   My name is Thelma Purchase and I live in Welwyn Garden City.  What I really had the hearing aids for was that if I went to a talk then I could pick it up, but if I find that my family are here, and they're all talking together, and I miss what they're saying quite often.

Behind the ear hearing aidAlan -   So how do hearing aids work and how could engineers improve them to give Mrs Purchase a better listening experience?  I asked Soren Laugesen, a Research Engineer at the Eriksholm Research Centre, a part of the Oticon hearing aid company, what a hearing aid is.

Søren -   Basically, it consists of a microphone which picks up the sound so the digital circuitry that processes this sound and a small loudspeaker that delivers the sound back to the ear.

Alan -   So, is a hearing aid basically a pair of glasses for your ears?

Søren -   The most obvious thing you notice about people with hearing loss is that they can't hear the very soft sounds.  A big part of what a hearing aid does is to amplify sounds to make these soft sounds loud enough to be heard by the user.  But it's not as simple as that because if you try out a loud sound, you'll actually recognise that a hearing aid or a hearing impaired person hears this sound perfectly well.  So, the hearing aid doesn't need to amplify the loud sounds, only the soft sounds.  And in order to do that, we have some circuitry, which is called a compressor, which makes sure that the soft sounds are amplified but keeps strong sounds basically where they are.

Alan -   To give you an idea of what this sounds like, here's an uncompressed sentence...

"Hello.  I'm speaking quite quietly at the moment. But sometimes I raise the level of my voice..."

I just amplified the whole thing, the quiet bit would indeed be easier to hear, but the loud part would become painfully loud. Instead, the compressor circuit in the hearing aid makes the quiet bits louder, but leaves loud sounds untouched so the overall sound is a consistent volume.

"Hello.  I'm speaking quite quietly at the moment. But sometimes I raise the level of my voice"

...but what if there's background noise?  How can we help a hearing impaired person listen to one sound among many?  For instance, I'm now sitting in a busy café using a microphone that picks up all the sound around you.  The overall sound you're hearing is loud enough, but my voice is not very clear due to all the background noise.  How can we alleviate this problem?

Søren -   In order to improve on that, we put various helping systems into the hearing aid and the most powerful is definitely the directional microphone.  Can you imagine that if you have 2 microphones in a line and that sound is coming straight behind you then you'll have the same sound picked up by the 2 microphones with a little bit of tiny delay between them.  So if you now take the 2 signals and delay them with respect to each other and subtract them from each other, you'll end up with nothing.  So, the microphone will pick up nothing from behind, but will still be able to pick sound up from the front direction where you want to hear from.

Alan -   And so, if we now switch the microphone to direct mode, you should be able to hear me more clearly because the background noise is not being picked up by the microphone.  But what about the noise that's left, such as low frequency sounds?

Søren -   The other form of noise reduction that we have in a hearing aid, is a mechanism that looks at the sound that is coming into the hearing aid and tries to figure out whether certain bits of it is actually noise or actually a signal that the user wants to listen to.  And if you have noise let's say, at very low frequencies, then you can turn down these low frequencies where the noise is dominating, in order to allow speech that you want to listen to, to come through more clear in the other frequency bands.

Alan -   So that's how we remove unwanted background noise, but why is the sound quality from a hearing aid...which sounds a little like this, not as good as a CD player for instance?

Søren -   In a CD player, there's a sounding rate of what we call 44,100 Hertz which essentially means that the system is measuring the actual sound 44,000 times each second, but that has a cost in terms of processing power and the simple electrical power you need to feed into the system.  And we can't really afford that in the hearing aid because the battery is so small.  So, a typical modern hearing aid will operate at a sampling rate of a little less than half of the CD player, which means that we have to say goodbye to the upper part of the frequency range.  But what really makes a difference to the sound quality you can get from a hi-fi system and hearing aid is the loudspeaker that we're using, which for obvious reason has to be very, very tiny.  So there's really a limit to how loud it can play and also again, the bandwidth of what it will be able to produce.

Alan -   So the smaller loud speaker and the reduced number of samples of sound taken each second by the hearing aid reduces the higher frequencies in the sound, and produces a lower sound quality.  We can also improve hearing aids by having them communicate information with each other about the sounds coming in to each of them.  The difference in volume between your ears is one way of discerning where sound is coming from.  By having the hearing aids communicate, we can preserve this difference, even when amplifying and compressing sounds.

Søren -   In some situations, it can create some problems if the amount of amplification on the 2 are different.  To avoid that problem, we could coordinate the 2 hearing aids to that they act in synchrony.

Chris -   Soren Laugesen, explaining the workings of a hearing aid to Alan Boyd.  Alan, so you actually work on this.  So, that was really interesting, what he was saying about the linking together of the 2 hearing aids in the right and left ear so they literally talk to each other.  Is that actually being implemented yet or is that still experimental?

Alan -   The implementation of that is actually available on the market at the moment, but largely just for what's called changing programme.  So just pressing buttons, you don't have press 2 buttons on either side.  But the actual linking in terms of sound information is coming in and is a kind of cutting-edge part of the research.

Chris -   So listen up for that one.  It's coming to a hearing aid near you quite soon.

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