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A behind the ear hearing aid

Hearing aids

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by Chris Woodford. Last updated: April 3, 2017.

Hearing is one of our most important senses because it alerts us to dangers we can't always see. You might not notice a lorry backing towards you, but if you hear the beep-beep-beep of a reversing alarm you'll look up and get out of the way. Hearing evolved in animals as an early-warning system, but for humans—the most evolved animals of all—it's so much more than that. We communicate largely by speaking and listening to one another and we listen to music so we can experience deep emotions whenever we wish. Imagine how frightening and isolating it can be, then, if you're unlucky enough to be deaf from birth or if your hearing starts to decline as you get older. Thankfully, science and technology—in the shape of hearing aids—can help many people who suffer from hearing problems. Let's take a closer look at hearing aids and find out how they work.

Photo: A typical BTE (behind-the-ear) analog hearing aid (left), with its ear mold (right), and button battery (bottom). The ear hook, the other essential component that links the hearing aid to the ear mold, is not shown on this photo.

How our ears hear sound energy

Sound is simply a kind of energy we can hear. Things make sounds when they vibrate (move back and forth), setting air in motion around them. Think what happens when you bang a drum: the taut drum skin vibrates very quickly, pushing and pulling on the air molecules (atoms joined together) that are next to it. These air molecules move about more energetically and start crashing into other air molecules too. That's how waves of sound energy race out from a drum in all directions and that energy (the same energy you gave to the drum skin by hitting it in the first place) keeps traveling through the air until it reaches your ears.

What happens then? The pinnae (big outer flaps) of your ears are shaped so they can gather sounds coming from different directions and funnel them into the ear canal (the hole that leads to your inner ear). At the end of your ear canal, there's a tiny drum-like skin called the eardrum. When incoming sound waves hit the eardrum, they make it vibrate. Three tiny bones called the hammer, anvil, and stapes (or stirrup) in your skull detect those eardrum vibrations and pass them on to a snail-shaped organ called the cochlea, which is filled with fluid and tiny hairs called cilia. The sound vibrations make the fluid in the cochlea wash back and forth, agitating the cilia. The cilia detect those vibrations and send electrical signals to your brain, which you hear as sounds of different frequency. In short, then, hearing is all about sound energy entering your ears and being turned into electrical impulses by tiny hairs inside your cochlea.

Anatomy of the human ear

Artwork: Anatomy of the human ear. Hearing begins with the outer ear (pinna), but all the really clever apparatus that lets us sense and recognize sounds is actually concealed inside our skulls. Why do we have strange folds in our outer ears? They help us distinguish sounds coming from different directions. Picture by courtesy of National Institute on Deafness and Other Communication Disorders (NIDCD) and National Institutes of Health Photo Galleries.

How we can lose our hearing

The path between your outer ear and your brain can be blocked or damaged in many different places and in a number of different ways, so people can become deaf or lose some or all of their hearing for lots of different reasons. One of the most common types of hearing loss happens when the hairs in the cochlea become damaged. If there are fewer hairs, sounds produce less stimulation in your brain—so things need to be louder for you to hear them. That's where hearing aids come in. They can't help everyone with impaired hearing, but they can often make a difference to hearing problems caused by a loss of cochlear hair cells, medically known as sensorineural hearing loss (SNHL).

How hearing aids work (the simple version)

A behind the ear hearing aid

Photo: A BTE (behind-the-ear) hearing aid. You can clearly see the pink-colored case that sits behind the ear and the clear plastic tube leading to the ear mold at the bottom. Photo by Tyler W. Hill courtesy of U.S. Marine Corps.

There's a tired old joke in television sit-coms where people shout at a deaf person (usually a deaf, elderly person) to make themselves heard. What happens if you shout at a deaf person is that you transmit sound waves of greater amplitude (volume) and energy into their ear canal. Their cochlear hair cells are more likely to detect these more energetic sound waves and, consequently, they're more likely to hear you. Old-style ear trumpets work a slightly different way. Effectively, they make the outer ear much bigger and concentrate the energy in incoming sounds into a smaller area. That increases the pressure that sounds make on the eardrum and, again, improves the person's chances of hearing.

While shouting louder and using ear trumpets are crude, mechanical solutions to the problem of hearing loss, a hearing aid is a much more sophisticated electrical solution. A hearing aid is simply an electronic sound amplifier. You've seen people on stage speak into a microphone and have their voices hugely amplified by giant loudspeakers so crowds can hear them? A hearing aid works exactly the same way, except that the microphone, amplifier, and loudspeaker (and the battery that powers them) are built into a small, discreet, plastic package worn behind the ear or just inside the ear canal.

An Acousticon hearing aid from 1925

Photo: Hearing aids as they used to be. This Acousticon aid dates from 1925. You wore the top, headphone part over your ear. The bottom part contained the microphone, battery, and a control (left) for adjusting the volume. This is an exhibit at Think Tank, the science museum in Birmingham, England.

One of the most common types of hearing aid is called a BTE (behind the ear) and consists of two separate pieces. Behind the ear, there's a hard plastic case that contains a small microphone, amplifier, and loudspeaker. This is linked, via a tube, to a softer plug called an ear mold shaped to fit just into the person's ear canal. When you wear a hearing aid like this, the microphone picks up sounds around you and turns them into an electric current, the amplifier (using one or more transistors) boosts the size of the current, and the loudspeaker turns the boosted current back into a much louder sound. This amplified sound flows through the tube and the ear mold into the person's ear. A different style of hearing aid called a CIC (completely in the canal) has all the same components but fitted into a small plug that pushes right into the ear canal. In between the two extremes of BTE and ITC, you can get somewhat larger hearing aids called ITC (in the canal) (which means partly in the canal and partly in the outer ear) and ITE (in the ear), which fill the canal and part of the outer ear but don't sit behind it like a BTE. CIC, ITC, and ITE hearing aids are now so discreet that you may not even notice someone is wearing one.

Hearing aids come in two main kinds. Analog hearing aids simply convert sound into electric currents, boost the currents, and turn them back into louder sounds. Digital hearing aids are more sophisticated (and cost much more). They convert the sound into a numerically coded signal and, depending on how they are designed, process and refine the signal before turning it back into a sound. Digital hearing aids can be tuned so they emphasize sounds of particular frequency or block out unwanted noise more effectively, whereas analog hearing aids tend to amplify everything (background noises as much as important sounds) by the same amount.

Although a hearing aid can never restore hearing completely, it can make a huge difference to a person's life by helping them converse more normally and enjoy everything from TV and radio to recorded music and birdsong. It's a great example of how science and technology (often much maligned) can really improve the quality of our everyday lives!

A basic pocket-style analog hearing aid with earpiece attached by wire. The electronic components inside a pocket-style analog hearing aid.

Photo: 1) This basic "body" hearing aid (probably dating from the mid-1950s to the mid-1960s) fastens to your clothes with the brass spring clip at the top or slips inside a jacket pocket. The microphone holes above and below the clip pick up sounds, which are amplified by the circuit inside and fed out through the earpiece (bottom). 2) The circuitry inside is fairly simple. The most important components are the four amplifying transistors (black) on the left, which boost the sounds from the microphone. I haven't managed to identify the exact make and model, but the spring clip detail looks very like ones made by Fortiphone of London, England.

How an analog hearing aid works

Artwork showing the components in a hearing aid and how they amplify sound

  1. Sound waves travel toward your ear (pink) and the hearing aid you're wearing behind it (blue).
  2. A small microphone picks up the sounds and turns them into an electric current.
  3. An amplifier circuit (containing one or more transistors) increases the strength of the current.
  4. A small button battery powers the amplifier circuit and other components.
  5. The amplified current drives a small loudspeaker.
  6. The loudspeaker plays its sound into a tube called the ear hook.
  7. The ear hook plays the sound through the ear mold into your ear canal.
  8. Sound waves of greatly increased volume travel to your inner ear.

A digital hearing aid works in much the same way, except that the amplifier chip digitizes the sound signals from the microphone, then processes and filters them before it amplifies them—producing much clearer sounds. It can be much more closely tuned to your particular hearing difficulties and it automatically adjusts itself to different environments (noisy restaurant, quiet home, or wherever you might be).

How hearing aids work (the more complex version)

What you read up above was a hugely over-simplified explanation: hearing aids are much more than just basic amplifiers! For those of you who'd like more detail, here's an in-depth explanation of how real hearing aids actually work, how digital aids differ from analog ones, and why they cost so much more.

What problems does a hearing aid have to solve?

You might be wondering, if a hearing aid is just an amplifier, and an amplifier is just a few transistors, how come it costs so much? But of course, it's not quite that simple! First, there isn't simply one type of hearing impairment. You can lose hearing for three different reasons (or a combination of them):

The next thing to consider is that there are different types of hearing loss. With a sensorineural impairment, you might lose only low or only high-frequency sounds, for example, so you would need a hearing aid that amplified frequencies very selectively. Not only that, but in an environment where there are many sources of sound (someone talking over the sound of a jackhammer, perhaps), you'd want to amplify only the soft sounds that you can't hear rather than making the louder sounds painfully unbearable. Finally, you need a hearing aid to work in subtly different ways in different environments (at home, in a concert hall, on a busy street, or wherever you happen to be).

How does a hearing aid cope with all these problems? In theory, a simple analog hearing aid is too crude to do all this. If a hearing aid were just a miniature microphone and loudspeaker, it would amplify all sounds by the same amount. That's not what most people with hearing difficulties actually want. When you have your hearing tested, the audiologist (the person who does the test) will establish the exact pattern of frequencies you can or can't hear, known as your audiogram. If you choose to have an analog hearing aid, it will be designed to boost some frequencies more than others to match your needs as closely as possible. Even so, if it's a basic analog hearing aid, t will still essentially boost all incoming sounds by a certain amount, so it will amplify irritating background noises as well as voices you want to hear—people chattering in the background as well as the TV program you're watching.

Programmable analog hearing aids

The next level of sophistication is a known as a programmable analog hearing aid, and has various different settings you can select to give different kinds of amplification for different everyday environments. The settings are preset when your hearing aid is manufactured, according to your personal audiogram, and usually you have to select between them yourself using a small, discreet switch somewhere on the hearing aid. Although more sophisticated than basic hearing aids, programmable aids are still analog: they boost all incoming sounds without processing them in any intelligent way.

Digital hearing aids

Digital hearing aids, which have been widely available since the 1990s, are very different to analog ones: they analyze incoming sounds intelligently, do their best to figure out which sounds and sound frequencies you want to hear, and boost those selectively.

Now there's nothing automatically superior about digital technology: there are many audiophiles who swear that analog record players have superior sound quality to digital CD players, while lots of photographers still prefer analog film cameras to digital cameras. What makes the difference is that digital music or digital photos can be processed in all kinds of ways that are much harder with analog information—so you can quickly and easily remix music on your computer, digitally enhance your photos, and so on. In much the same way, digital hearing aids aren't superior to analog ones because they amplify sounds better—analog aids may, in fact, do that job just as well—but because they turn sounds into digital information that can be enhanced to make speech that's easier to understand, music that's more pleasant to listen to, and so on.

How do digital hearing aids work? They use a combination of different techniques very broadly referred to as DSP (digital signal processing), including:

Are digital hearing aids better than analog ones?

Digital hearing aids can cost twice as much as analog ones, but are they worth it? That's obviously a very subjective question. To some people, the very discreet nature of a CIC hearing aid is the most important consideration, irrespective of whether it is analog or digital and even if a larger, more intrusive BTE hearing aid would provide better performance. But comparing like for like, analog for digital, which gives the best overall performance in varied listening environments? Various studies have been done:

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Since digital aids cost more, they can be sold more profitably, and that risks their benefits being overstated or "overhyped" by private hearing clinics. There's no sure way of knowing whether a digital hearing aid will be better for you—unless you try one and see. And if you never try one, you'll never know!

References

  1. One year follow-up of users of a digital hearing aid by S. Arlinger and E. Billermark. Br J Audiol. 1999 Aug;33(4). p223.
  2. Most dispensers in Journal's survey report greater patient satisfaction with digitals by David H. Kirkwood. The Hearing Journal, Hearing Journal, March 2001, Volume 54, Issue 3, p21.
  3. Blinded comparison of three levels of hearing aid technology by Donald J. Schum D and Randi R. Pogash. Hearing Review, 2003, Volume 10 Issue 1, p40.
  4. MarkeTrak VIII: Customer satisfaction with hearing instruments in the digital age by Sergei Kochkin. The Hearing Journal, September 2005, Vol. 58 No. 9, p30.
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Text copyright © Chris Woodford 2008, 2014. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008/2014) Hearing aids. Retrieved from http://www.explainthatstuff.com/hearingaids.html. [Accessed (Insert date here)]

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