Howstuffworks "How Hearing Aids Work"

For the estimated 28 million Americans with hearing loss, life is a series of missed opportunities. Frustrated friends and family members often tire of shouting and will just give up on communicating altogether. This can make the world a very lonely place for the hearing-impaired. Hearing aids -- small electronic devices that amplify sound -- can help restore many of the sounds that hearing-impaired people are missing. However, research finds that very few people who need hearing aids actually use them. Only one out of every five Americans who could benefit from a hearing aid wears one, according to a 2006 study from Duke University [source: Medical News Today].

Some people pass on hearing aids because of the cost, while others do so because they are embarrassed to be seen wearing them. What they don't realize is that many hearing aids are relatively inexpensive, and many of today's hearing aid styles are so small that they are nearly impossible to spot.

In this article, we'll find out exactly how hearing aids work, and learn about new technologies that are providing clearer, more natural sound for people with hearing loss. But first, let's look at what causes hearing loss.

What Causes Hearing Loss?
When you listen to something, whether it is a car alarm or a dog barking, the sound travels through the opening of your outer ear and causes your eardrum to vibrate. Three small bones in your middle ear carry this vibration to the cochlea -- the shell-shaped structure in your inner ear. The vibration stimulates hair cells, which create an electrical current in the auditory nerve. This current transmits the sound via nerve impulses to your brain, where it is processed into noise, like the sound of a car alarm or that of a dog barking.

Photo courtesy NASA

People lose hearing in one of two ways:

Conductive hearing loss occurs when sound doesn't move as it should through the eardrum, ear canal or the three bones of the inner ear. It can be caused by earwax, a punctured eardrum, fluid in the ear, a genetic defect or an infection. The result is a sensation as though your ears are plugged. Conductive hearing loss can be treated with surgery.

Sensorineural hearing loss involves damage to the cochlea. It's the most common type, affecting about 90 percent of people with hearing loss. Sensorineural hearing loss can be a byproduct of aging, or it can occur due to infections, genes, head trauma, exposure to loud noises or fluid buildup in the inner ear. This is the type of hearing loss that a hearing aid can help.

Hearing Aid Parts

Hearing aids are fairly simple devices, consisting of four basic parts:

A microphone picks up sound from the environment and converts it into an electrical signal, which it sends to the amplifier.
An amplifier increases the volume of the sound and sends it to the receiver.
A receiver/speaker changes the electrical signal back into sound and sends it into the ear. Then those impulses are sent to the brain.
A battery provides power to the hearing aid.

Hearing aids aren't effective for everyone. Hair cells in the inner ear must pick up the vibrations that the hearing aid sends and convert those vibrations into nerve signals. So, you need to have at least some hair cells in the inner ear for it to work. And, even if some hair cells remain, a hearing aid won't completely restore normal hearing.

How are hearing aids made?
To make a hearing aid, an audiologist or hearing aid dispenser will make an imprint of the patient's ear by pouring silicon material into the ear. Once it hardens, the silicon imprint is removed from the ear and sent to the manufacturer to make the hearing aid.

The imprint is used to make a silicon mold, which is filled with acrylic and hardened in an ultraviolet oven. This creates the shell of the hearing aid. Holes are drilled into the hearing aid, and the electrical components -- volume control, microphone and speaker -- are placed inside. A group of wires is attached to all of the different electronic parts and the battery is installed. When the hearing aid is finished, it is polished smooth and then analyzed to make sure that it fits the patient's hearing prescription.

Types of Hearing Aids

Photo courtesy National Institutes of Health

People with hearing loss have several hearing aid styles from which to choose. Generally, the smaller the hearing aid, the more it costs and the shorter its battery life.

In the ear (ITE): This large hearing aid works well for people with mild to severe hearing loss. It fits completely in the bowl of the ear. Because it is so large, the ITE hearing aid is among the most visible of styles, but the battery lasts longer than in smaller aids, and it can accommodate directional microphones (more on that later) and other added features.

In the canal (ITC): The ITC hearing aid works only for mild to moderate hearing loss. It is customized to fit the size and shape of the person's ear canal. Although this hearing aid is inconspicuous, its small size makes it difficult to adjust and change the battery. Some ITC hearing aids come with a remote control to make changing the settings easier. Also, users sometimes experience feedback noise with this type of hearing aid because the microphone and receiver sit close together.

Completely in the canal (CIC): This hearing aid is also appropriate for mild to moderate hearing loss, and it's even smaller than the ITC hearing aid. In fact, it's so small that a user must pull on a small wire to remove it from his ear. Because it fits completely in the ear canal, the CIC hearing aid is barely visible. Again, though, its small size makes it difficult to adjust and use added features. It's also more expensive than larger hearing aids.

Behind the ear (BTE): The BTE hearing aid can help with all types of hearing loss, from mild to profound. The electronics are in a case that sits just behind the ear. The case connects by a piece of clear tubing to a plastic piece called an earmold, which sits inside the ear. Sound travels from the earmold into the ear. This powerful and adjustable hearing aid can accommodate progressive hearing loss, as well as growth, which makes it ideal for children. However, its size and the tubing attaching the electronics to the earmold make it very visible. Also, the BTE hearing aid can cause feedback if it's not fitted correctly.

Behind the ear open fit: This newer version of the BTE model solves the problem of visibility. It is so small that it can barely be seen. A tiny device sits behind the ear and is attached by a tube to a miniscule speaker inside the ear canal. This hearing aid doesn't require an impression for fitting, and it provides better sound quality than ITE hearing aids. The downside is that it works only on people with mild hearing loss who can still hear low- and mid-frequency sounds.

Digital vs. Analog
Analog hearing aids amplify sounds -- the trouble is they amplify all sounds equally. So while your sister is recounting stories of her recent vacation, her voice is being drowned out by the competing sounds of the television, air conditioner and vacuum cleaner. Some analog hearing aids are programmable, meaning that they can hold separate settings for different listening situations. You can "change the channel" to adjust certain sounds louder or softer, depending on your preferences and hearing loss. But because they aren't as sensitive as digital hearing aids, analog hearing aids are being gradually phased out.

The vast majority of hearing aids sold in the United States today are digital. Although they are more expensive than analog hearing aids, digital hearing aids provide much clearer sound quality. Digital hearing aids contain a computer chip, which analyzes the sound based on the person's hearing loss and listening situation, and then amplifies it in a way that accommodates for the volume and pitch of incoming sounds. It even adjusts for feedback. Like analog hearing aids, digital aids can be programmed for a variety of listening environments.

Cochlear Implants and New Hearing Aids

Cochlear Implants
Traditional hearing aids work well for people with varying degrees of hearing loss, but they can't restore sound to people who are profoundly deaf. Cochlear implants work better in people with more severe ear damage because they bypass the damaged parts of the ear and send the sound information -- as electrical signals -- directly to the auditory nerve. Cochlear implants can be used alone or with traditional hearing aids in people who have mild hearing loss at some frequencies but more severe hearing loss at other frequencies.

Photo courtesy National Institutes of Health

The cochlear implant is made up of two main systems: an external system and an internal system. The external system is composed of three parts: a sound processor, a microphone and a transmitter. The internal system features a receiver and an electrode array.

The microphone, which is attached to the sound processor, captures sound and sends it to the sound processor, a small device that can be inconspicuously clipped over the outer ear. The transmitter sits behind the sound processor and connects to the scalp directly outside where the internal receiver is implanted under the skin. The transmitter is fitted with a magnet that holds it onto the internal receiver. (Instead of being fastened to the scalp and ear, the external system can also be carried in a shirt pocket or hip pouch.) The receiver, which is about the size of a quarter, is implanted under the skin in the bone behind the ear. And the electrode array is a wire that runs from the implant into the cochlea.

The microphone picks up sound and sends it to the sound processor, which translates sound into digital information. It sends this digital information to the implanted receiver, which changes the digital information into electrical signals and sends these signals to the electrode array. The electrode array sends these signals to the auditory nerve, which then passes along the signals to the brain.

New Hearing Aids
New bone-anchored (baha) hearing aids bypass the normal hearing process to help people with severe sensorineural, conductive or mixed hearing loss who can't be helped by regular hearing aids. Instead of merely amplifying sound, these surgically implanted devices attach to the bones in the middle ear. Bypassing the auditory canal and middle ear, baha hearing aids create vibrations in the skull and transmit those vibrations directly to the cochlea through a process called direct bone conduction.

Is one hearing aid enough?
For many reasons, chief among them cost, some people want to wear jonly one hearing aid (monaural amplification) rather than two (binaural amplification). For people with hearing loss in one ear, one hearing aid may be sufficient. However, if you have hearing loss in both ears, experts say it's better to wear two aids because they will help you distinguish between different sounds and identify from which direction sounds are coming. Research finds that wearing one hearing aid can actually deteriorate hearing in the "good" ear.

Hearing Aid Accessories

The most basic and essential component of any hearing aid is the battery. Without it, the hearing aid won't work. Most hearing aids use zinc-air cells, which are powered by oxygen, but a few use mercury batteries. The size of the hearing aid dictates how long a battery will last. Larger hearing aids have bigger batteries, which can last for about two weeks. Smaller batteries work for three to five days, on average.

David Hallett/AFP/Getty Images
This wristwatch doubles as a hearing-aid remote.

In addition to the basic components of the hearing aid, you can add a couple of special features to enhance the quality of sound you're receiving:

A directional microphone allows you to hear sounds directly in front of you more clearly than sounds behind you and to the sides, so that you can focus on conversations without worrying about annoying background noise. You can switch between a directional microphone and regular microphone, depending on your listening situation. Directional microphones will fit only on the larger hearing aids.

A telecoil allows you to switch from your normal hearing aid microphone to the "T" setting in order to filter out environmental sounds when you're on the phone. Telecoils work only with hearing-aid-compatible phones. (All wired phones manufactured today must be hearing-aid-compatible, but many cordless and cell phones aren't compatible.) Many theaters, places of worship and auditoriums have induction-loop systems, which will also work with your hearing aid's "T" setting.

Telecoils aren't the only devices that can help people with hearing loss use the phone. Amplified phones have a louder ringer, as well as flashing lights, to signal incoming calls. Text phones transcribe the call into text for people who are severely hearing impaired. A loopset has a wire loop that goes around the neck and connects to a mobile phone. The loop sends sound from the phone to the hearing aid and filters out background noise.

If you find that you still have to turn the TV volume way up to hear it, you might benefit from direct audio input. Many hearing aids can be plugged directly into the television, computer, CD player, and radio for clearer sound.

Research to Improve Hearing Aids
Hearing aids available today are smaller and more powerful than ever, and researchers are aiming for even higher sound quality in the future. They're looking for ways to better amplify the sound signal and reduce feedback using the latest computer technology.

One avenue of research is focusing on a very unusual subject -- the fly Ormia ochracea. This tiny fly has very precise hearing, which allows it to determine the source of a sound with uncanny ease. Scientists are modeling directional microphones after the fly's ears. The new microphones will help amplify sounds from certain directions, while suppressing sounds from other directions to help people focus on conversations when they are surrounded by background noise.

Buying a Hearing Aid

If you think you need a hearing aid, the best place to start is with a visit to an ear, nose and throat (ENT) specialist and/or an audiologist. The ENT will examine you to make sure there's no underlying problem causing your hearing loss, such as an infection or buildup of earwax. Then the audiologist will assess your hearing in a soundproof booth to see how well you can pick up sounds at different frequencies. The audiologist will also fit you for your hearing aids by making an impression of your ears. After the hearing aids have been made, you'll return to the audiologist for a fitting and to have the hearing aid programmed. You'll learn how to clean your hearing aids, adjust the volume and change the batteries.

Hearing-aid costs can vary significantly. Big analog hearing aids can cost a few hundred dollars, whereas small digital hearing aids can run into the thousands of dollars. Plan to spend anywhere from $500 to $6,000, depending on the options you choose. Because they involve surgery, cochlear implants are much more expensive -- about $40,000.

Although Medicare and many private insurance plans won't cover the cost of hearing aids, Medicaid does cover it, and Veterans Affairs will pay for qualified veterans. Some private insurance companies will cover the hearing tests used in the initial evaluation. Low-income patients can get help paying for their hearing aids through Audient Alliance.

Although price is important, it isn't the only issue to consider. Reliability and appropriateness are crucial when your hearing is at stake. Also remember that some prices include an evaluation and checkups.

If they are well cared for, hearing aids should last for five to seven years. Most of the problems that send hearing aids in for repairs are caused by dirt, earwax and oil from the skin that blocks the microphones and receivers.

Tips for Buying a Hearing Aid
Beware of companies making promises that seem too good to be true. No hearing aid will restore complete hearing, no matter how much it costs. Also watch out for companies that sell only one brand of hearing aid. Instead, visit a licensed audiologist.

Ask about a trial period. Most states require that manufacturers give customers at least 30 days to try out their new hearing aids. If you decide to return them, you’ll only have to pay back 5 to 20 percent of your purchase price. For mail-order hearing aids, federal law requires that companies give you the option of canceling your order for a refund.

Check the warranty. Most hearing aids have a one-year warranty, but longer warranties are available. The warranty should cover parts and labor. Also find out if you can get a loaner hearing aid if yours needs to be repaired.

Hearing Aid History

John Franklin/AFP/Getty Images
This long-distance ear trumpet
was custom-made in the
19th century.

The first hearing aids were enormous, horn-shaped trumpets with a large, open piece at one end that collected sound. The trumpet gradually tapered into a thin tube that funneled the sound into the ear.

The development of the modern hearing aid might not have been possible had it not been for the contributions of two of the greatest inventors of the late 19th and early 20th centuries. Alexander Graham Bell electronically amplified sound in his telephone using a carbon microphone and battery -- a concept that was adopted by hearing aid manufacturers. In 1886, Thomas Edison invented the carbon transmitter, which changed sounds into electrical signals that could travel through wires and be converted back into sounds. This technology was used in the first hearing aids.

The Industrial Revolution allowed for the mass production of hearing aids and created a new middle class that could afford the technology. In the 1800s, several companies, including George P. Pilling and Sons of Philadelphia, and Kirchner and Wilhelm of Stuttgart, Germany, produced their own versions of hearing aids. In 1898, the Dictograph Company introduced the first commercial carbon-type hearing aid. A year later, Miller Reese Hutchison, of the Akouphone company in Alabama, patented the first practical electrical hearing aid, which used a carbon transmitter and battery. It was so large that it had to sit on a table, and it sold for $400.

In the 1920s, vacuum tubes were introduced to hearing aids, which made sound amplification more efficient, but enormous batteries still made them cumbersome.

1952 ushered in the age of the transistor hearing aids. The addition of these simple on/off switches finally enabled the advent of a smaller hearing aid. Early transistor hearing aids were designed to fit within the frames of eyeglasses. Later, they were adapted to fit behind the ear. The first transistor hearing aid to hit the market in late 1952 was sold by Sonotone for $229.50.

In the 1990s, hearing aids went digital. Sound quality improved and became more adjustable. Also during this time, programmable hearing aids were introduced.

At the turn of the 21st century, computer technology made hearing aids smaller and even more precise, with settings to accommodate virtually every type of listening environment. The newest generation of hearing aids can continually adjust themselves to improve sound quality and reduce background noise.

For more information on hearing aids and related topics, check out the links on the following page.

Lots More Information