According to the Royal National Institute for Deaf People, one in 10 adults in the UK would benefit from wearing a hearing aid, yet only one in 30 does so.
Hearing aids work by amplifying surrounding sounds so they can be heard by the user. Available in different shapes, sizes and types, they are worn either behind or in the ear, and are available free from the NHS or bought privately from registered hearing aid dispensers.
Recent years have seen a move from “analogue” to “digital” hearing aids – in the latter, the sound is processed by a tiny computer, allowing the aids to be customised very precisely to an individual’s level of hearing loss.
Many digital hearing aids can also be programmed with different settings for different sound environments – ranging from a quiet living room to a noisy restaurant. Most hearing aids can be switched to link in with hearing loop systems at counters and in venues including cinemas and theatres.
There are many types of hearing aids (also known as hearing instruments), which vary in size, power and circuitry. Among the different sizes and models are:
Body worn aids
This was the first type of hearing aid invented by Harvey Fletcher while working at Bell Laboratories, thanks to developments in technology they are now rarely used. These aids consist of a case containing the components of amplification and an ear mold connected to the case by a cord. The case is about the size of a pack of playing cards and is worn in the pocket or on a belt. Because of their large size, body worn aids are capable of large amounts of amplification and were once used for profound hearing losses. Today, body aids have largely been replaced by Behind-The-Ear (BTE) instruments.
Behind the ear aids (BTE)
BTE aids have a small plastic case that fits behind the pinna (ear) and provides sound to the ear via air conduction of sound through a small length of tubing, or electrically with a wire and miniature speaker placed in the ear canal. The delivery of sound to the ear is usually through an earmold that is custom made, or other pliable fixture that contours to the individuals ear. BTEs can be used for mild to profound hearing losses and are especially useful for children because of their durability and ability to connect to assistive listening devices such as classroom FM systems. Another benefit when used with children is cost, when the child is growing quickly a new mold can be made for a fraction of the price of a new ITE. Their colors range from very inconspicuous skin tones to bright colors and optional decorations. Recent innovations in BTEs include miniature “invisible” BTEs with thin hair-like sound tubes (see open-fit devices below). These are often less visible than In-The-Ear aids (ITEs) and some keep the ear canal more open so listeners may still utilise their residual natural hearing (most helpful for those with normal hearing in the lower frequencies). Ideal for high frequency losses, these miniature versions are generally used for mild to moderate hearing loss.
In the ear aids (ITE)
These devices fit in the outer ear bowl (called the concha); they are sometimes visible when standing face to face with someone. ITE hearing aids are custom made to fit each individual’s ear. They can be used in mild to some severe hearing losses. Feedback, a squealing/whistling caused by sound (particularly high frequency sound) leaking and being amplified again, may be a problem for severe hearing losses. Some modern circuits are able to provide feedback regulation or cancellation to assist with this. Traditionally, ITEs have not been recommended for young children because their fit could not be as easily modified as the earmold for a BTE, and thus the aid had to be replaced frequently as the child grew. However, there are new ITEs made from a silicone type material that mitigates the need for costly replacements.
Receiver In the ear aids (RITE)
At a first glance, these devices are similar to the BTE aid. There is however one crucial difference: The speaker (‘receiver’) of the hearing aid is placed inside the ear canal of the user and thin electrical wires replace the acoustic tube of the BTE aid. There are some advantages with this approach: Firstly, the sound of the hearing aid is arguably smoother than that of a traditional BTE hearing aid. With a traditional BTE hearing aid, the amplified signal is emitted by the speaker (receiver) which is located within the body of the hearing aid (behind the ear). The amplified signal is then directed to the ear canal through an acoustic tube, which creates a peaky frequency response. With a RITE hearing aid, the speaker (receiver) is right in the ear canal and the amplified output of the hearing aid does not need to be pushed through an acoustic tube to get there, and is therefore free of this distortion. Secondly, RITE hearing aids can typically be made with a very small part behind-the-ear and the wire connecting the hearing aid and the speaker (receiver) is extremely inconspicuous. For the majority of people this is one of the most cosmetically acceptable hearing device types. Thirdly, RITE devices are suited to “open fit” technology (see below) so they can be fitted without plugging up the ear, offering relief from occlusion.
In the canal (ITC), mini canal (MIC) and completely in the canal aids (CIC)
ITC aids are smaller, filling only the bottom half of the external ear. You usually cannot see very much of this hearing aid when you are face to face with someone. MIC and CIC aids are often not visible unless you look directly into the wearer’s ear. These aids are intended for mild to moderately-severe losses. CICs are usually not recommended for people with good low frequency hearing, as the occlusion effect is much more perceivable.
Bone Anchored Hearing Aids (BAHA)
The BAHA is a auditory prosthetic which can be surgically implanted. The BAHA uses the skull as a pathway for sound to travel to the inner ear. For people with conductive hearing loss, the BAHA bypasses the external auditory canal and middle ear, stimulating the functioning cochlea. For people with unilateral hearing loss, the BAHA uses the skull to conduct the sound from the deaf side to the side with the functioning cochlea.
Individuals under the age of 5 typically wear the BAHA device on a headband. Over age 5, a titanium “post” can be surgically embedded into the skull with a small abutment exposed outside the skin. The BAHA sound processor sits on this abutment and transmits sound vibrations to the external abutment of the titanium implant. The implant vibrates the skull and inner ear, which stimulate the nerve fibers of the inner ear, allowing hearing.
Recent hearing aids include wireless hearing aids. One hearing aid can transmit to the other side so that pressing one aid’s program button simultaneously changes the other aid and both aids change background settings simultaneously. FM listening systems are now emerging with wireless receivers integrated with the use of hearing aids. A separate wireless microphone can be given to a partner to wear in a restaurant, in the car, during leisure time, in the shopping mall, at lectures, or during religious services. The voice is transmitted wirelessly to the hearing aids eliminating the effects of distance and background noise. FM systems have shown to give the best speech understanding in noise of all available technologies. FM systems can also be hooked up to a TV or a stereo.
In developed countries FM systems are considered a cornerstone in the treatment of hearing loss in children. More and more adults discover the benefits of wireless FM systems as well, especially since transmitters with different microphone settings and Bluetooth for wireless cell phone communication have become available. Many theatres and lecture halls are now equipped with assistive listening systems that transmit the sound directly from the stage; audience members can borrow suitable receivers and hear the program without background noise. In some theatres and churches FM transmitters are available that work with the personal FM receivers of hearing instruments.
Directional microphones are currently – after FM systems – the best way to improve the signal to noise ratio, and thus, improve speech clarity in noise for the wearer.
Many hearing aids now have directional microphones, which can be a major improvement in crowded places such as restaurants and open-plan offices, because the directional microphone allows the user to focus on whoever is directly in front with reduced interference from conversations behind and to the sides. It is common for such a hearing aid to have both a directional microphone and an omnidirectional microphone and a switch that lets the user choose between hearing in all directions versus hearing only in the direction his or her head is facing. Some more-advanced models can electronically subtract signals so the user hears the directional signal minus the omnidirectional signal for improved background noise rejection.
Adaptive directional microphones are a further sophistication of the concept. The hearing aid processor is able to distinguish noise as opposed to speech and automatically reduce the particular noise source from a certain angle. The limitations are at the identification level, where a noise that behaves similarly to a speech signal is difficult to identify, thus reducing efficacy. In severe background noise, the directional microphone is less efficient, however benefits may still exist.
Directional microphones unfortunately work best when the distance to the talker is not so large. But at close range there is often less need for directional microphones. At longer range and when there is more background noise, an FM system is currently the best technology that can bridge distance and suppress background noise at the same time.
Telecoils (T-coils), sometimes referred to as “Telephone Coils”, allow audio sources to be directly connected to a hearing aid, which is intended to help the wearer filter out background noise. They can be used with telephones, FM systems, induction loop systems and public address systems. In the UK, and some Scandinavian countries, hearing loop systems are widely used in churches, shops, railway stations, and other public places. Within the US, such installations are uncommon, and are often ineffectively and/or inconveniently installed.
A T-coil consists of a metal core (or rod) around which ultra-fine wire is coiled. T-coils are also called induction coils because when the coil is placed in an electromagnetic (EM) field, an alternating electrical current is induced in the wire (Ross, 2002b; Ross, 2004). The T-coil detects EM energy and transduces (or converts) it to electrical energy. T-coils can also be used to pick up magnetic signals, just as a microphone picks up an acoustic signal; the T-coil then sends the signal to the hearing aid circuit or processor for amplification.
Since T-coils are effectively a wide-band receiver, interference is common. Such interference manifests as a buzzing sound, which varies in volume depending on the distance the wearer is from the source. Sources are electromagnetic fields, such as computers, electric cables, mobile phones, electric motors, airplane equipment, etc.
Since, by design, the sound processed through a T-coil is markedly different from a hearing aid’s microphone. The frequency characteristics can differ, especially at the mid and low frequencies.