What types of aid are available?
Hearing aids can be either digital or analogue.
Digital hearing aids
In truly digital aids, the sound is converted from analogue to digital form. In this form, numbers represent the electrical signals, to which it is possible to apply mathematical techniques. The digital signals are then converted back to analogue form and sent to the receiver (in some aids the digital signal is sent directly to a special receiver). This makes it possible to process the sound in ways that would be difficult or impossible with analogue aids.
Digital hearing aids may include several features that are not found in analogue aids, such as systems for reducing:
- feedback (the annoying whistling sound that occurs when the sound generated by the aid leaks back to the aid microphone)
- the loudness of steady background noises, improving listening comfort
In addition, they may contain more sophisticated automatic gain control systems than analogue aids, and they may have directional microphones with enhanced directionality. Digital aids are also generally highly adjustable to suit the individual.
One potential problem with digital hearing aids is that they introduce a slight delay in the sound, typically between 1 and 10 milliseconds (thousandths of a second). Usually, this small delay is not noticeable. However, if a single digital aid is fitted, the time difference of the sound at the two ears may disturb the ability to localise sounds. Therefore, it is recommended that digital aids are fitted to both ears, not just one ear.
Analogue hearing aids
"Traditional" hearing aids are of this type. The term "analogue" means that they use conventional electronics. In these aids a microphone picks up sound, which is amplified using transistors, and is then reproduced by a miniature loudspeaker (receiver).
There may be a choice of settings, for example to allow for noisy situations, and these may be selected automatically.
Many analogue aids incorporate some form of Automatic Gain Control (AGC) or compression system which can give a lot of amplification for weak sounds while preventing loud sounds from becoming uncomfortably loud. Sometimes, the AGC operates independently in two or more frequency ranges, called "bands" or "channels". This can be advantageous for a person whose amount of hearing loss varies with frequency.
Digitally programmable hearing aids
These are analogue aids that use a digital memory to select between various settings. This makes it much easier for the audiologist or dispenser to provide the user with a range of suitable settings for different listening environments. In some models, the user can select settings using a remote control. Programmable aids have a number of advantages. They can be more precisely tailored to the individual's needs, settings can be changed and then restored and, as more settings may be available than in conventional analogue aids, more appropriate settings may be found for different situations.
How are hearing aids fitted?
Behind-the-ear (BTE)
Behind-the-ear (BTE) aids are the most common type in Europe and are available both through the NHS and commercially. The hearing aid sits behind your ear and the sound is directed into your ear canal via a custom made earmould. Some BTE models can be fitted onto the arm of your spectacles.
In-the-ear (ITE)
In-the-ear (ITE) aids fit entirely into the ear, filling the entire external part of the ear but with no components sitting behind the ear. Smaller versions of these fill only a small part of the external part of the ear, and are known as in-the-canal (ITC) hearing aids. Even smaller versions sit entirely in the ear canal, and are known as completely-in-the-canal (CIC) hearing aids. ITE hearing aids are not suitable for all degrees of hearing loss. They may be available through the NHS, depending on local policy, and are widely available from private hearing aid dispensers.
Body-worn (BW)
Body-worn (BW) aids consist of a small chest worn unit with a lead connecting this to an earphone and earmould. Because of their size, these aids are easier to operate than other types, and can provide higher amplification. BW aids are available through the NHS and commercially but are now less common than other types of aid.
Bone anchored
A fairly recent development, the bone anchored hearing aid is helpful to patients who are unable to use a conventional aid. This may be for a variety of reasons, such as if the patient's ears are absent or malformed, or if they have a disease that regularly causes the ears to discharge, clogging up their hearing aid. It is estimated that some 30,000 people fit into this category.
These types of aids works by transmitting sound through the bones of the skull to the nerves of the inner ear.
The aid is attached to a small titanium screw which is drilled into the mastoid bone behind the ear. The operation is quite simple and is carried out under local anaesthetic. It takes about 30 minutes to perform. The screw is then left for three months to allow the bone to grow back, locking the screw firmly into place, after which a plastic socket is attached. The bone anchored hearing aid fits onto this, and can be easily removed for sleeping or bathing.
Directional microphones
Several models of hearing aid incorporate directional microphones. These selectively amplify sounds coming from the front, while sounds coming from the sides or rear are amplified much less. In a noisy situation, if the user looks at the person they want to hear, the directional microphone can help them to pick out the desired voice from the background noise. Directional microphones are available in both behind-the-ear and in-the-ear hearing aids, but they are not available (and could not work) in completely-in-the canal hearing aids. With some models of hearing aid, it is possible to select between a directional microphone and a conventional "omni-directional microphone", with a button or switch on the aid or via a remote control. The omni-directional mode may be useful in some situations, for example when listening to music or when crossing the road.
