Hearing Aid

Hearing aid



Definition:


Hearing aids are electroacoustic instruments that improve the audibility of speech and other important signals for the person with hearing loss. (Doyle, 2002)

The aim is to amplify the volume of the signal to a level that is, on average, above the individual’s hearing threshold and below that individual’s level of discomfort, so that the signal is comfortably audible, and to do so in a way that maximizes the intelligibility of speech signals.


Parts of a hearing aid


Hearing aids come in many shapes and sizes but they all have the same basic components.

The main parts of any hearing aid are:

1) Microphone
2) Amplifier
3) Lead (or cord)
4) Receiver (earphone)
5) Earmould
6) Battery
7) Volume control
8) On-off switch

In addition to the above components many hearing aids also have:

9) Tone control (may be inside)
10) Telecoil (T switch)
11) Maximum output control (Inside)
12) Gain control (Inside)
13) Audio input connection

The inside controls are for the audiologist to set, not for the wearer to adjust himself.

How does it work?

1) The microphne picks up sound from the environment and converts it from an acoustic signal to an electronic signal.
2) The amplifier processes and amplifies the electronic signal.
3) The receiver works like a speaker and converts the amplified electronic signal back to acoustic signals.
4) The sound is delivered to the ear canal through a tube in the earmould or via the hearing aid shell
5) All hearing aids require a battery to supply power to the system
6) The amount of amplification (or gain) is varied by means of the volume control.
7) The on/off switch turns the hearing aid on or off (Some times there is no separate on/off switch but the aid is turned off by the volume control)
8) When telecoil is in use the aid can pick up electromagnetic signals from a telephone or inductive loop.
9) Low cut tone control gives more high frequency emphasis by cutting the low frequency signals. It is sometimes used to cut background noise.
10) High cut tone control gives more low frequency emphasis by cutting the high frequency signals. It is sometimes used to reduce background noise.
11) The maximum output control (or peak clipping) limits the maximum intensity of sound that the aid will deliver.
12) The gain control reduces the maximum amplification.
13) A means for conection the aid directly to an FM radio receiver or other external device.

Types of hearing aids:


Hearing aids are most commonly classified by style, which is closely related to the size of the instrument. There are five basic types of hearing aid available today. These are-

1) Body-worn or Pocket hearing aid
2) Behind-the-ear (BTE) or Post-aural hearing aid
3) In the ear (ITE) aid
4) In the canal aid
5) Eyeglass or Spectacle aid

The Bone conduction hearing aid, Radio frequency (RF) or Frequency modulated (FM) are additional hearing aids.


When electroacoustic hearing aids were first developed several decades ago, the body aid was only type available. In the ensuring years, the other types of instrument were developed, with BTE hearing aids being the most common in the 1970s. During the 19980s, the trend has been for ITEs to capture an increasingly larger portion of the hearing aid market. The increasing popularity of the ITE and ITE and ITC hearing aids is a result of both consumer pressures to improve the cosmetic appeal of the devices and rapid developments in the field electronics. High fidelity electronic components and the batteries to power them have been drastically reduced in size, making the in the ear devices possible.


Description of various types of hearing aids:


1) Body-worn or Pocket hearing aid:

The largest and least frequently fitted aid is the body level hearing aid, which is worn clipped on to clothing, generally at chest level. The signal that is picked up and amplified in the body worn aid is fed to a receiver attached to a standard earmould. Because the microphone (Input transducer) and the receiver (output transducer) are a considerable distance apart, very powerful amplification may be provided with little risk of acoustic feedback. Even though powerful amplification is now available in much smaller aids, body-level aids remain appropriate for some individuals, particularly those who require very large controls because of reduced manual dexterity and/or poor vision.

2) Behind the ear (BTE) or Post-aural hearing aid:

In behind the ear (BTE) hearing aids all components are worn at ear level. The microphone, generally level with the top of the ear, is thus in a more natural position than it is in body aids. Even individuals with profound hearing losses may be fitted with BTE aids. Earmould styles and materials in BTE aids can vary greatly and are an important part of the sound delivery system. The same hearing aid can produce very different results with different earmoulds.

3) In the ear (ITE) aid:

In the ear (ITE) aids house all components in a custom made shell, which fits into the concha and external auditory meatus. The size of ITE aids varies with the size of the individual’s ear and the desired features. In the canal (ITC)


4) In the canal aid:

In the canal (ITC) hearing aids provide all components in a custom made shell that occupies the ear canal and less of the concha. The smallest aids are completely in the canal (CIC) aids, which are effectively not visible. There are some exciting developments in amplification with CIC. Deep canal hearing aids sit partly down in the bony portion of the ear canal and this appears to provide relief from the occlusion effect experienced by aid users who complain about how they hear their own voice.

Many smaller hearing aids cannot provide features (such as telicoil) beyond the standard microphone, amplifier, battery and receiver because of space limitation.

5) Eyeglass or Spectacle aid

Eyeglass or spectacle aids provide amplification through hearing aids built into the frame of spectacles. The amplified signal is fed via a tube to the ear canal. Spectacle aids may or may not be worn with earmoulds, depending on amplification.

Others:

Radio frequency (RF) or frequently modulated (FM) hearing aids are instruments in which the microphone is positioned at the sound source rather than at the ear of the listener. A type of aid that is fitted very infrequently is the bone conduction hearing aid, in which an oscillator placed on the mastoid and held in place by a spring steel band over the head replaces the output transducer. An advance on present situation is the implantable bone conduction aid (IHA) or bone anchored hearing aid (BAHA).


Hearing aids also can be classified as:

1) Analogue
2) Digital

1) Analogue:

Most hearing aids currently use ‘analogue’ circuits. These have developed a lot from the old ‘class A’ amplifiers and ‘class B’ push-pull amplifiers found in more powerful hearing aids (eg powermaster). Many aids now have ‘class D’ amplifiers in compression circuits.

2) Digital:

In the last few years a new range of hearing aids has been developed which use digital programming of analogue circuits. The aids are adjusted with the help of a computer. This often makes it easier to modify the frequency response according to the user’s audiogram.

The latest hearing aids use digital signal processing (sometimes called DSP or 100% digital). The electrical signal is converted into digital information and then ‘processed’ before being converted back to an analogue electrical signal and then into sound by the receiver.

One advantage is the lack of internal electrical noise. It is also possible to devide up, manipulate or ‘process’ the signal much more easily than with an analogue circuit.


Advantages of hearing aids:

Body worn hearing aids:

• Does not break easily.
• Cheap in rate
• Battery is available
• Easy to carry
• Safe for the client
• Easy to identify the problems

Behind the ear hearing aids:

• Not very visible
• Nice to look
• Need not wear in the body
• Can receive sounds from around of the client
• Modern pattern
• Very lightening
• Comfortable


Disadvantages of hearing aids:

Body worn hearing aids:

• Very visible
• Old fashion
• Does not look nice
• Not modern pattern
• Need to wear like a dress
• Not very comfortable
• Cannot receive all the sounds

Behind the ear hearing aids:

• Not cheap
• Battery is not available
• May break easily
• Repairment is not very easy
• Difficult to clean


Care and Maintenance of hearing aids:

Microphone:

a) The microphone is most effective within a distance of about 3 feet. Small children should wear their pocket hearing aids in a specially made belt or harness. This keeps the aid securely in the correct position.
b) With small children it must be protected from dirt, spiled food, drink etc by a baby cover
c) The microphone works on M setting (not T)
d) Switch off when not in use to save batteries.

Amplifier:

a) This consists of electronic components inside the case. They must be kept clean and dry.
b) The volume control determines the amount of amplification (or gain)

Cords (or leads):

a) A break in the lead sometimes causes an intermittent signal. (Sometimes on, sometimes off)
b) There should be no knots in the lead.
c) When the aid is not being used, the cord should be coiled correctly and loosely.
d) Different length cords are usually available. Try to get short cords for little children.

Receiver:

a) The receiver may be either 2 or 3 pin. It is matched to the aid. Do not allow children to exchange receivers.
b) If possible use a small size receiver with little children
c) Receivers are not easily damaged unless they get wet. Do not allow children to put the mouth.

Earmould:

a) This must be tight fitting or feedback and whistling occurs
b) Earmoulds must be kept clean. They should be removed from the receiver or hearing aid elbow and washed carefully in warm soapy water. They must be dried very carefully before replacing.
c) The tubing will need to be replaced about every six months

Batteries:

a) Pencil batteries run down gradually and must be changed frequently before they are drained.
b) Dead batteries must not be left in the hearing aid.
c) Post aural button batteries cut out suddenly. Spare batteries should be carried always.
d) Note that small button batteries contain poison. They must not be swallowed. Keep them away from babies.








References



a) Doyle, J., (2002), Practical audiology for Speech-language therapy, Whurr Publishers, London.

b) Bess, F.H.; Humes, L.E.; (1990), Audiology the Fundamentals, Wiliams & Wilkins, Baltimore.

c) Tucker, I.; Powell, C. (1991), The Hearing Impaired Child and School, A condor book, London