HEARING AIDS AND 
HOW THEY WORK
A hearing aid is a small electronic device that amplifies sound.  No matter what style, size, or brand, all hearing aids have the same basic components: a microphone, an amplifier, a receiver and a battery.  All the parts of a hearing aid work together to amplify sound.  The hearing aid has internal and external controls.  The internal controls are set by the audiologist based on the users frequency needs and the external controls can be set by the user, such as the "on" and "off" switch and volume control. Some hearing aid users depend on a Telecoil switch to be able to use a telephone or an assistive listening device, which can be controlled by the user.

The three basic sizes of hearing aids are the behind-the-ear, in-the-ear, and in-the-canal.  A behind-the-ear aid fits behind the ear with a soft plastic ear mold which fits inside the ear.  The plastic tube, or ear hook is connected to the earmold and curves around the top of the ear to attach to the microphone, battery, amplifier and receiver that are all inside the case behind the ear.
 
 

image of ear with behind-the-ear hearing aid
Behind-the-ear
Behind-the-ear is suitable for all ages and all degrees of hearing loss.  This aid is most powerful and therefore it is used more with people who have a moderate to profound hearing loss.  Unfortunately, it is not as cosmetically appealing as the other two options.
 
 
image of ear with in the ear hearing aid
image of in the ear hearing aid
In-the-ear
In-the-ear hearing aid fits completely in the outer ear.  The hard earmold holds all the parts within its small case, which means the microphone and receiver are close together increasing chances of feedback.  The in-the-ear hearing aid is not suitable for children or for people with a profound hearing loss.  Also, it can be easily damaged by earwax and/or ear drainage; however, people prefer this aid due to its size and it being cosmetically appealing.
 
 
 
image of ear with in canal hearing aid
image of in the canal hearing aid
In-the-canal
In-the-canal hearing aid is similar to the in-the-ear hearing aid, but it is extremely smaller because it fits in the ear canal.  There are two options for in-the-canal aid.  There is the in-the-canal aid which mainly fits in the ear canal or an even smaller aid which fits completely in the canal and cannot be seen by others.  Both sizes are only suitable for mild to moderate hearing loss because the chance of feedback is increased.  The in-the-canal aid has similar disadvantages to the in-the-ear aid, but one main obstacle with in-the-canal aids is that the size of the aid does not allow easy access to the volume control and hte battery door.

For more information:
Hearing aids and How they Work
Demystifying Assistive Listening Devices: The Devil is in the Detail

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