The normal healthy human ear can hear sounds with a frequency as low as 20Hz (20 vibrations of the eardrum per second), all the way up to a frequency as high as 20,000Hz (20,000 vibrations of the eardrum per second)!
The ear is able to hear the very slightest whisper of sound, 0 dB SPL (0 decibels Sound Pressure Level) or less, and yet, is able to tolerate very high intensity sounds in excess of 115 dB SPL (115 dB exposes the eardrum to millions of times more pressure than 0 dB SPL) for short periods of time.
ITE models are the largest of the models that fit completely in your ear -- only the BTE is larger. As the picture shows, they tend to fill the ear completely. They are less powerful than BTE aids, but more poweful then ITC or CIC aids. Usually, they are large enough to offer features such as a directional microphone, telephone switch or larger amplifier.
Completely in the canal hearing aids are the smallest of all the models that are available today. They fit completely into your ear canal (hence the name!) and are virtually invisible to other people -- usually only a small plastic wire protrudes from your canal and maybe be visible to people who are looking very closely at your ear. The plastic wire is necessary to enable you to pull the hearing aid out of your ear -- it sits too far down the canal for you to be able to pull it out without the wire.
In the canal models are smaller than ITE aids, but larger than CIC ones. They tend to fill roughly half of your ear, whereas the ITE models usually fill the entire ear. ITC models are usually visible to other people; although they do come in range of skin-matching colours that make them difficult to spot unless someone is looking directly into your ear.
Behind the ear hearing aids
have a main shell section, an earmold and a
connecting tube. The main shell houses the
electronics and the battery -- the shell
sits at the top of your ear and runs down
behind. On the latest hearing-aids the tube
that runs from the shell to the ear-piece is
very thin and is almost invisible to the
casual observer. The design of the earmold
varies from model to model; in some it sits
in the hollow of the ear, like an ITE; in
others it fits more like a ITC aid.