One complaint of most patients with cochlear loss is a decrease in dynamic range, also called cochlear recruitment. This is because the loudest sounds are still perceived with the same intensity, whereas patients with cochlear loss are unable to hear quiet sounds. Therefore the range between comfortable listening and uncomfortable loudness is much smaller than in normal subjects. Aids must then be able to supply adequate gain to the user without creating sounds which are too loud. Most aids sold today use linear amplifiers, which increase the signal amplitude by a constant amount, regardless of input level.
Earmold design is an important factor in aid design.
With BTE type aids, a silicone earmold is used to deliver sound to the ear. Completely blocking the canal results in an increased amplification and perception of low frequencies, especially in the sound of the wearer's own voice. Feedback occurs when a microphone is placed very near a speaker. The microphone picks up the speaker output, which is amplified and sent to the speaker, which is picked up by the microphone and amplified again, etc. The capacity of the amplifier is quickly overloaded, and the high-pitched sound called feedback results.