Different levels of deafness have been defined according to the level of hearing loss on conversational frequencies (500, 1000, 2000 Hz):

mild deafness: hearing loss of 20 to 40 dB

moderate deafness: hearing loss of 40 to 70 dB

severe deafness: hearing loss of 70 to 90 dB

profound deafness: hearing loss above 90 dB (can be of type I, II or III)

total deafness or cophosis.

The cause of the hearing deficiency may originate in different parts of the auditory system. A distinction is made between two types of deafness. This will be developed later:

Conductive deafness

Perceptive deafness (or sensorineural deafness)

It is important to be able to distinguish these in order to understand the possibilities and therapeutic difficulties.