One can wonder how the various levels of hearing impairment impact speech and language.
Professor J.C. LAFON helps us on this: « …the audiometric classification of hearing impairments issued by BIAP (International Bureau for Audiophonology) summarises how the various levels of impairment affect speech and language. The key elements of this classification are presented below.
It refers, obviously, to a probability of occurrence that will depend on when the deafness arises, how early the child is equipped with hearing devices, on parental educational guidance, on speech therapy, on the child’s intellectual capacities, personality and behaviour, on the social and educational environments, on psychological and medical monitoring, on family care…
Mild hearing loss : average tone loss (a.t.l.) from 20 to 40 dB. Speech is perceived if the voice is normal, but some phonetic components are missing in the child’s perception, in particular if the voice is low-pitched.
Moderate hearing loss : a.t.l. from 40 to 70 dB. Speech is only perceived if the voice is loud, lip reading is employed, the patient encounters serious issues without hearing devices.
Severe hearing loss : a.t.l. from 70 to 90 dB. Speech is hardly perceived and only if the voice is very loud, lip reading is heavily employed, hearing aids are formally required and speech therapy is essential.
Very severe hearing loss : a.t.l. over 90 dB. Muteness is systematically observed if the child does not receive appropriate education and hearing devices. Lip reading is essential. If the average tone loss is over 100 dB at the 250 Hz, 500 Hz et 1000 Hz frequencies, then the hearing loss is considered to be almost total. This is cophosis. This does not mean that nothing can be perceived, but the informational content is so low that no acoustic structure can be identified through hearing. »(1)
No doubt you noticed Professor J.C. LAFON only refers to children here, but many of these lessons could also be used in adults.
(1) Professor J.C. LAFON « les enfants déficients auditifs » (« hearing-impaired children ») page 75.