Severe to profound, pre-lingual deafness means that before the person learned his or her first language, s/he had become significantly deaf. Pre-lingual can mean from birth (congenital) or it can be from early life, up to about the age of three.
The degree of deafness is measured by testing both loudness (decibels) and pitch (frequency) and plotting responses onto a graph called an audiogram.
Severe deafness occurs when a sound must reach between 71 and 90 decibels before it is heard. Profound deafness occurs when a sound has to be louder than 91 decibels to be heard.
It is important to place this information into its proper context. The sound made by a lawn mower is estimated to be 100 decibels. This level of sound hurts our ears and, in fact, damages our hearing.
Someone with severe deafness would hear the sound of a lawn mower as a quiet one and someone with profound deafness would barely register the sound or possibly not hear it at all.
For English, the so-called “speech range” for the various sounds (or phonemes) of English fall roughly between 25-65 decibels, across a frequency range of roughly 250 hertz to 4,000 hertz.
This means that someone with severe or profound deafness cannot hear English speech at all (remember: sounds under 71 decibels are not heard at this level of deafness). Hearing aids may help someone with severe to profound deafness to hear some sounds in the English language.
This is why someone with significant, pre-lingual deafness often faces a challenge when trying to develop English literacy skills; s/he is trying to learn to read and write a language that s/he does not know and cannot readily access through listening.