Hearing is tested by qualified professionals called audiologists.  The test takes about 30-45 minutes and the results are recorded onto a graph called an audiogram: https://en.wikipedia.org/wiki/Audiogram

The left axis of an audiogram shows the loudest decibel level required in order for the individual to hear the sound at a particular frequency.  These range from 0 decibels to 120 decibels.

The bottom axis shows the frequencies of sound ranging from 250 hertz to 8,000 hertz.

What we hear – or don’t hear – is broken into categories, which are:

Normal hearing:  0-25 decibels
Mild hearing loss:  26-40 decibels
Moderate hearing loss:  41-55 decibels
Moderately severe:  56-70 decibels
Severe deafness:  71-90 decibels
Profound deafness:  greater than 90 decibels

This is a simulation of hearing levels, from someone with normal hearing, through to someone with a profound hearing loss:

here

Hearing loss and the various tests involved are complex to interpret and cannot be fully explained here.  Nonetheless, there are some generalisations that can be drawn.

Hearing aids are not usually fitted until the deafness reaches at least a moderate level.  Someone who is severely or profoundly deaf is said to have significant deafness.

English speech sounds occur at a range of frequencies in a loudness range of between approximately 30 and 60 decibels. This then means that someone with severe to profound deafness cannot hear English speech.  With hearing aids, some people with this level of deafness can hear enough speech – note that what they hear of the speech will be incomplete – to be able to understand.  Often, they will support what they can hear by lipreading.

With significant deafness from birth or early life – prior to learning English – it becomes very difficult to learn English by listening.

 

References:

https://en.wikipedia.org/wiki/Audiogram