To quote Prof. J.C. LAFON:
« The level of hearing impairment opens various prospects.
Naturally, the hearing threshold is only one of the characteristics that allow sounds to be perceived by human beings based on their spectral composition. In other words, that information only tells us there may be a signal, but by no means whether that signal is perceived properly, if it is identified properly, and all the less so if it is useful in understanding the global message. Actually, those levels, calculated and expressed in decibels, provide information on what cannot be perceived: they provide adverse data.
It is generally acknowledged that hearing devices only become necessary for patients to perceive their surroundings when the general impairment reaches a level of 35 dB of hearing loss, and a level of over 30 dB of hearing loss in the 2000 Hz octave band. And vice versa: hearing aids are considered to be properly fitted and to provide a reliable improvement when maximal acoustic capacities display a hearing loss of about 30dB, as opposed to normal hearing. Basically, an average level of hearing loss inferior to 40 dB is enough to maintain a perception of the surrounding environement: of course, some useful information will be altered, but to a degree that is sufficient to maintain the capacity to create a coherent mental representation, compatible with a normal social life.
And this is all the more so because the ambient backgroung noise often reaches those levels in daily life: it has a similar effect to an increase in hearing threshold. The onset of acoustics events during the hearing process does not display major variations compared to a normal hearing function. Indeed, the main relevant characteristics of speaking out loud – the average level of which is 60-70 dB HL at proximity to the speaker lips – can be perceived… and a bit of lip reading provides the complementary information. This explains why, nowadays, we still come accross teenagers suffering from a light sensoneurinal hearing loss that is only diagnosed when they get to university, and that remained undetected till then (such cases occur less often than in the past, though, due to a systematic control of hearing function at earlier stages). However, the impairment does exist and results in an incapacity to hear whispering voices, remote sounds or rustles: all of this does cause some inconvenience in social life and restrain the environment. The combination of an additional impairment in the child will have major consequences: the amblyopic patient will lack the complementary information for a proper hearing, a patient with mental disease will suffer from a stronger sensoneurinal impairment, disabled people will have more trouble to structure their environmemnt, etc. It is thus crucial to check the hearing function in all patients suffering from sensory, intellectual or physical disabilities, and to treat any hearing impairment no matter how light it is.
The next level of hearing loss is set at 50 dB, the average level of hearing disabilities. As a result, we distinguish an up-to-50 dB group and a beyond-50 dB group of average hearing impairment. This is the point where most of the acoustic information contained in speech is not accessible to the hearing impaired. Below that average threshold, and in particular if the test shows a better hearing of deep or high-pitched sounds, the relevant sounds that are perceived allow an autoadjustment of the patient’s voice. Indeed, those patients can control their voice, tune, pace, vocal timbre and phonatory production. Above the 50 dB level of hearing loss, some timbres are missed. An absence of high-pitched tones alters the quality of the voice, as if it was timbre-depleted, the movements of the soft palate cannot be controlled properly and the articulation of consonants – those with a high-pitched tone in particular – is not as sharp as normal, it is diminished by as much as what the patient can hear. This 50 dB threshold of average hearing loss will be addressed at a later stage, when the disabling aspects of hearing impairment will be discussed. This is when we get to the world of deafness as the society conceptualises it: the verbal and social impairment, the impact on schooling, the attitude of the hearing impaired…»(1).
(1) Prof. J.C. LAFON « les enfants déficients auditifs »(« hearing-impaired children ») page 107.