"Speak up please ... ok, there's no need to shout!"
If you have a relative or acquaintance with hearing loss, you may have heard those words before...or perhaps you've had to say them yourself?
Most people understand 'hearing loss' to mean that more volume is required in order to hear. However it is not only about adding more volume; many people with hearing loss perceive loud sounds in a similar way to normal hearing individuals; and may even be more sensitive at certain frequencies or pitches.
The ability to manage the range of incoming sound volume is referred to within the world of psycho-acoustics as a person's 'dynamic range'. Psycho-acoustics is the study of how sound is perceived; and is central to developing products designed to help with hearing loss such as hearing aids. So, in other words, dynamic range is the range of hearing between hearing threshold (i.e. just audible) and uncomfortable listening levels.
Being able to process different volumes is controlled by the cochlea, which is a part of our inner ear. This is a non-linear organ, in that it handles sounds with lower intensities differently to sounds with higher intensities. Within the cochlea, tiny hair cells pick up various aspects of the incoming 'wave' of stimulation. Hearing loss occurs when these hair cells are damaged. If this happens, we often see a reduced dynamic range which indicates that the ear's ability to process loudness is impaired.
For the normal hearing ear:
10dBHL is 'I can only just hear it' ; 100dB HL is 'very very loud!'
Therefore the dynamic range is 90dB.
For the ear with sensorineural hearing loss:
60dBHL is 'I can only just hear it' ; but 100dBHL is 'very very loud!'
The dynamic range is 40dB.
Hearing aids can not (yet!?) replace a normally functioning cochlea however they try to imitate the function of the hair cells, particularly when it comes to restoring normal loudness growth. For a hearing aid to do this well means that a hearing aid wearer should perceive soft sounds as soft, medium sounds should be comfortable and loud sounds should sound loud, regardless of their dynamic range. Digital hearing aids therefore aim to provide what is referred to as non-linear amplification so that low-intensity inputs are given more increased volume more than high-intensity inputs.