Speech is made up of a range of frequencies. The lower frequencies are made up of vowels which provide the volume in speech. The higher frequencies are mainly the consonants that provide the clarity in speech. This is why people with age-related hearing loss (presbyacusis), where high frequency hearing is more affected than the low frequency hearing, can quite often still hear people speak but are unsure of what they are saying.
Hearing aid channels separate speech and other incoming sounds into their different frequency components. By doing so, the hearing aid can then provide the correct level of amplification (and compression) for each frequency of sound based on an individual’s hearing loss and listening needs.
In total, there are approximately 17-20 frequency regions (‘critical bands’ based on the Bark scale) that relate to speech within the organ of hearing (cochlea). As a result, the majority of hearing aid manufacturers deem this to be the optimum number of channels within a hearing aid.