HEARING
The sensation of sound reaching the ear, and the process of converting it into neural impulses which can be analysed into the component features of speech.
Sound waves are first processed by the peripheral auditory system. After reaching the pinna (the visible part of the ear), they are channelled into the ear canal, which acts as a resonator, amplifying the waves which pass along it. It can provide resonance for a range of frequencies from 500 Hz to 4000 Hz, but is especially sensitive to those between 3000 Hz and 4000 Hz. It is thus well fitted for transmitting the most important information in a speech signal.
The sound reaches the eardrum, and sets in vibration three small bones behind it known as the ossicles. This has the effect of producing a focus of acoustic energy which is much greater than that at the ear drum. The sound waves pass into the inner ear as variations in pressure, and produce movement along the basilar membrane. This in turn causes movement in a set of highly sensitive hair cells which rest on the membrane; and the movement is converted into neural impulses by the auditory nerve. The nerve consists of a bank of around 28,000 receptor neurons, each finely tuned to a particular frequency.
The brain then decodes the impulses. However, it is not easy for researchers to determine which pieces of the encoded information actually contribute to the identification of a particular speech sound.
See also: Deafness
Further reading: Ball and Rahilly (1999: Chaps 10–11)
