Cochlea
[see page 6, Converts] sound waves into electrical signals for the brain to process. Essentially a very advanced form of frequency-analysis.
The cochlea is 35mm long and covered in a colourless liquid called perilymph and
another liquid called endolymph. It consists of two membranes:
- Basilar membrane
- Reissner's membrane
It's divided into 2 regions along its length by the cochlea partition.
There're [see page 6, hairs] all around the cochlea. As the sound travels through the cochlea hair cells are excited depending on their position in the cochlea; with the highest frequencies hairs being near the start and the lowest near the end.
Think of the cochlea like one long sound graph, with hairs at different points along the unrolled cochlea being excited due to different frequencies. The excitation is measured and sent to the brain.
Note: The actual conversion of mechanical movements into pulses is done by the organ-of-corti.
Hearing Loss
Is a complication where we steadily have difficulty hearing things. This is due in part to the hair cells in cochlea dying, especially with higher frequency cells which're active much more often than lower freqency ones.
Hearing aids work around these issues by amplifying sound to higher frequencies so they assist when low-frequency hair cells have died. A cochlear implant on the other hand works by bypassing the hair cells altogether, stimulating the nerves directly.
Signs
Possible [see page 20, signs] for hearing loss include:
- Soft sounds are difficult to hear quickly.
- Hearing threshold in frequency increases.
- Loud sounds become too loud.
- Dynamic range in frequency is increased.
- Speech in noise is no longer understood correctly.
- Psychoacoustic benefits declined (eg. Frequency/Time resolution).