1) The outer ear - a) it might simply not be there. Some people are born without auricles, and some without the ear canal.
a) the canal might be blocked by wax, or a tumour or a polyp, or a boil, or a bead.
2) The middle ear - a) the ear drum might be damaged. This might be because of a sudden change in pressure, eg in an aeroplane, in an explosion or a punch in the head. It might also be because an infection weakens or tears it.
a) b) the ossicular chain might be defective or absent
b) the bone of the ossicles might keep growing and fuse them together so that they can't articulate against each other. This is called otosclerosis.
c) the ossicles might be dislocated eg by a blow on the head.
d) the eustachian tube might be blocked by an infection, so that the ossicles can't move freely.
3) The inner ear - a) the hair cells might not be there
e) b) the cells might be damaged or even destroyed
b) the auditory nerve is not working.
4) The brain isn't working. Sometimes everything in the ear works fine, but the brain can't receive, or decode the message that the ear sends. Sometimes this is because of physical problems. It can also be psychological, when the condition is sometimes called "hysterical deafness", although it's not a fashionable term. There is also a strange languages disorder called "word deafness", when a person can hear everything fine (eg birds, dogs, cars, the wind) but can't extract any information about words from it.
Deafness due to the brain not working is one type of deafness. The other two main types of deafness are "Conductive" and "Perceptive" deafness.
Conductive deafness occurs because sounds cannot be conducted into the inner ear. This means that the outer or middle ears aren't working. It is sometimes called "middle ear deafness". The result of conductive deafness is that everything sounds quieter.
Perceptive deafness occurs because the inner ear can't cope with the information it receives, and can't pass it on up the auditory nerve. Because it is in the inner ear, it is sometimes called "inner ear deafness". The result of perceptive deafness can be that some frequencies are heard less well than others. There are also problems like recruitment.
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