(Check out the diagram in session 1)
The auricle (or pinna) gathers sound and channels it into the ear. It also serves to magnify the sounds, so that at 3 kHz it can magnify sounds by up to 15 dB. Many mammals can move their ears to catch the sound better, but we don't have these localising abilities.
Sound caught is then channelled down the auditory canal towards the eardrum. The auricle and the canal together are called the "outer ear".
The vibrations of the molecules strike the ear drum, which vibrates back and forth, moving the ossicles (Latin for "little bones") that are on the other side (in the middle ear). Thus far, the transmission of sound is mechanical. The ossicles magnify the vibrations (by up to 30 dB). If the sound is going to be too loud, the vibrations are dampened by muscles attached to the stirrup. A reflex action (the acoustic reflex) is set up if the noise is too loud, making the muscle contract and holding the bones tightly so that they vibrate less hard against the ear drum. However, the muscles can't stop really strong vibrations from getting through, so some loud sounds can still damage the ear.
It is vital that the ossicles can move freely, and that the pressure on both sides of the ear drum is the same (otherwise it would be pushed in or out too far). There is an important channel that runs from the middle ear to the throat that helps keep the air pressure the same, when air-pressure changes. This is the eustachian tube. (It's odd that we should have the evolutionary equipment to cope with pressure changes, when we've only just learned how to go up in lifts and aeroplanes, and down mine shafts. Perhaps the people who claim we evolved in the water are right. When you dive, the pressure changes on your ears and you need someway to equalise this).
The last ossicle (the stirrup) is attached to the oval window in the cochlea. The area from the ear drum to the oval window is called the middle ear.
The cochlea is the inner ear. It has an oval window, that is in contact with the stirrup. It is a shell-shaped organ. Below the oval window, there is another, round window. When the stirrup vibrates against the oval window, it causes the fluid in the cochlea to move in ripples . So now the information is being transmitted hydraulically. The pressure in the fluid is allowed to dissipate by the round window, much as the waves in a swimming pool would get too big if the water couldn't lap into the trough at the sides, or over the top.
The cochlea is divided into an upper and lower chamber by a membrane called the basilar membrane. This contains the organs of Corti which contain 20,000 hair cells. As the ripples in the fluid in the cochlea passes along the chambers, it causes the membrane to shear and the hair cells to bend. This bending is only of about 0.1 nanometre but it is enough to trigger electrical impulses which are then gathered up into the auditory (VIIIth) nerve. At this stage the transmission of the information has become electrical.
The hair cells are not all the same. There is not total agreement about exactly how it works, but it is clear that they play an important part in frequency perception. The cells nearest to the oval window are most important for high pitch. Cells further into the cochlea seem to be most sensitive to low pitch, or it is possible that cells all the way along the cochlea respond to low frequency vibrations.
The cochlea isn't a passive receptor of sound. The hair cells control the response of other hair cells, allowing complex messages about the sound to be transmitted to the brain. We used to think that the brain did all the "clever" analysis of sound, but now it seems that the cochlea does quite a bit of it. The hair cells are grouped into inner and outer cells, and it is possible that they have very different controlling tasks. It is very possible that the cochlea actually amplifies the sound. However, we really don't know how it does it. Certainly, researchers have recently discovered that the ear actually emits sound. These seem to be echoes of some sort, produced by the contraction of the muscles in the ear.
Once the impulses are picked up by the auditory nerve, they pass up the brain, to the auditory cortex, where the perception and identification of sound occurs. The brain also has to join up the information from both ears. The two ears deliver slightly different information, and this is used to give extra information about where the sound is coming from.
Localisation of sounds is important so that we can work out where a sound source is, relative to us. This is useful if we want to spot danger coming, or if we want to see who is speaking.
Localisation is done by
a) time differences. The sound will reach the ear that is nearer the source first. This is especially useful for low frequency sounds which have long wavelengths.
b) loudness differences. The head is pretty solid, and can block some sound so that the sound is louder for the ear nearer the source. This is particularly useful for high frequency sounds which have short wavelengths and don't bend very well around the head (long waves bend quite well). At high frequencies, the loudness difference can be as high as 20 dB.
Localisation of sound is an important reason for having two ears. A second very important reason is that if one stops working, we have a spare. This is not to be underestimated! We have a lot of duplicated organs, so that we can manage surprisingly well with one eye, ear, kidney, lung, arm, leg etc. Obviously, we can't manage as well as with two, but very much better than without any at all.
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