Sound waves approaching the ear enter either directly or are reflected by the pinnae down the meatus and are conducted to the cochlea by the three auditory ossicles (i.e.: the malleus, the incus and the stapes).
The ossicular chain produces a pressure amplification of about 20:1. Their vibrations are conducted up the cochlea by the basilar fluid which excites about 30,000 small hair cells on the surface. It is from the motion of these hair cells that the brain interprets sound.
How the Ear Works
Sound waves travel through the meatus (auditory canal) to the tympanic membrane (ear drum). The auditory canal can resonate and amplify sounds within a frequency range of about 2000 Hz to 5500 Hz by up to a factor of 10.
Successive compressions and rarefactions of air reaching the eardrum result in a change in pressure between the outer ear and the middle ear. The Eustachian tube helps to keep the middle ear at atmospheric pressure.
The difference in pressure between the sound wave striking the outer surface of the eardrum and normal atmospheric pressure on the inside of the eardrum causes the eardrum to vibrate.
Within the middle ear, vibrations travel through three small bones (the hammer, anvil, and stirrup) to the cochlea. The bones act as interlocking levers which amplify the force of the eardrum striking the hammer. The oval window of the cochlea is smaller than the eardrum. This causes a further amplification of the sound vibration, up to 20 times at some frequencies.
The semicircular canals act as miniature accelerometers. They also help to maintain a sense of balance by responding to gravity and changes in acceleration. The hair-like structures (dendrites) in the cochlea resonate at various different frequencies. The vibrations stimulate neurons to produce electrical impulses which are sent along the auditory nerve to the brain for processing.
The small pressure fluctuations we perceive as sound waves are imposed on a relatively stable atmospheric pressure. Given the way the ear is constructed, it is not sensitive to this constant pressure only the smaller fluctuations. (average atm = 101.325kPa). As an exercise using the the diagram above, try to determine just why the ear is not sensitive to atmospheric pressure (look at pressure equalisation via the Eustachian tube and the oval and round windows).
Direction Perception
The brain is able to detect the relative direction of a sound using the following mechanisms:
- Interaural delay
Depends entirely upon time delays between similar excitement levels in each ear. The distance between each ear can be taken to be about 150mm. This means that there exists a vertical plane, running through the centre of the head, within which sound reach each ear simultaneously. - The Effects of the Pinnae
These are designed to collect frontal sound and reflect it down the aural canal. Sound entering from above and behind must have been diffracted by the pinea and, as a result, slight spectral changes to the sound will have occurred (more on diffraction later). - Subtle Head Movements
These assist in determining the height of a sound source in the medial plane (as described above).
Distance Perception
The brain is also able to perceive the relative distance of a sound source as follows:
- Loss of intensity due to inverse square law and molecular absorption.
- Changes to the spectral content resulting from molecular absorption and diffraction around objects.
- The level of direct Vs indirect sound - the age old trick of increasing reverberation as a song finishes in order to give the impression it is fading away into the distance, sounds like its in a large cave.
Audible Range
The ear can hear sounds ranging from 20Hz to 20kHz. It is most sensitive to frequencies between 500Hz and 4000Hz, which corresponds almost exactly to the speech band. It should be noted that the ear is particularly sensitive to lower frequencies more than higher frequencies, an important point that will be the subject of more detailed discussion later.
Related Links
- Sensitivity of Human Ear
- http://hyperphysics.phy-astr.gsu.edu/hbase/sound/earsens.html
- Hearing and Equilibrium
- http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/Hearing.html