Our world is full of sounds such as someone
talking, music, a car engine, a cat purring, an aircraft flying overhead, a ball being hit and
thousands more. The sound makes the air molecules vibrate and this vibration (actually a
longitudinal wave) travels through the air until it reaches your ear.
The sound is
"received" by your outer ear, rather like a dish aerial, and then travels along the outer ear
canal until it reaches the eardrum. The eardrum is just what its name suggests – a thin
membrane like the skin of a drum. This vibrates when the sound waves hit it. Pressing on the
other side of the eardrum are the middle ear bones.
These bones act like a series of levers, magnifying the
movement produced by the eardrum until the final bone – called the stirrup bone because of
its shape, starts vibrating.
The stirrup bone transmits the vibrations to the inner ear –
a hollow organ filled with liquid and with the inside covered in fine hairs. The hairs are
sensitive to movement and convert it to a series of electrical impulses. These signals are
then carried by the nerve to the brain – you can hear the sound.
The inner ear is
also important in giving you a sense of balance. The movement of the fluid in the three
semicircular canals tells you whether your head is upright or not and if not what position it is
in.
The ear is a remarkable organ – your ear can detect a range of loudness from the loudest to the softest sound of about 1012. This is about 1000 times greater than the range of intensity that your eyes can detect.
Your eardrum vibrates
when the sound waves hit it but only very slightly. At the threshold of hearing for a sound of
3000 Hz it only vibrates about 10-7 m - less than the wavelength of visible light. Using a stethoscope or an old fashioned ear
trumpet helps to collect and concentrate the vibrations and so makes it possible for you to
hear fainter sounds than if you just used your ears with no extra help.
The human
ear is also sensitive to a large range of frequency from about 20 Hz to 20 000 Hz – a
difference of 1000 times. As you get older the upper limit of hearing drops. You can notice a
difference between the ages of 11 and 19 and adults cannot usually hear any sound above a
frequency of 17 500 Hz.
People exposed to loud sounds for long periods (like pop
groups or those working with very noisy machine tools) are likely to find that their hearing is
getting worse more quickly. The ability of your ears to detect high frequencies (say around
4000 Hz) drops off much more than their ability to pick up low frequency sounds.