Vibrations from a sound source will be funnelled by the outer ear into the ear canal towards the ear drum. As the drum is made to vibrate in unison with the air molecules it moves some tiny bones in the protected middle ear. These act like levers and magnify the effect of the vibration. These in turn stimulate the liquid in the cochlea which is situated in the inner ear. There are tiny hairs in the liquid inside the cochlea which respond to different vibrations. It is these hairs which enable an electrical impulse to be sent to the brain which then registers the sound.

Amplitude of vibrations and loudness

Some of the properties of sounds can be understood by thinking about the size of the vibrations made. The size of a vibration or its amplitude is a measure of how far the object moves outwards from the starting position. An object vibrating with a large amplitude requires a large amount of energy and produces a loud sound. If a drum is hit hard or a string is plucked with great force a relatively loud sound will be made. A whisper or a feint scratching sound will be made by vibrations of relatively small amplitude. In the case of a guitar string or a plucked elastic band it is possible to see the amount of to and fro movement that produces a loud or quiet sound.

Fig 4 Diagram to show a large amplitude along with a representation in graphic form

Figure 4 shows an object vibrating with a large amplitude and how this might be represented on an oscilloscope in graphical form. Notice that the wave on the graph is not like the wave that a sound produces.

Loudness is measured in decibels (dB). This is a measure of the energy which is producing the sound and also how far away the sound detector is from the source. A prolonged sound level of 100 dB can ultimately cause damage to the human ear, while a level of about 30 dB would represent a quiet room with the television switched on.

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