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Sound waves are longitudinal waves. They cause particles to vibrate parallel to the direction of wave travel. The vibrations can travel through solids, liquids or gases.

The speed of sound depends on the medium through which it is travelling. When travelling through air, the speed of sound is about 330 metres per second (m/s). Sound cannot travel through a vacuum because there are no particles to carry the vibrations. The sound travels faster in solids than it does in liquids or gases as the speed depends on the density of the material. In water, sound travels at 1,400 m/s, in wood at 4,000 m/s and in steel at 5,790 m/s.

The ear

The human ear detects sound. Sound waves enter the ear canal and cause the eardrum to vibrate. Three small bones transmit these vibrations to the cochlea. These bones transmit most efficiently frequencies of 1 kHz to 3 kHz. The vibrations in the cochlea produce electrical signals which pass through the auditory nerve to the brain, where they are interpreted as sound.

Properties of sound

The frequency of a sound wave is related to the pitch that is heard:

• high-frequency sound waves are high pitched
• low-frequency sound waves are low pitched

The amplitude of a sound wave is related to the loudness (volume) of the sound:

• high-amplitude sound waves are loud
• low-amplitude sound waves are quiet

Oscilloscope traces showing the following sounds:

1. quiet, low-pitch sound
2. loud, low-pitch sound
3. loud, high-pitch sound

The cochlea is only stimulated by a limited range of frequencies. This means that humans can only hear certain frequencies. The range of normal human hearing is 20 Hertz (Hz) to 20,000 Hz (20 kHz).