| Sound is a mechanical wave. It travels as compressions (increases in pressure) and rarefactions (reductions in pressure) through a material medium like air, water or a piece of wood. Sound won't pass through a vacuum. This compression and rarefaction is due to the particles in the medium moving backwards and forwards along the direction the sound is travelling. Sound is thus a longitudinal wave. |  |
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Like all waves, sound has frequency, wavelength and amplitude. Frequency is more commonly known as pitch. Audible sound ranges from around 20 Hz to 20 kHz. Middle C on the piano has a frequency of 261.6 Hz. Going up an octave in pitch corresponds to a doubling of the frequency. |
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| The loudness or volume of a sound wave is dependent on its amplitude. The threshold of human hearing corresponds to a pressure wave amplitude of around 2 x 10-5 Pa which is less than 1 billionth of normal atmospheric pressure (~100 kPa). The loudest sound we can hear, without it becoming painfu,l is around 60 Pa. | ||
The speed of sound is around 340 m/s in air at atmospheric pressure. Generally, sound travels faster in liquids than in gases and faster still in solids. In pure water, sound travels at 1440 m/s. Applying the rule that v = f l, the frequency range of human hearing corresponds to wavelengths of between 0.2 and 17 metres in air. |
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| When two sound waves pass through the same medium at the same time, their amplitudes add together. This is called superposition and it leads to the phenomena of interference and beats. | ||
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Two sources of sound of the same frequency but in different positions in a room will produce constructive and destructive intereference in different parts of a room depending on whether the waves add together or cancel each other out. Two sounds, of slightly different frequencies will add together to produce beats as the amplitudes add together and cancel over time. The beat frequency is equal to the diffeence between the two source frequencies.
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| You will find an interesting website on sound, if you click here. | ||
