Sound is the name for longitudinal waves of pressure, caused by the oscillation of particles within solids, liquids, or gases, which can stimulate specialised receptor cells in our organs of hearing and thus be detected ("heard") by the brain.
Sound cannot travel through a vacuum, such as space, and types of sound exist which we cannot hear, but other animals may do (for example, ultrasound).
The change in the note of the sound we hear is because of a change in frequency of the waves - higher frequencies giving higher pitch - whilst a change in volume is due (mainly) to changes in amplitude.
The range of human hearing is from 20 - 20000Hz, and the speed of sound in dry air at 0°C is 331 metres per second.
Anne Evans
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