Sound is produced by vibrations of a stimuli, the vibrations of the stimuli cause vibration within the air particles around it. The frequency of sound produced by a stimuli remains at a constant frequency until it reaches the ear and enters the ear at that same frequency. i.e. the vibrations produces by the stimuli are the same as the vibrations travelling through the air and the vibrations heard within the ear. e.g. If a tuning fork vibrates at 256Hz it sets the air particles through which it is travelling at 256Hz also, the vibrations are then heard by the ear at 256Hz.

Figure 15.1 represents the constant frequency of sound as it travels. If this image were extended in both directions it would represent a sound wave and the frequency of the wave would always remain the same.

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