If you throw a stone into the water, the waves spread out in a circle. In exactly the same way, a sound source creates local pressure changes in the air. These rapid changes in air pressure are perceived by the ear as sound.
Sound waves are fluctuations in air pressure caused by sound events. The length of the sound waves defines the frequency, the height of the sound waves the level. Long sound waves have a low frequency and are perceived as deep tones. Short sound waves have a high frequency and are perceived as high-pitched tones.
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