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Sound consists of pressure changes which propagate through a medium such as air. A sound can result from anything which causes changing pressure waves by forcing molecules alternately closer together and farther apart. In the following illustration, for example, a sound source is represented by the little red dot in the center of the image. A sound source causes the disruption of air molecules in wave-like patterns propagating outward from the sound source. This outward propagation is depicted by the red line (with the arrow).
Like waves that result from dropping a stone into a still pool of water, air pressure waves (herein called waveforms) consist of patterns of crests and troughs. The crest is the high point of the wave and the trough is the low point of the wave. This phenomenon is pictorially rendered in the above diagram by the wavy red line: the top part of the line constitutes the crest, while the lower part of the line constitutes the trough.
The distance between successive crests and troughs is refereed to as the wavelength of the waveform. In most natural sounds, the crest and trough characteristics are highly complex: they don't exhibit the clean wavy behavior depicted in the above picture. When such complexity is extreme, the perceived effect is one of noise. The sound of waves crashing on the beach is an example of such a noisy sound. Sounds like this produce only minimally distinguishable pitch characteristics. This is because there is a low level of periodicity in the waveform they produce. Waveforms that are periodic allow the ear to detect patterns whose perceptual response is one of pitch. However, even noisy sounds like waves crashing on the beach, have some periodicity, by virtue of which some waves sound high while others sound low and rumbling.