What is a time period in regards to sound?

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Sound is a vibration of some medium: gas, liquid, or solid. It propagates through a medium as a wave, causing the compression and extension of that medium.

When a portion of a medium is compressed from some direction it tends to stretch out. This stretching occurs in the opposite direction and in the same direction. This way a wave goes through a medium farther and farther. Such a wave is called longitudinal.

For the most simple sounds (pure notes) the compression and extension have a periodic character. Some time is required to compress a portion of a medium, and then the same time is required for this portion to stretch out (and to compress the next portion). The time required for one complete cycle of compression and stretching is called a period of a sound.

The quantity inverse to a period is called the frequency of a (sound) wave.

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