The act of controlling the dynamic range of a piece of audio - bringing the loudest and quietest parts closer together in amplitude/volume
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Compression waves happen when particles within a wave are close
together rather than spread out. Anything that has a sound is a
compression wave that can be heard.
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The compression ratio for the 1995 Mustang is: 9.0:1
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9.1:1 is the compression ratio for the 8.1
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In terms of particles, "compression" means that particles move
closer together.