Why does a fart make a sound?

The muscles of the anus is doing its best so that feces may not leak.
When feces come out, it opens greatly, and when a fart comes out, it opens a little.
If gas passes along the anus with sufficient vigor, membrane will vibrate and sound will come out.
The strength and loudness of the sound depend upon the passage speed of gas, and the quantity which comes out.
The number of a fart increases by the old person is due to the weakness of the anus muscles.

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