Wikipedia.org presents a most thorough and surprisingly detailed explanation on the subject of flatulence right here. I confess I’m quite surprised at the space devoted to this. But, of course, we all know how this is VERY important information:
…as a result of the voluntary or involuntary tensing of the anal sphincter, the rapid evacuation of gases from the lower intestine occurs through the anus, which, usually depending upon the relative positioning of each buttock, may or may not result in an audible crackling or trumpeting sound. This is often perceived as being humorous or socially awkward. The process may also result in the emission of odor, often deemed foul in character…
And Wikipedia’s page explains the “social considerations” involved!
While the act of farting is generally considered to be an unfortunate (but sometimes intentionally contrived) occurence in public settings, a flatus may, in casual circumstances, be used as either a humorous supplement to a joke, or as a comic activity in and of itself. usually accompanied by a distinct odor and sound, a process known informally as farting, or as passing gas.
And Wikipedia further explains the “mechanism of action”, and keywords like “vibration”, “tightness”, “pitch”, even… and the fact that flatulence can occur accidentally… oh my!
The noises commonly associated with flatulence are caused by the vibration of the anal sphincter, not by the buttocks. The sound varies depending on the tightness of the sphincter muscle and velocity of the gas being propelled, as well as other factors such as water and body fat. The pitch of the flatulence outburst can also be affected by the anal embouchure. Among humans, sometimes farting happens accidentally, such as incidentally to coughing or sneezing; on other occasions, intentional farting occurs through the tensing and releasing of the anal sphincter.
Um, well… you’ll have to read the whole page. It’s pretty thorough!