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Why Are Some Farts So Loud? A Scientific Inquiry

A doctor, a sound designer and a professor of biology weigh in on Fartgate

’Twas not the falling of a gavel convening the fourth round of presidential impeachment hearings, nor the desperate pleas of a spiraling president, that cut through our national psyche this week. Nope, that honor belongs to a big, giant fart.

Was it host Chris Matthews? Rep. Eric Swalwell? An unknown producer? We may never know. Many of my media colleagues have attempted to get to the bottom (pun unavoidable, so my apologies) of Fartgate, but I found myself pondering something much deeper: Why do some farts make such a clatter in the first place?

“One of the byproducts of digestion is gas, many kinds of gas, but gas,” retired physician Marc Leavey tells me. “Now, how interior and exterior sphincter muscles relax to let out that gas is the critical move that may result in a sound.” The same goes for the amount of gas waiting to get out: “A little gas expelled slowly may be silent — smelly, but still silent.” But, Leavey adds, if there’s a lot of gas being expelled — for example, because you’ve been sitting in an eight-hour congressional hearing clenching your butthole — then, “your anal verge becomes like a brass musician’s embouchure.”

An embouchure, for any non-band-geeks out there, is the lip formation brass musicians use to properly push air through their instrument. Here’s video of a trumpet player’s embouchure, so just replace his lips with, well, you know, a butthole:

Contrary to popular belief, the flopping of the butt cheeks have nothing to do with the toot sound. “Only if someone is manipulating the cheeks!” Leavey explains. Again, it mainly just comes down to the amount of gas building inside of you and how relaxed your sphincter muscles are. “If the opening is held tightly, a high squeaky noise is heard; looser makes a lower and often louder sound,” Leavey says. 

Photo via Wikimedia Commons (gas added)

Much like playing trumpet, the air passing through your sphincter causes vibrations in the anal opening. And when any object vibrates, the alternating pressures cause an oscillation, which becomes a sound wave. Humans detect these changes in air pressure as sound, and the frequency with which the object is vibrating is measured in hertz (Hz)

“From analyzing a few fart SFX from the libraries I have on hand, I’d place a loud fart somewhere in the low-to-mid range on the frequency spectrum, probably between 100 Hz to 1,000 Hz,” says Seth Parker, a sound designer in Chicago. For reference, he adds, “100 Hz would be a low-sounding bass note, 1,000 hertz would be a few octaves above that, perhaps something you’d hear from the middle range of a flute.” 

“Presumably, if one had good control, and if there were enough gas to expel, a melody could be played,” Leavey reasons. “But who would want to hear it?” 

Meanwhile, the volume of sound is measured in decibels. “The different uses of ‘decibels’ in audio can get complicated,” Parker continues, but for simplicity’s sake, he says the average fart likely falls around 80 decibels of SPL, or sound pressure level. “I can’t say for sure what the sound pressure level of a fart would be without actually taking a measurement, but normal conversation falls around 60 dB and a bus driving by might be closer to 80 dB, so I’d place an average fart somewhere around that.” 

Finally, I needed to know why — i.e., is there a reason why human farts are so audible? It couldn’t be a design flaw, could it? I mean, surely evolutionary science plays some kind of role and an echoing fart from a sleeping Homo sapiens warded off predators lurking in the night. 

Unfortunately, Richard Klein, professor of biology and anthropology at Stanford University, offers no relief. “I doubt that anyone has studied why human flatulence is sometimes audible, but I don’t think there’s a scientific answer,” he admits. Much like with other species, he adds, a fart is just a “way of releasing gas trapped in the colon.”

Which means your asshole makes a lot of noise for no good reason.