Question: What do sharts sound like?
Short answer: A muted trombone
Long answer: Last week’s post featured an interview with the newest member of the College, Two-Tone Malone. TwoTone has already produced some remarkable farts, and among them was something that we have not yet documented in our database of over 3000 farts: A shart. For those who are new to the field of flatology, a shart is a fart that accidentally leads to the production of solid output. Experienced farters rarely shart, but it is an occupational hazard that is difficult to avoid entirely.
Here is the shart produced by TwoTone:
The actual shart is clearly audible toward the end of the clip, but it’s interesting to ask whether it could have been predicted from the sound frequencies registered before the event. The answer to this question requires a review of some of our previous work on fart sounds.
We have previously seen that fart acoustics can be captured by a model in which the rectum is effectively a tube that is closed at one end. In that case, the sound frequencies are given by:
Here v is the speed of sound, L is the length of the tube, d is the diameter of the tube, and n is the (odd) harmonic number. This kind of equation allows us to predict, for example, the dominant sound frequencies in the farts of different animals. Specifically, because L increases with the size of the animal, fart frequencies decrease according to a general scaling law.
Now, a similar effect should be observable if the diameter d of the tube increases. This would explain how sharts escape, and it would predict a decrease in the fundamental sound frequency f. To test this hypothesis, we plotted the power spectrum of the fart in question at various times relative to the onset of the shart. Here are the results:
We see that one second before the shart (blue line), we have normal flatoacoustics, characterized by a peak frequency around 60 Hz and smaller peaks at higher harmonics. As the shart approaches (red and yellow lines), the peak frequency does not decrease, and in fact it actually seems to increase slightly, falsifying our hypothesis.
A second possibility is that the approach of the solid material that defines the shart alters the sound frequencies, in much the same way that the bell of a brass instrument emphasizes lower sound frequencies. It does this by simply obstructing the flow of air. We have previously seen that normal farts sound very much like tenor trombones.
This hypothesis seems to be consistent with the available data, which shows that the higher harmonics are sharply attenuated as the shart approaches. Here are the findings of a recent paper by Campbell, Gilbert, and Myers (2021) (reproduced in Campbell et al., Acoustics Today, 2021):
The spectrogram in the bottom figure shows the frequency content when the plunger is open (o) versus closed (+). Consistent with the shart effect, the higher harmonics are effectively eliminated by the closing of the instrument’s aperture. Thus, it appears that, contrary to our first intuition, a shart can best be predicted by the decrease in amplitude of the higher acoustic harmonics, which is most likely caused by the presence of solid material making its way toward the exit. These results suggest that an early warning system for sharts could be devised, based on the principle of tracking the abrupt disappearance of higher harmonics in ongoing farts.