Question: Is it possible to conceal fart sounds?
Short answer: Yes, to some degree.
Long answer: Benjamin Franklin famously suggested that science should develop food additives that render fart smells “agreeable as Perfumes”. As far as we know, this has not been accomplished. Consequently, those who wish to conceal the smell of their farts must rely on special undergarments that hide the odor, and it appears that these actually do work (Suarez et al., Gut, 1998). Concealing the sound is trickier.
One promising technology is active noise control, which has commercial applications in noise-cancelling headphones. The technology works by sampling a sound and reproducing it in antiphase, thereby creating destructive interference. In theory, a noise cancellation system that was embedded in a user’s undergarments could work in the same way, by emitting a noise that cancels fart sounds. If this were combined with odor-filtering technology, the user would have free license to fart, undetected. The device would have to include technology for recording farts and generating sounds, but it would surely be more pleasant to use than the current state-of-the-art.
There are two challenges to the development of fart-cancelling sound technology. First, the device must emit a noise that has the same volume as the fart. Second, the technology must produce the anti-fart with sufficient speed as to eliminate the fart sound before it is heard. The second issue is particularly interesting, because a long delay would effectively create two fart sounds, and we have already seen that two simultaneous farts sound like another fart. Indeed, this is the basis of our Fart Maker algorithm.
To see how challenging this problem is, consider a typical fart:
If we play this fart simultaneously with a copy of itself that is inverted in phase and shifted by just 23 milliseconds, we hear the following:
The cancellation has clearly backfired, as the two sounds together sound like a fart that is even louder than the original. It turns out that in order to cancel a fart successfully, the delay has to be closer to 20 microseconds. In that case, we achieve some degree of cancellation:
The plot below shows the effect of adding antiphase farts at various delays:
The red line indicates the volume of the original fart, and the other points show that the delay must be less than about 100 μs for the cancellation to be effective. As shown by the vertical gray line, analogue noise control systems can operate with a delay of about 27 μs (Pawelcyk, 2002), so it seems that the best attainable cancellation is on the order of 50 – 60% of the original fart amplitude.