Question: What happens when a lot of people fart for a long time in a small space?
Short answer: They see ghosts.
Long answer: There are many stories about haunted places, and many of these places happen to be taverns. Presumably this relates to the unsavory activities that have always taken place when strangers come together to eat and drink. People who visit these taverns report various experiences of a supposedly supernatural variety, including sensory hallucinations and vague feelings of danger, paranoia, etc.
Of course, as scientists our interest in the supernatural is negligible, but it is interesting to ask whether there are any explanations for these phenomena beyond the mundane quirks of human psychology. One such explanation was proposed by someone named Vic Tandy some 25 years ago.
Tandy was an engineer who worked in a laboratory where several people had reported eerie experiences similar to those described in supposedly haunted taverns, including strange apparitions and “a feeling of depression and occasionally a cold shiver.” After some investigation, Tandy discovered that a fan in one of the laboratory’s cleaning systems was emitting a powerful hum at 19 Hz. This is just below the lower limit of human auditory sensitivity, yet it is thought to be in the right range to exert other physiological effects on the nervous system. Indeed, switching off the fan dispatched the ghost. In other words, the experience of the paranormal could be attributed to exposure to infrasound.
We have previously documented the ultrasonic facets of farts, but this episode, along with several other related investigations, led us to formulate a somewhat unusual explanation for the prevalence of haunted taverns. The explanation follows from the indisputable fact that people fart a lot in taverns.
Now, as noted in previous posts, room acoustics can alter the sound of farts, emphasizing some frequencies and suppressing others. To a first approximation, a patron positioned at one end of a tavern will produce sounds that are amplified at a particular frequency:
where f is the frequency, v is the speed of sound, and L is the length of the room. Given that v = 343 m/s, this equation indicates that a sound frequency of 19 Hz will be amplified when the room is 9.03 m in length, which is about right for a typical tavern.
Note that this is true even if multiple people are farting out of phase with one another, because almost all human farts have approximately the same peak frequency. When adding two sinewaves of the same frequency ω, we obtain another sinewave:
where, A is the amplitude of the original sinewaves, ω is the frequency, and ∅ is the phase offset.
We used our Fart Maker technology to simulate 5 people farting at the same time, with different starting and stopping times. Here is the result:
As expected from the above reasoning, the combination of these 5 farts preserved the characteristic frequency spectrum of individual farts, with a dominant peak around 275 Hz, along with higher, odd harmonics. But farts clearly have some power in the infrasonic range as well, as evidenced by the power spectrum of the composite fart, shown in blue below:
Using methods developed in our recent work on room reverb, we can simulate the interaction between the multifarious fart and the acoustics of a typical tavern.
Using a room resonance calculator, we can recover the following frequency-domain impulse response for a typical tavern:
This has a prominent peak at 19 Hz (leftmost red bar). Convolving this impulse with the power spectrum of the fart yields the red curve in the above plot (“fart+ room”). Clearly, the room acoustics selectively amplify the infrasonic components of farts around 19 Hz.
From the simulation, we can also see that this effect would be strongest when the farters are standing close to a wall (red and blue below):
This makes sense given the standing wave calculation above, which captures the relationship between sound wavelength and room length. Thus we can conclude that farting in a space the size of a tavern will create standing waves at a frequency of approximately 19 Hz. Since previous work has shown that this frequency is capable of inducing seemingly paranormal experiences in people, it does appear that farts are capable of producing ghostly apparitions.