Question: What does a fart sound like to a bat?
Short answer: See below.
Long answer: One of the most influential philosophical studies of consciousness asked what it is like to be a bat. As far as we know, this question has not been answered, but an important facet of the question concerns the bat’s sensory experience, which revolves largely around its perception of high-frequency sounds. As mentioned in previous posts, human farts produce sounds primarily in the range of 200 – 300 Hz, but they also contain power at much higher frequencies.
The importance of high frequencies was first noted informally by Lord Rayleigh, who wondered why he had trouble understanding his assistant when they weren’t facing each other. He wrote, “Probably the turning away of the speaker softens…high elements in the sound…The repetition and extension of these observations would be of interest.” We now know that high frequencies are indeed important for understanding speech.
In a previous post, we suggested that high frequencies are also important for understanding fart sounds, since they seem to be amplified by the opening and closing of the external sphincter. As noted in our previous report, farting is in this regard similar to shouting through the anus.
A review of research on speech indicates the surprising finding that speech sounds often extend even to the ultrasonic range. These are frequencies beyond the range of normal human hearing, which in adults has an upper limit of about 16 kHz. Higher frequencies can be heard by children, as well as animals like bats and dolphins, which use ultrasonic frequencies to communicate.
To determine whether farts contain ultrasonic frequencies, we turned to our database of more than 3000 farts. For technical reasons, these were sampled at 44100 Hz, which places the Nyquist limit of our analysis at around 20 kHz. Nevertheless, we can ask whether fart sounds contain frequencies that would be inaudible to the human ear, by analyzing the power of frequencies in the range of 16 – 20 kHz. Here is the mean result for a sample of 392 farts:
The red line shows that the power in the ultrasonic range is elevated well above baseline (blue), indicating that farts do indeed create ultrasonic sound frequencies.
To understand what this might sound like to a bat, consider the example fart in this audio clip:
Most of the power in the waveform is contained in the lower frequencies, but from the spectrogram below, it can be seen that moments of high sound amplitude generate broadband frequency spectra that extend well beyond 16 kHz (dashed lines).
By band-pass filtering the signal, we can isolate these components from the rest of the audio clip and slow them down, to create the following audio:
This is what farts sound like to a bat.
Bats primarily use ultrasound for echolocation, which raises the interesting possibility that humans could use farts to navigate. This will be the topic of a future post.