The brown note is a hypothetical infrasonic frequency that would cause humans to lose control of their bowels.

Frequencies supposedly involved are between 5 and 9 Hz, below the lower frequency limit of human hearing. The Brown Note's effects on the bowels are controversial, though, and many studies report that the frequency doesn't cause diarrhea but nausea.

On February 2005 the television show MythBusters used twelve Meyer Sound 700-HP subwoofers—a model and quantity that has been employed for major rock concerts. Normal operating frequency range of the selected subwoofer model was 28 Hz to 150 Hz but the 12 enclosures at MythBusters had been specially modified for deeper bass extension. Roger Schwenke and John Meyer directed the Meyer Sound team in devising a special test rig that would produce very high sound levels at infrasonic frequencies. … The hosts on the show tried a series of frequencies as low as 5 Hz, attaining a level of 120 decibels of sound pressure at 9 Hz and up to 153 dB at frequencies above 20 Hz, but the rumored physiological effects did not materialize. The test subjects all reported some physical anxiety and shortness of breath, even a small amount of nausea, but this was dismissed by the hosts, noting that sound at that frequency and intensity moves air rapidly in and out of one's lungs. The show declared the brown note myth "busted".