The brown note is a supposed infrasonic frequency, usually placed somewhere between 5 and 9 Hz and so below the range of human hearing, that would make anyone exposed to it lose control of their bowels. It is a modern myth with no scientific support. Its origins are uncertain: one account traces it to a 1974 spoof article about a giant horn, another to vibration tests in the US space programme, which produced nausea and disorientation but nothing of the kind the legend describes.
When the television programme MythBusters tested the idea in 2005, using an array of powerful subwoofers, the effect did not occur; participants felt at most mild anxiety, breathlessness or slight nausea. Air is a poor carrier of very low vibration into the body, and a 2007 review in Military Medicine found no evidence that infrasound causes diarrhoea. For audiences of drone and doom metal, used to feeling bass physically at loud concerts, the brown note is a useful case of how the bodily power of sound tends to be exaggerated into folklore.