The brown note, also sometimes called the brown frequency or brown noise, is a hypothetical infrasonic frequency capable of causing fecal incontinence by creating acoustic resonance in the human bowel. Considered an urban myth, the name is a metonym for the common color of human faeces. Attempts to demonstrate the existence of a "brown note" using sound waves transmitted through the air have failed.
Frequencies supposedly involved are between 5 and 9 Hz, which are below the lower frequency limit of human hearing. High-power sound waves below 20 Hz are felt in the body.