The brown note, according to an urban legend, is an infrasound frequency that causes humans to lose control of their bowels due to resonance. There is no scientific evidence to support the claim that a "brown note" (transmitted through sound waves in air) exists. (Wikipedia)
It's such a silly concept. There are a number of sonic weaponry in development, I suppose for "crowd control". The 7hz tone that's usually attached to the brown note is a particularly interesting frequency for other reasons.
No doubt "the brown note" is a myth, but I have a contension with MythBusters, which I haven't seen too much of (don't have a TV), is that not everything can be demonstrated in the context they set up. In some of these 'myths', there's a grey area which their psuedo-'controlled experiment' methods don't account for. Thus their myth-framing is a self-fulfilling prophecy.