You're thinking of the brown NOTE, which has not been proven to actually exist.

It goes something like this:

• White noise is a mixture of all frequencies audible to humans, which sounds quite high pitched. It's excellent for focus, against tinnitus and just for it not to be too quiet. Helps a lot of people sleep too.

• Pink noise is the same thing except tweaked to sound lower yet still "crisp". It's been shown to generally be even more effective for falling asleep than white noise.

• Brown noise is the same deal yet deeper still. It's been shown to be effective against anxiety and, at least in my own anecdotal experience, against anxiety-induced insomnia.

On top of all these benefits, a combination of all three is what headphones and earbuds use for active noise canceling, an effect you can also achieve to some degree yourself.