white noise

from WordNet (r) 3.0 (2006)

white noise
    n 1: a noise produced by a stimulus containing all of the
         audible frequencies of vibration; "white noise is a good
         masking agent"

from The Collaborative International Dictionary of English v.0.48

White noise \White noise\, n.
   a mixture or random noise sounds extending over the entire
   audible frequency spectrum with approximately equal intensity
   at all frequencies. It is used in certain experiments, as in
   psychology, to prevent subjects from hearing meaningful
   sounds.
   [PJC]