Subject: Backwards Masking: setting things straight
From: sandell@ferret.ils.nwu.edu (Greg Sandell)
Date: 1990-09-16, 15:36
Newsgroups: alt.tv.twin-peaks

Okay, this is the second time I've seen the phrase "backwards masking"
being used here...but not once has it been used in the context of
what backwards masking really is.  Backwards masking, a phenomenon
of auditory perception has NOTHING to do with sounds being played
backwards.  Backwards masking has to do with the *temporal ordering*
of sounds.

"Masking" refers to the reduction in audibility of a signal in the presence
of another sound.  You can hear a pin drop in a church, but not when
someone coughs at the same time...even though the absolute amplitude
of the pindrop has not changed.  The cough masked the pin drop.

If the cough follows the pindrop by about five seconds, the pindrop is
still audible.  But as you move the two sounds closer together in 
time, the audibility of the pindrop is affected by the following
cough.  If the pindrop is followed by the cough just .25 seconds later,
the pindrop is no longer audible:  the cough has "backwards masked"
the pindrop.  

A reference on this:  Elliott, L.L. (1971), "Backward and Forward Masking,"
AUDIOLOGY 10, 65-76.

Greg

****************************************************************
* Greg Sandell (sandell@ils.nwu.edu)              Evanston, IL *
* Institute for the Learning Sciences, Northwestern University *
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