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Minor thirds and background noise

In general, environmental sounds have a 1/f spectrum, which was the subject
of much discussion on this list a couple of years ago.  However, the
spectral shape of an individual instance of masking background can vary
widely, from speech-shaped noise to broadband noise or, theoretically, even
harmonic (say, if you were sitting in the middle of a horn section).

At 12:46 AM 1/29/2005, Christian Kaernbach wrote:
Dear List,

In a very speculative way one could assume that one reason for
minor-third calling is to avoid possible masking by unexpected
environmental sounds. Assuming that many environmental sounds have a
spectrum varying strongly as a function of frequency (that is with the
amplitude in one frequency band not correlated strongly to the amplitude
in other frequency bands) it would be advisable to code your message
such that it involves two different frequency bands so that if one of
these two bands is masked only half of the message is lost. That would
be better than a 50% chance that the entire message is lost because
speech is redundant, and because the recipient might solicit a resend of
the message.