[ Ref. 050907 MMDigest, "Player Piano Information from the Past"

Being a student of William Braid White, I was taught that "reduced
pressure" is the correct term since a vacuum is a "total absence"
of anything.

Of course, there are some instruments that use the reduced pressure
on the inside and some that use the lower pressure on the outside.
Some, like the Aeolian player pipe organ, actually pressurize the
spoolbox so the actually the lower pressure is behind the paper roll.

Others, such as the Artizan band organs, use a raised pressure so that
the air stream for any given note actually travels from behind the paper
roll and out the front.  This system, however, uses a special pressure
roller in front of the paper roll so it isn't blown away from the
tracker bar.

Hal Davis