John Hutchison, who posed the original question about building a
singing bird mechanism in the 101122 MMD, wrote me offline to ask:

    "When you say pneumatic cloth should not be used, are you meaning
    that black neoprene-coated cloth that player pianos would use?  And
    if so, is that because it is not strong enough?"

The rubberized cloth is too thick.  Vacuum systems work with tens of
inches of negative pressure, usually twenty inches or more.  This means
that the rubber cloth does not need to seal as tight.  Pressure systems
work on single inches of positive pressure with high flow rates.
Something like a bird whistle may not produce even an inch of wind.  It
does however need quite a few cubic inches per minute.

While leather is porous, it remains flexible and can fit a better seal.
Zephyr and the thin leathers used in theater organs are as thin as
tissue paper.  These can also take a lot of flexing.  The tanning
process of zephyr skin or the red "Morton" leathers works to make the
leather air tight.  I think it was an older posting to the MMD which
indicated that zephyr was used to make the gas bags of the lighter-
than-air ships.  This would indicate that the membrane will not pass
hydrogen easily.  Zephyr is made from the stomach or intestine of cows,
sheep, or goats.

At the moment quality zephyr skin is hard to get.  Old skins are more
like parchment.  For recent work I had to use the red "Morton" leather
which is available from theater organ suppliers.  In this case, the
thinner the better.

Julie Porter