On Thu, 17 Jan 2013 23:20:48 -0600, Ignoramus24985 wrote:

I started using a compressed air dryer. This dryer, as part of the
operation, opens a valve every minute or so for a couple of seconds.
....
This valve opening produces a very annoying noisy blast of air. To
combat this, I routed the polyethylene tubing to the outside of the
building, by drilling a hole in the masonry.

This "resolved" the problem by moving the blast from the inside to the
outside. The outside is the back of the building and faces the
railroad. So it is not bad. But, I would like those blasts to be
quieter on the outside also. What can I do, realistically, to silence
them?

Routing the blast of air through a baffle box would help. See eg
http://physics.stackexchange.com/questions/28441/can-sound-be-separated-from-air
and http://wiki.diyfaq.org.uk/index.php?title=Baffle_box for some
discussion, and also look for "sound baffle box" (without quotes)
on google images. As shown in some pictures, the surfaces in a
baffle box should have sound-absorbing stuff (eg fiberglass
insulation) on them. The box could be outside, eg a tall thin box
fastened to wall, with exit opening pointed down.

--
jiw