This brings up something interesting that i noticed with my special
Quasi Constant Wave (QCW) SSTCs. I can control the rate of growth for
the spark, and also the rate of decay. Slowing down the growth time
does make the spark sound kinda "fluffy", but giving it a sharp turn
off (removing the power from the spark fast, like 20uS) makes it quite
a bit louder. I think the answer is simply that it cools down a lot
faster, so you still get a significant dp/dt (change in pressure) due
to cooling more rapidly. I cant imagine there being any sort of
vacuum being generated. But yes, the cooling part of the cycle makes
noise too ;-). The sparks are still not nearly as loud as a
conventional tesla coil making similarly sized sparks, so im guessing
its the heating part of the cycle that makes most of the sound.