Wow, what an inspiring thread - thank you all and above all King Korg for starting it 5 years ago!
but then in my brain a small bell started tinkeling from way back when I was a trainee at Broadcasting School in Nürnberg. I checked with my old "Handbuch der Tonstudiotechnik"and indeed there had been mention of the need for the teeeniest opening even in the otherwise completely closed pressure-capsule, to compensate for ... well ... the weather.
No, honestly!
If you seal the capsule completely and the weather gets better, the increase in atmospheric pressure would result in a rise the membrane tention, right?
I attach the concerning paragraph as a .jpg, it translates as:
"fig. 4/12 shows in sectional view the basic structure of the capsule of a pressure receiver. The interior of the capsule is separated soundproof from the surrounding by the mernbrane; in the capsule, due to the pressure compensation through a small opening, the external air pressure prevails, but the rapid pressure fluctuations of the sound are not compensated."
So maybe k browns idea ...
could be the solution to that problem.
best wishes from Bremen!
Wulf