You can do it when you're only producing real low fundamental frequencies
and can control the phase. In a pulse jet or any kind of rocket motor, it's
a ton of high-frequency broad-band noise. You might be able to control one
or maybe two frequencies (assuming they are correllated), but forget about
the rest of the junk.

"AL" wrote in message
...
Submarines use a device (the name escapes me just now) to do exactly
what you guys are talking about.