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Re: sound hole cover for loops

> When I used to play traditional jazz on an Ephiphone hollow body, I
> dropped black balloons inside the f-holes and blew them up. Worked great
> and didn't look weird.

This is brilliant!
    As for how these soundhole covers work, my take is this: Acoustic
resonant feedback occurs with acoustic guitars in two main frequencies - 
cavity resonance and the top resonance. The cavity resonance can be found 
singing a low "huuuuuu" into the soundhole. When you hit the resonant
frequency, you'll feel the body "come alive" and accentuate the note.
Dreadnoughts resonate around a low G, big jumbos ring at F, concert size
boxes ring at A, and my Ovation super-shallow rings at a prim C#. This
resonance creates the "foghorn" tone feedback, and is like blowing across
the top of a bottle (or a flute). The top resonance is hard to experience
without actual feedback (in fact I know of no way to "excite" it) but when
it feeds back, you'll hear a higher pitch, often with overtones, that is
very direction-dependant. This is the one that is easliy removed with a
"phase" switch on either the guitar's system or the preamp.
    I've found the best cure for both to be the heavy rubber soundhole plug
sold as "Feedback Buster" by Ovation/Bruno. Thinner, flexible covers can
still vibrate and allow the cavity resonance to occur. I tried another
soundhole cover with a little adjustable port once and it was too thin and
brittle, and even with the little port closed, it allowed too much air
through. (I suspect the balloon solution above works so well because it
seals the body so well and adds a large mass of dead air as well.) The "FB"
not only totally mutes the cavity resonance, it also seems to weigh down 
top, effectively retuning the top with its rubbery, non-resonant mass and
damping the top resonance. It also keeps the soundhole lips from vibrating,
which is a factor in both these feedback scenarios.
    Another whole level of discussion involves the actual acoustic tone of
your instrument, what pickup system you use, what you use to amplify it, 
how big a room you're playing to. And probably some other stuff...
Douglas Baldwin, coyote-at-large