Does anyone know what this black, filter-like thing is that professionals put on the hold of their acoustic guitar? From what I can tell, it looks like some kind of filter that eliminates excess hum or something. Any help is appreciated. Thanks
Do you mean "hole" (instead of "hold")?
Like the thing Jerry Cantrell is using in this video?

It's designed to eliminate feedback when you're playing an electroacoustic guitar through an amp.
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It's just a soundhole cover mate. Plain and simple.
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Its the Lydian mode; formed in Eastern Arabia when the Persians invaded England.

Quote by Blind In 1 Ear
try the sexolydian scale.
i've always heard it called a feedback buster.
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I just looked in my Oxford English Dictionary and under "Acoustic Guitar", there was your Avatar and an email address!
^thats what I've always heard them called too.

It's just a chunk of rubber that fits into the hole. Plugging the hole dampens the acoustic guitars vibrations and reduces feedback.
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