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Question from a Audio Masterclass visitor...
"How can I eliminate feedback from an acoustic guitar on a live show? sometimes I use a Shure SM57 to mic congas because my band has an acoustic African set up and it also sometimes give me feedback. What shall I do to get rid of the feedback?"
OK, this is two questions for the price of one. Firstly, the acoustic guitar...
Feedback happens when the sound from the loudspeakers gets back to the source microphone or instrument and is re-amplified, returning again at an even higher level. The familiar oscillation of feedback (howlround) is heard.
Feedback is a problem with microphones, acoustic guitars with pickups (which I assume that you are talking about), and also record decks.
Concentrating on the acoustic guitar, the first part of the solution is to make sure that the loudspeakers are pointing away from the guitar. If you do this, there is less sound to come back and be re-amplified.
Secondly, the guitar can be made more feedback resistant.
The body of the guitar is designed to resonate and transfer the energy of the strings to the air efficiently.
Unfortunately it will work very efficiently in the other direction too. It will take sound energy out of the air and transfer it to the strings.
So feedback is a likely occurrence.
The solution therefore is to make the body of the guitar less efficient as a resonator.
One solution is to block up the sound hole. Proprietary sound hole blockers are available, but you could easily improvise your own.
This should be enough. If it isn't, then the guitar can be stuffed with damping material. I would recommend plastic foam for this. It's easily available, cheap and works well.
Now, the congas...
Damping won't work because you are using a microphone to pick up the acoustic sound of the congas.
The trick here is once again to point the loudspeakers away from the congas.
But also point the microphone away from the loudspeakers. Rearrange your set up so that this is possible.
Also, place the mic as close to the drums as you can. This might mean some compromise in sound quality, but it will give greater resistance against feedback.