Anybody who plays by using an semi acoustic or electro acoustic guitar will be aware you will be continually walking the tightrope of unrestrained, ear piercing feedback. Not the controlled splendor that's decorated many a stage show or record, the monster making you really feel physically sick any time it invaded your own aural calm.
As somebody who has frequently encountered this I had to identify why this takes place and the things I could do regarding it. You'll find so many elements which might lead to you having feedback from a instrument, but once we're able to realise why it takes place we might have the capacity to regulate it and after that eradicate factors including the room, effects, body shape even the clothes worn as they will all have an affect on it.
So, everything has an optimum sound frequency that it will vibrate at. An electro acoustic guitar was created to easily vibrate at a whole range of different sound frequencies e.g. so you can get a louder sound. Basically this is what resonance is. The science bit is that once the strings vibrate on the acoustic guitar they move the air around them which then goes into the guitar hole and changes the air pressure in the guitar. As the strings are attached to the neck and bridge this causes the guitar to flex which again causes the air around the guitar to move.
Every guitar has frequencies that it is better at amplifying than others. When a loud sound is played with the frequencies that it likes, the guitar will amplify those much more than other frequencies. There is a transducer that takes that sound and feeds it back into the guitar through the amplifier, thus you have a feedback system. This is known as positive feedback as there is a net gain in amplification.
What happens is that your guitar will feedback at a certain frequency which the guitar resonates at. If you play a sound without this frequency in it or a sound much lower than the others it will resonate but not as much, because generally resonance works on a logistics scale where the feedback cycle grows very quickly then reaches a point where it just can't go any higher.
So the question is can you control this type of feedback? One way is to find the resonant peaks of the guitar and equalise them down which is very difficult and you would probably end up having issues with your guitar. A better way is to change the position of the guitar in relation to the amplifiers. A guitar is three dimensional and its resonating frequencies exist in 3D space. They are not symmetrical and the frequency may be dampened by the human or guitar body along its path to the guitar's cavity. By changing the amount of acoustic resonance you can change the resonant frequencies or dampen them enough to lower the feedback. You can do this by adding/removing weight on the guitar or adding material in the sound-hole of your guitar that alters the shape of the sound-hole. Have you ever seen bands where the sound-hole in the guitar is blocked up by tape?
Even solid body electric guitars can experience feedback and reduced amp level is often the only way to eventually manage it. In fact there's a stage where regardless of what you are doing you can expect to always acquire positive feedback.
Nevertheless, amplifiers can be created for reduced feedback from the electro acoustic guitar. A high gain can have an inclination to feedback greater than a low gain one.
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