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Anyone who plays with an electro acoustic or semi acoustic guitar will know that you are perpetually walking the tightrope of uncontrolled, ear piercing feedback. Not the controllable beauty that has adorned some a stage performance or recording, the beast that makes you feel physically sick when it crashed through your aural calm. As somebody who has regularly experienced this I wanted to find out why this happens and what I could do with regards to it. There are some elements that may bestow to you getting feedback from your guitar, but if we may understand why it happens we may be competent to control it and then eliminate parts such as the room, guitar effects, body shape even the clothes being worn as they will all have an effect on it. So, everything has an optimal sound frequency that it will vibrate best at. An electro acoustic guitar is designed to effortlessly vibrate at a whole range of dissimilar sound frequencies e.g. so you may get a louder sound. Basically this is what resonance is. The science bit is that when 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 piercing 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, thence 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 sure 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 resonant but not as much, because in general resonance works on a logistics scale where the feedback cycle grows very speedily until it reaches a point where it just can’t go any higher. So the question is may 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 modify the position of the guitar in relation to the amplifiers. A guitar is three dimensional and it is resonating frequencies subsist in 3D space. They are not harmonious and the frequency may be dampened by the humane body or guitar body along it is path to the guitar’s cavity. By altering the amount of acoustic resonance you may modify the resonant frequencies or dampen them sufficient to reduce the feedback. This may be done by adding/removing weight to the guitar or adding material into the cavity of the guitar that modifies the shape of the cavity. Have you seen bands where the sound hole in the guitar is blocked up by tape? It is not just the electro acoustic guitar that suffers, even a solid body electric guitar may experience feedback and a low amp volume is in truth the only way to at long last control it. In actual fact there is a point where no matter what you do you will always get positive feedback. However, amplifiers may be designed for low feedback. A high gain will have a tendency to feedback more than a low gain one. |
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