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Old-school method like to put lots of paper into body of guitar is not recommended in here =))

Anyway, I saw on youtube that one guy having delay+compressor+overdrive+reverb played with epiphone DOT, and his amp was not so far from his guitar infact.. how it's possible? When I turning on my distorsion and especially reverb, speakers goes mad about feedback that squicky sound (don't know how it called in english).

How can I control this kind of stuff and what the theory of this "effect"? My semi-acoustic guitar is twice fatter then epi-dot.

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The "Theory" is called resonance. I'm sure you've seen this video before: Tacoma Bridge

Every object have frequencies which they vibrate easiest at. You can think of it as if you shook it at a certain frequency it will be easier than at other frequencies.

An acoustic guitar is designed to vibrate easily at many frequencies(a whole range) i.e., so you can get a louder sound. This is basically resonance at work. When the strings vibrate on an acoustic guitar they move the are around them which goes into the guitar hole and change the pressure in the guitar. The strings are also attached to the neck and bridge which causes the guitar to "flex" which causes the air around the guitar to move too.

Now a guitar has some frequencies that it is better at amplifying than others. When a loud sound is played with those frequencies in it near the guitar it will amplify them much more than other frequencies. Since you have transducer which takes that sound and feeds it back into the guitar(Through the amp's speakers) you have a feedback system. This is called positive feedback because it results in a net gain in amplification.

Notice how your guitar seems to feedback at a certain frequency? This is one of the frequencies that the guitar resonates at. If you play a sound without this frequency in it or much lower than others it will resonant but not as much. Why? Because generally resonances works on an exponential scale(or better a logistics scale). The feedback cycle can grow very quickly then reaches a point where just can't go any higher(the high cannot produce any louder sound).

So, how can one control such feedback? One is to find the resonant peaks of the guitar and EQ them down. This may not be preferable because of the tonal repercussions.

Another way is to change the position of the guitar relative to the speakers. The guitar is 3 dimensional and it's resonating frequencies are actually modes that exist in 3D space. They are not symmetric. Also the frequency may be dampens by the human body or guitar body along it's path to the guitar's cavity.

Changing the amount of acoustic resonance can shift the resonant frequencies or dampen them enough to reduce the feedback. You can do this by adding/removing weight to the guitar or adding material into the cavity of the guitar that alters the shape of the cavity.

The problem is, is that acoustic guitars are designed to amplify pressure waves. Your amp is creating pressure waves and your pickup is picking them both up. The acoustic guitar cavity is basically taking the sound from your amplifier and adding it into the pickup's. This creates a vicious cycle. If you were to draw the way a sound would move through the signal chain you would see that some of it would go into a circle.

Sound out of guitar -> into Amp -> out of Speakers -> (some) Into Guitar's Acoustic Cavity -> Picked up by transducers -> Becomes part of the sound out of the guitar -> Sound out of the guitar

Each time you go through the cycle though you get more and more sound out of the amp since at first you start with no sound from the amp and each time the amp's output gets louder.

Even solid body's can have a lot of feedback. Low amp volume is the only way to ultimately control it(there is a point where no matter what you do you'll get feedback and it will always be positive).

Note though, Amp's can be designed for low feedback. If your amp is a high gain amp then it will have a tendency to feedback more than a low gain one. They can't completely eliminate the problem but an Amp's circuitry can also experience positive feedback and resonate. This can easily compound the problem of acoustical feedback.

No one will be able to offer you a specific solution because it will depend on way to many factors. You are simply going to have to sit down and figure out how to reduce the feedback. For all you know the guy was running his amp very quite and just enough to mic it? Or maybe he did use paper in his guitar? Or maybe his guitar was just not very resonant? The Guitar, Amp, Room, Effects, Body shape, Cloths you were, etc all have an effect on it.

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+1 Fantastic answer! Well done mate. –  Ali Maxwell Feb 20 '11 at 21:07
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From my on the road experience, the best way to control feedback is to reduce stage volume. I understand this is not easy to do on a crowded stage (if you play small bars and whatnot), and especially if your bassist enjoys turning his amp to thunderous volume levels (been there) but from my limited experience, there's a couple of things you can do. AbstractDissonance nailed the reason why you get feedback, and here's a couple of strategies that I have used to reduce it:

In Ear Monitors. These things are expensive, but will help you reduce stage volume. Wedges tend to generate muddy mixes anyway--unless you have a really nice set.

Turn all amplifiers away from stage front, or isolate them in a different room. This is just good common sense. If you are using your amplifier as a monitor then you likely aren't hearing the rest of the band enough. That's a mistake. Never live in "guitar world" when playing with a full band. Also most amplifiers have a tendency to "beam" certain frequencies at different positions on the speaker cone which is a natural effect from the physical properties of the speakers. Having a blasting amplifier pointed straight at your audience is a sure way to deafen some folks, and will likely annoy your sound engineer (they're a controlling lot when it comes to volume).

Turn it down. Even if your amp goes to 11, that doesn't mean you have to play with it there all the time. Although most tube based amplifiers sound good at higher volume levels (there's some physical properties at work there) you can definitely find a happy medium if you must use your amp as a monitor, else isolate the amp and turn it as loud as you want.

All of these strategies basically function to lower your stage volume. These won't work in 100% of cases, but they're a good starting point.

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Feedback (technically 'positive audio feedback') is the noise you hear when an input (guitar pickup/mic) sends a signal to an output (an amp) and then picks up the amplified sound again, and sends that down to the output and outputs that. This loop continues, creating that sound. Feedback increases in volume over a short period of time because of the different levels of re-ampfliciation happening as the signal travels around the loop. There are more technical reasons to do with why feedback occurs, but I do not have enough knowledge to be able to explain them :(

How much feedback occurs can depend on a number of factors. It can differ depending on the frequencies that the mic/pickup picks up, the frequencies the amp is set at, how much gain is added, room resonance and direction of the input device in relation to the output.

The guy you mention may have turned the guitar pickup volume down, and the amp volume up to compensate for the loss in volume. By doing this, the pickups would not have been 'inputting' as much signal as if they were at full volume, hence it does not pick up the amp sound as much.

To reduce it yourself, you could try the trick I outlined above (it seems to work for me), as well as not having your guitar face your amp when you play. Feedback also depends on room frequencies, so experiment with the EQ on your amp and find works best to reduce feedback both on your guitar, and in the environment in which you play. In my experience I have found that generally boosting the mids causes a greater feedback effect.

Hope this helps.

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