Feedback is a blessing and a curse. Sometimes it is the sweetness of sustain, while other times it is killing your performance. Let’s a take a look at what it is and how to channel it for musical nirvana.
Feedback: Understanding It
Before we can really know how to either defeat feedback or use it, let’s take a moment to understand it (for the uber geeks among us, here is a rather technical discussion of feedback in control systems). You may know it can be positive or negative.
The Plus
Positive feedback means the signal or sound is adding to an existing signal. That will increase the strength of the signal and loudness of the sound. If you take that new signal and plug it into the input again (i.e. feed it back in), the sound will grow. Keep that up and you either have your audience yelling to turn it down or jamming along with you.
The Minus
Negative feedback, on the other hand, takes away from the signal and is a darn good way of controlling an otherwise unruly sound. The gain control on an amp is a feedback control in the circuit, for example. The knob controls how much signal is fed back to the input. In the case of negative feedback, the signal polarity is reversed and fed back to the input. Feed the same signal strength to the input in the reverse polarity and the original signal is cancelled. Use smaller amounts of reversed signal and the original will be quieter.
The Context
Some amplifiers use feedback in the circuits to control the size (or amplitude) of the signals in the circuit. Vox is one manufacturer of guitar amp that used no feedback in what is called an open loop circuit. The one common element in all sound systems that provide feedback no matter what is YOU. The input is your guitar or microphone and the sound comes out the speaker only to be picked up by the guitar or mic again and run back through the amplifier. This regenerative process builds until the amp can no longer go any louder. This is always positive feedback.
Finally, there is the aspect of resonant frequency. Microphones have one element and a single frequency they will best resonate. That is the frequency that will scream when the mic is pointed at a speaker. There is enough ambient noise around to start the mic vibrating and then it is off to the races – Scream!
Guitars, on the other hand, have strings at different resonant frequencies that can be caught in a feedback loop. The volume of the amplifier is the main determinant in getting the desired string and note to vibrate and – Scream!
Defeat Feedback or Use It?
Now the question arises, what do we do with it? If your mic is feeding back, that is bad and can be an ear piercing experience for your audience. Your acoustic instruments may also need some attention to avoid unwanted sound emanating from them. Let’s start with how to quell the unwanted.
Quieting the Scream
As mentioned earlier, microphones have a single frequency they will usually resonate best on. Defeating that one frequency typically requires cutting it with a graphic equalizer or turning down the tone control for that range of frequencies. Without a graphic equalizer, it will be a bit of a compromise between sound quality and feedback elimination. Once you find the right frequency and cut it sufficiently, you can point the offending mic right at a speaker and it will be quiet as a mouse.
For acoustic guitars, the resonant frequency is usually the soundboard resonant frequency. Some acoustic preamps have feedback canceling circuitry that works quite well. Another common trick is to fill the soundhole to 1) keep the sound from entering back into the guitar body and 2) deaden the vibrating top. Anything from socks to commercially available soundhole covers will do.
Lastly, employ the same technique you would as with a microphone: notch out the offending frequency or frequencies. That can be done with a pedal graphic equalizer thereby maintaining a good sounding signal to your preferences. Again, everything is a compromise. Remember, It’s All About the Tone, Man.
Singing With Feedback
Ok, so you want to get that searing lead that sustains forever and screams in the night like that lonely howling wolf, right? Just crank your amp and let ‘er rip! Not so fast! Controlling the scream can be a bit trickier than it looks. Sometimes you want it to take your note to the next level and other times, it is a nuisance. Let’s learn to control ourselves.
Guitar feedback is a matter of volume and proximity. The louder the amp or the closer you are to the speaker, the more likely you will get feedback. As the signal gets stronger near the guitar, the strings will vibrate on their own. Once you get the strings to vibrate, you have feedback i.e. the signal from the guitar is going through the pickups to the amp, out the speaker and vibrating the string or strings to repeat the process all over. The limiting factor here is how much the string can actually vibrate.
Volume
Simply pushing your volume for your solo can give you enough volume to get the ball rolling, so to speak. Pedals are handy here but you can also use the volume control on your guitar. Strats are particularly good for this with their control right up near the strings in easy reach. Otherwise, a good volume pedal will do the trick or use a boost pedal and click it on at the right time.
Proximity
Another technique is to use proximity to your advantage. This also makes for a good show as you move around to the music accentuating notes and phrases with a bit of feedback. Simply facing your amp can get a response or moving closer may do it. This takes some practice and finesse as you find the sweet spot. Jimi Hendrix was an early master of this technique.
Concluding Feedback
Whether you are trying to defeat or sing with it, understanding feedback is an important part of a guitarists knowledge. There will be times you will need to employ one or the other: kill it or enjoy it.