How does feedback happen and how do I avoid it?

Everyone is familiar with the unpleasant, unwanted noise of feedback: a high-pitched screeching sound come from the speakers. Feedback is always a possibility when working with amplification because it happens when a pickup or microphone “hears” the sound signal from the loudspeaker or amplifier and that signal is then bounced back between the two in what is called a “feedback loop.”

The simplest thing to do to avoid feedback is to properly shield the pickup or microphone and keep it from directly facing the sound source so that the feedback loop physically can’t happen. However, sometimes feedback can also occur when sound waves from the speaker of amplifier vibrate the part of the instrument that the pickup is attached to. In this case, feedback sounds more like a low boom than a high-pitched screech.

In general, feedback becomes more of a problem the louder you are playing. This makes sense because higher volume produces more powerful sound waves, which can reach farther and have more energy to vibrate your instrument. Certain pickup placements are more prone to feedback depending on where they are on the instrument.

Undersaddle pickups are quite feedback resistant because they are wedged under the strings next to the saddle and this area does not vibrate much. Bridge plate pickups (installed under the bridge, inside the guitar) are also quite resistant to feedback because the bridge plate is relatively thick and does not vibrate as much as other parts of the guitar. Sound board pickups, on the other hand, are more sensitive to feedback because the 1/8" (or less) thin sound board can act like a diaphragm and vibrate along with loud sound signals from the speakers. (Read more about the difference between undersaddle and bridge plate pickups.)

We’ve done tests with our FanTaStick undersaddle pickup and the Pure Pickup on the bridge plate, and were able to achieve about the same gain-before-feedback with both systems. But of course, undersaddle pickups with their very “direct” and string driven tone cut better through a loud mix.

Dieter's Sound Bite

I have feedback problems, phase issues etc.: This podcast describes why acoustic instruments may experience feedback and how to avoid it. I also talk about phase issues and what characteristics you can expect from the Pure Pickup. Right-click to download: Feedback and Phase Issues Podcast

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