Few things are more disruptive to a soothing song or spirited speech than the painful shriek of feedback. Amplifier feedback occurs when the microphone picks up its own sound projected from the speaker and loops that sound back into the system, amplifying it again and again. This cycle happens so fast, with sounds looping back and forth at an estimated 1,000 times per second, that what comes out is feedback. Depending on the frequency that’s looping, the amplifier feedback can be a high-pitched screech or a deep, booming roar.
When amplifier feedback begins, the audience flinches and typically looks to the back of the room, hoping the person in the sound booth can help. The sound engineer can adjust volumes and frequencies, and should have taken key steps ahead of time to prevent feedback. However the person holding the microphone may also play a role in causing amplifier feedback.
If the person holding the microphone steps in front of the speakers, the loop is almost certain to begin. This is why amplifier feedback is even more likely with wireless microphones, which tend to be omnidirectional and allow the performer to walk in front of the speakers. The closer the microphone gets to the speaker, the louder the feedback is likely to be. Also, if the speaker or singer holds the microphone far from their mouth, the sound engineer has to turn up the volume to accommodate, increasing the likelihood of feedback occurring.
While it's impossible to completely safeguard against amplifier feedback, there are many things that can be done to prevent it or stop it once it’s occurring. Tips include:
- Turn the volume down. This is the most immediate fix if feedback is taking place. The volume should be kept only as loud as it needs to be for the audience to hear.
- Keep the microphone behind the speakers, so that it cannot pick up its own sound. Point the speakers away from the person speaking or singing.
- If working with an equalizer, lower the frequency where the sound is feeding back.
- Turn off any microphones not in use.
- Encourage the person on the microphone to hold it close to their mouth.
- Use unidirectional microphones, which only pick sound up from a limited direction, typically within about 60 degrees. Omnimicrophones pick up sounds from all directions.
- If possible, soundproof the room by laying down carpeting and other materials that will deaden the sound bouncing back to the microphone.
- Purchase and install a feedback controller. These electronic processors monitor the sound and automatically reduce the levels if a feedback frequency is reached.