Prevent Microphone Feedback
Microphone feedback can be damaging to your sound system and painful to hear. It is produced when the signal from the mic is amplified and picked up again from the speaker, creating a continuous loop. The signal then continues to amplify at an extremely fast rate until it causes an unpleasant sound. Here are a few steps you can take to prevent feedback from occurring.
Steps
- Place the microphone behind the main speakers and in front of the monitors. If the monitors are too far off to the side of the mic, there can be feedback due to the pickup pattern of live vocal mics. It's best to place the monitor directly behind the mic.
- Do not cup the microphone. Many singers have the tendency to place their hand around the microphone when performing which can cause unpleasant, high-pitched feedback. Keep your hands at the back of the microphone. If you are walking around on stage, be careful not to go past the main speakers or point the microphone at the monitors at all.
- Utilize a feedback eliminator. There are rack-mountable units that can be hooked up in the mains or monitor chain. Feedback eliminators have the ability to detect when feedback is about to increase and cut the exact frequency, thus eliminating the feedback.
- This can be very helpful if your act involves switching vocalists and moving the microphone around on stage.
- Use a graphic equalizer. A graphic equalizer allows a sound engineer to effectively prevent feedback from occurring during the performance -- a process known as "ringing out" the microphones. This is done before the show, during soundcheck.
- During sound check, the vocalist sings through the microphone while the engineer increases the level slowly until feedback begins. Once it starts, the engineer will find the correct band on the graphic equalizer and try to lower the gain.
- This process should be done with every microphone during soundcheck. On a high quality systems, there are usually 2 31-band graphic equalizers, 1 for the main mix and 1 for the monitor mix.
- Utilize the parametric equalizer on the channel strip. Most high-end mixing boards have a mid-sweep parametric equalizer that can help if you need to pull out a specific frequency.
- The bandwidth of a parametric equalizer is often much smaller than the bandwidth on a graphic equalizer, and allows for precise frequency control. Thus, it allows the engineer to pull out the frequency that's feeding back without compromising tone.
- Treat the acoustics of the room. This is only applicable if the venue belongs to you. Treating the acoustics can prevent excessive reverberation, which can potentially increase the chance of the mics feeding back.
- Place foam above and on the back wall behind the stage to reduce stage volume. Consequently the monitors won't have to be as loud, thus decreasing the chance for feedback to occur.
Tips
- Get to the venue a few hours early. Taking time to ring out everything, reposition speakers and place mics correctly on stage can go a long way ensuring good sound quality during your performance. With enough time, a good sound engineer can effectively troubleshoot any potential feedback issues.
