Rock guitarists are known to use feedback as an effect but for most of us, it’s almost always considered undesirable.
What is feedback?
Also known as Audio Feedback, Acoustic Feedback or Larsen Effect. It is a positive feedback loop that exists between audio input and audio output of a PA system.
By now, we have all experienced that the easiest way to cause feedback is to point a microphone (audio input) directly at a loudspeaker (audio output). This causes the preamp and amplifier to repeatedly amplify the sound of the loudspeaker picked by the microphone causing a ‘loop’ which we hear as feedback. Feedback occurs at one frequency at a time, but there could be multiple instances of feedback taking place at the same time at various frequencies.
A feedback loop
Factors affecting feedback
Now that we know a mic and speaker can cause feedback, let’s learn how to avoid it. There are four factors that come into play when we have feedback:
- Frequency response
Let’s talk about each in detail.
The sound pressure level drops with an increase in distance. The closer the input (microphone) and output (speaker), the higher are your chances of feedback. To maximise gain before feedback, the mic and speaker must be as far apart as possible. Keeping the large main PA far from the stage will help a long way.
The tighter the directivity of speaker and polar patter of mic, the lesser are your chances of feedback. Your chances of feedback are lowest when the speaker points to the back of the microphone. Speakers are more directive in the higher frequency regions and lose directivity in the lower end. With the help of directional subwoofer setups (viz – Cardiod), we will be able to achieve a higher gain before feedback even in the lower frequencies of audio.Use microphones with tight polar patters like Cardiod and Hyper-Cardiod instead of omni-directional or figure-of-eight to avoid feedback from stage monitors firing directly at the back of the microphone. Choose your mics wisely.
Tip: Don’t cover the mic with your hand. By covering a mic you are making it an omni-directional mic, which will increase your chances of feedback.
- Frequency Response
Almost all PA systems are not ‘true’ flat response. They have bumps of energy which combine with room modes and ringing (resonance) to trigger feedback at the slightest excuse. Now add a mic that has bumps of energy in those same frequency regions and you’ll have the perfect recipe for disaster. Start with ‘ringing out’ the system. More on this a little later.*
Gain before feedback is the maximum gain you can achieve before causing feedback on a given system. Here, the system includes the microphone, amplifier, speaker and everything else in between. Change any one component and the system changes. You could achieve more gain before feedback by replacing a microphone for one with a tighter polar pattern or by replacing the stage monitor loudspeaker with a better one. Always keep your ears and eyes open for changes you can make to achieve the maximum ‘gain before feedback’.
Ringing out the system*
This is one of the most effective ways to maximise gain before feedback. Any good live sound engineer would start by doing this first. As I mentioned earlier, feedback occurs at single frequencies. There may be multiple instances of feedback if you’re seeing more than one on your frequency RTA. These bumps where feedback happens is usually a by-product of the resonant frequencies of the ‘room’ and system.
Start with the right monitor-mic placement. As shown in the illustration below, it is best if the monitors face the back of the microphone.
Point monitors at the back of a microphone to minimise chances of feedback
1. Open a mic on stage one monitor at a time and slowly gain up till you begin to hear feedback, identify that frequency by ear (this takes some practice), or you could use a portable RTA like Phonic PAA3 or buy AudioTools by StudioSixDigital for iOS. RTA apps on android phones are not accurate but work fairly well only in quiet environments.
2. Reduce the identified ‘feedback frequency’ or the frequency closest to it by half a dB at a time on the graphic EQ for that monitor.
3. Repeat this process for all the monitors. Beware of cutting out too much or too many frequencies while you reach the limit of your system’s gain before feedback. By cutting out too much, you are essentially reducing the output level of the system.
Two instances of feedback occurs in this at around 45Hz and 250Hz. On a 31 band graphic EQ I would cut 40Hz and 250Hz as necessary