How to Prevent Feedback in a PA System

The article below is reproduced from my website, you can find it here:- www.soundtruth.co.uk/soundbasics.shtml

We get asked this all the time, and we're afraid that there is no very short answer (other than “turn it down!”). However, there are steps that can be taken to greatly reduce the risk of feedback. Reduce the risk almost to zero in some circumstances.

• Reduce on stage volumes where possible - then you can reduce monitor gain and hence reduce the possibility of feedback. Prime culprits here are on stage bass guitar amps, and drums... drums being more of a challange to 'turn down'!
• Encourage vocalist to sing really close up to the mic. Not all singers are used to, or like this technique, but from a monitor engineers point of view it works best! A good singer can still pull the mic away a bit for louder syables, words or sections. This tip is especially hand for vocalists that are asking for more in the monitors than you can give them without feedback.
• Eliminate simple causes of feedback – for example a guitar or bass left plugged in, turned up and on a stand. Inexperienced musicians are prone to doing this and need to be educated!
• Make sure your FOH speakers are positioned not to bleed into the microphones.
• Be aware of the pickup pattern of your microphones, and use this to your advantage. Most mics are cardiod, and so are pretty directional. Make sure they are not pointing at monitors or front of house speakers.
• Don’t put unnecessary mics such as the drum overhead into the monitor system.
• Don't use more mics on stage than nessasary. Every time you double the number of mics on stage, you reduce the gain before feedback by 3dB (the system will start to feed back at a perceieved volume about 30% lower than before).
• Use good quality microphones. If you’ve ever used a really cheap mic, you will know how prone they are to feedback and handling noise. Having said this, many people won't find much difference between an £80 microphone and a £350 microphone. Just avoid the cheapo £20 mics and you should be OK.

Shure SM58 Mic, a good industry standard vocal microphone

• Use good quality monitor speakers. Many venues try to save money and buy cheap monitors for the band – this is a shortcut to feedback problems, and a poor stage sound.
• Use good quality FOH speakers. Poor quality FOH speakers can also give feedback problems due to comb filtering and un-even frequency response.
• Hearing loop systems can cause feedback, especially into electric guitar pickups. Solve the problem by removing the guitar from the loop mix, or reducing the loop mix level.
• Follow a standard sound-check procedure (for an example, read running a soundcheck) and make sure to do the FACE / FLAT HAND / CUPPED HAND test, with all the mics ON, both in the monitors and FOH.
• Listen to the feedback and identify the frequency. This can be done with skill – or when an audience is not there, you can ‘sweep’ the mid band EQ and ‘find’ the offending frequency. Cut the frequency by 3dB and the feedback problem will reduce. More cut can be used in some cases, but will of course start to influence the sound of the mic. Low frequency feedback (for example from a tie clip mic), can be reduced by cutting the bass. LF feedback from a kick drum or toms is normally countered using a gate.
• Use a graphic EQ on either the FOH or relevant monitor mix to cut the offending frequency. Some engineers ‘ring out’ a monitor or FOH rig prior to a show. This can result in up to 6dB more gain before feedback. However, the monitors can end up sounding rather dull, as one normally ends up with quite a bit of upper mid-range cut on the EQ (The bands on a normal EQ are not really narrow enough, and end up cutting more frequencies than are required).
• Use a feedback destroyer. In some ways a last resort… the cheaper f/b destroyers such as Behringer can have problems with noise levels. Sabine make better but more expensive models. They use very fine notch filters to cut feedback frequencies and need the system to be ‘rung out’ prior to use in order to work at their best. They should give about another 6dB gain before feedback, without the side effects of graphic EQ or channel EQ.