Eliminating feedback, Getting rid of feedback

One of the biggest problems young bands have when starting out is feedback. During rehearsals or whilst playing their first few gigs in pubs or small venues the backline amps are ear splittingly loud but no one can hear the vocals. The engineer is desperately trying to get more but he can’t because the pa is on the verge of feeding back.

This article attempts to explain in simple terms what feedback is, what causes it and how to get rid of it for good.

Feedback is normally heard during a live performance of music or spoken word. It is usually a high pitched squeak which rings out at tremendous volume causing members of the audience to clutch their ears in pain and flee from the venue if it doesn’t stop. 

Feedback is caused by the sound from the PA system over-spilling or feeding back through into microphones, instrument pick-ups or record deck needles and then being amplified again and again through the PA system, looping round and round becoming increasingly loud. At worst it can break loudspeakers and can ruin a live performance if it happens too often.

There are a number of ways to avoid it or eliminate it completely.

Speaker Placement:

If you point a mic at a speaker it’s likely to cause feedback. Make sure that the front of house speakers are at least a few feet in front of any on stage mics. Don’t ever walk in front of the house speakers whilst holding the mic. Face the speakers directly away from the band towards the audience. Obviously monitor speakers have to be pointing back at the band but we will cover this later. It’s a bad idea to place speaker stacks on the stage. You can get low frequency feedback from sound travelling through the floor of the stage from speakers and looping back through drum mics etc. Put the stacks on the floor in front of the stage so that they are not touching the stage at all.

Microphone Choice:

There are generally two types of microphone: dynamic and condenser.

Condenser mics pick up any ambient sound in a room. They don’t need to be near a sound source to pick it up. This makes condensers notoriously bad when it comes to feedback – if they are set with even a modest amount of gain on the desk they will feedback. In live sound applications these types of mics are most commonly used as overhead mics for the drum kit. They need very little gain and are usually just there to reinforce the sound of crashes and ride cymbals etc.

Dynamic mics have a limited area around them that is sensitive to sound. This area is often referred to as the cardioid pattern because of its heart like shape. These mics are ideally suited for vocalists where the singer will put the mic on or next to their lips. Dynamic mics do not pick up other on stage sound so the performance happening through the mic can be isolated in the mixing desk enabling a brighter, cleaner mix. The other advantage of dynamic mics is that they have high gain before feedback characteristics. In other words you can boost the gain on the vocals without causing feedback. If you go too far then feedback will happen but not as much as with a condenser mic.

Mixer Settings

When sound checking mics as a general rule don’t have the gain on the channel at any more than 11 o’clock. If you are not getting enough vocal in the mix and the output volume (Channel Fader) is set at full then either your vocalist is not projecting very well; the mic is too far from their mouth or the pa system is just not keeping up with the backline amps. Try turning the amps down or get a bigger pa.

The Room.

Some rooms are particularly bad for feedback. One club I know has a floor to ceiling mirror at the back of the stage! Sound travels in waves like light so sound coming out of the monitors bounces off the mirror and straight back into the vocal mics causing major feedback problems.  Even if it’s a shiny wall at the back of the stage this can make feedback worse. The solution is to paint it black or drape material to dampen the acoustic reflections.         

Monitors

Monitors go against all the common sense ideas about reducing feedback. They are pointing straight back at the band and generally get pushed harder and harder throughout the gig at the request of the lead singer who insist they can’t hear themselves. If you have followed the other rules and you still have feedback then 9 times out if 10 it’s the monitors.

EQ

Luckily there is an effective solution but it does involve buying (ideally 31 band) graphic EQs for every channel of monitor you will be using and for the left and right of the FOH (Front Of House) mix. A standard gig might have 4 monitor mixes and FOH so you would need 6 channels of EQ. They normally come in pairs. Some people think graphic EQs are simply for tweaking the overall sound of the PA system to suit a room, which is one use, but in fact their main application is in eliminating feedback.

When you get a feedback squeak it’s always at a particular frequency or at a number of different frequencies. This is generally governed by the acoustic properties of the room so it is different depending on which venue you are in. A graphic EQ splits the signal into 31 frequency bands from 20 Hz up to 20 kHz. If you know which frequency the feedback is occurring at then by notching that frequency down on the EQ you can stop it in its tracks. Experienced engineers can hear what frequency feedback is occurring at from having done so many gigs. Beginners can buy graphic EQs with a feedback detection system. This shows you what frequency feedback is occurring at with a little LED signal above the frequency range in question on the EQ.

Ringing out

Obviously it’s best if you can sort out any potential feedback issues before the audience arrive at the gig…this is how it’s done:

Set up the stage with all the mics you will be using and do a quick sound check and get rough levels for monitors.  Now ask the rest of the band to go outside and get some fresh air for 5 minutes while you ‘ring out’ the room.

Start with monitor mix one. Turn down the master faders for the other auxiliary sends.  If you already have feedback in mix one then eliminate the troublesome frequency or frequencies on the corresponding EQ channel. Note: don’t take out too much of any particular frequency or the monitor will start to sound rubbish – just notch it down enough so that the feedback stops. Now gradually turn up the master fader for Auxiliary 1. If you get more feedback on other frequencies then notch these out. You may need to adjust for the initial problem frequency more if it has reared its head again. Turn the auxilary master up a touch more and repeat the process. You don’t need to turn the auxiliary master right up because you will never be doing that during the gig. You just want to make sure you have room to manoeuvre in case the lead vocalist is wanting more and more in his monitor.

Now turn the auxiliary master back down to where you had it during sound check. Repeat this process for all other monitor mixes and for front of house.  

Now you have rung out the room. There should be much less chance of any feedback during the gig and you will be able to get far more out of the monitors and FOH.

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