Preventing Acoustic Feedback On Stage

Whether you're a performer or an engineer, these simple feedback-reducing tips should ensure your gigs remain free from squeaks and howl-rounds!

Anyone who's ever played or engineered a live music performance being amplified through a PA will be familiar with the sound of feedback — that nasty squealing or howling sound that gets louder and louder until either someone turns something down or something breaks! This unpleasant phenomenon is not only annoying for members of the audience, it can also be extremely distracting for the musicians and, if left unchecked, could easily ruin a performance.

In general, it's the job of the engineer to minimise the risk of technical problems, leaving the musicians to concentrate on what they do best, which is playing music! That said, however, a little knowledge on the part of the performers about what causes feedback, and the steps that can be taken to minimise it, will help the engineer enormously — not to mention the performers themselves.

Theory Lesson

Wherever possible, the front-of-house speakers should be placed in front of the stage, so as not to direct any sound back into the microphones.

The theory goes that if too much of the sound from the PA speakers leaks back into a microphone, it will circulate around the system, growing louder all the time and quickly building up into a continuous whine or whistle. With sufficiently high gain, where the overall loop gain is greater than unity (ie. the sound going into the mic from the PA is loud enough to loop back into the PA), an oscillation will occur, just as in the oscillator circuit of a synthesizer. This phenomenon is often called a 'howl-round'. Where the gain is high, but less than unity, the system can sometimes appear to 'ring', when, for example, someone speaks into one of the microphones. The more of the sound from the speakers that is able to find its way back into the microphone(s), either directly or via reflections from walls and ceilings, the more likely it is that feedback will become an issue.

It's very important to note that the onset of feedback is linked to system gain, not to the absolute volume level at which you're running your microphones. So a very loud singer working close to a mic may create no feedback problems, because the mic gain can be kept fairly low, while a singer with a quieter voice, or one who may not be quite as close to the mic, will be more likely to give rise to feedback. The quieter singer needs more mic-amp gain to bring up their level, and as the gain goes up, so does the risk of feedback.

Speaker's Corner

If you're using a cardioid mic (left), the monitor should be placed directly behind the microphone, as that is where the mic is least sensitive. For hypercardioid mics (right), that point is normally 45 degrees off the back of the mic, so you're better off keeping the mic horizontal and moving the monitor around to the side.

That's the theory covered, so now let's turn to the practical measures that can be taken to keep feedback problems to a minimum.

From what we've already explained, it should be pretty obvious that if your main PA speakers are behind the microphones, a lot more signal will find its way back into the mics than if the microphones are well behind the speakers. There are speakers, such as the columns made by Bose, that are designed to spread the sound over a very wide angle, and these can often be used behind the microphones if the amount of volume needed is modest, but the laws of physics still apply, so you'll be able to achieve more level if they're placed in front of the mics.

The quality of the speakers also makes a difference. Most PA speaker systems don't have a perfectly flat frequency response, and the off-axis response is invariably worse than the on-axis response. The frequencies most likely to cause feedback problems are the ones that are emphasised the most in the area where the microphones are placed, relative to the PA. So the more even a speaker's off-axis frequency response is, the better. But the quality of loudspeaker isn't the only important factor: any EQ boosts in the system, either from a graphic EQ or via channel EQ on a mixer, will essentially increase the gain at whatever frequencies are boosted — which, again, equals a greater risk of feedback.

Angle Delight

It's not just the main PA that can cause feedback, either. Monitor speakers are particularly problematic, as they tend to be even closer to the microphones and backline, but correct positioning can significantly improve the situation. Cardioid mics, such as the ubiquitous Shure SM58, are least sensitive to sounds arriving from the rear of the microphone, so these should be aimed directly away from the monitor. Some live vocal mics have a hypercardioid polar pattern, however, and these usually have their dead zone somewhere between 30 and 45 degrees off the rear axis, so the relative angle between them and any nearby monitors should be adjusted accordingly. Check your mic's manual or spec sheet for a polar diagram, see at what angle the best rejection occurs, and set the mic up with that area aimed at the monitor. Some people even go to the trouble of making a cardboard template cut at the correct angle, so they can align their mics with reasonable accuracy.

Mic Technique

Vocal mics should be held along the body, as above, and not in such a way that the singer's hand cups the basket (below). Doing this blocks the mic's internal tuning ports, making the mic more omnidirectional and increasing the risk of feedback.

That's all very well if the mic is placed on a stand and isn't going to be moved around, of course, but where the singer intends to hand-hold the mic and walk about on stage, they need to be aware of which part of the mic they're allowed to point at the monitors! In general, though, when the singer isn't actually singing into the mic, they should be holding the mic more or less upright, and not letting it point straight down towards the monitors, as this is almost certain to cause feedback if the monitors are playing at any kind of volume.

The chance of feedback can further be reduced if the singer holds the mic as close to their mouth as possible. This way, the amount of singing being picked up by the mic will be very high relative to the amount of extraneous sounds there are on stage (from drums, backline and monitors, for example), which means that the engineer will be able to keep the mic gain lower. They should avoid the temptation, however, to hold the mic right up by the basket. Most vocal mics have tuning ports inside the basket to make them directional, and if these are blocked by someone holding the mic too high up, this can change the directionality of the mic, essentially making it more omnidirectional and thus making feedback more likely.

On Reflection

Direct sound from the speakers isn't the only hazard, however. There's reflected sound to worry about, too, so if the stage has a hard rear wall rather than heavy curtains, especially if it is close to the mic, your feedback worries have just got worse. A low, solid ceiling over the stage area also reflects sound. If you can bring some heavy drapes to hang behind you at venues that are known to be problematic, you may be able to improve the situation.

Hard surfaces, such as walls behind the stage, will tend to reflect sound from the monitors back into the mics. Hanging some absorbent material, such as a thick curtain, behind the stage should help.Going back to your choice of speakers (both mains and monitors) for the moment, having a flat frequency response and good off-axis performance gives you the best chance of using higher gains without provoking feedback, but the design of your mains can also have a profound effect. For example, most simple 'one woofer plus horn' boxes radiate roughly the same amount of sound vertically as they do horizontally and all speakers, regardless of design, tend to widen their coverage pattern at lower frequencies, becoming almost omnidirectional at bass frequencies. Column speakers and small line-arrays differ from 'single woofer and horn speakers', in that the way the sound combines from the various drivers reduces the vertical angle of coverage, while widening the horizontal coverage. There are other benefits, too, such as better coverage towards the rear of the room, but in the feedback stakes, the fact that the sound is spread over a wider area means that reflections from surfaces that steer sound back into the microphones are likely to be less problematic. There will also be less reflection from the ceiling to worry about. Where the system has a separate sub, try to keep that as far away from the mics as possible, to prevent rumbling low-frequency feedback; switching in the low-cut filters on the vocal mic inputs will help with this too.

This doesn't mean to say that you can't achieve good results with more conventional boxes, but you have to position them carefully. A useful strategy is to place the speakers on stands above the heads of those standing near the front (which prevents the people from soaking up all the sound and also avoids deafening them!) and then angle the speakers slightly inwards and downwards so that they aim at around two-thirds of the way into the audience. Some speakers have angled tilt mounts built in, and if yours don't, it may be worth investing in stands that have tiltable heads.

Ringing Out

Assuming, then, that you have the best gear you can afford, and that it's set up in the best place in the room, what else can you do? The usual setup procedure for a PA system involves a process called 'ringing out' the room. This means turning up the gain on each mic until feedback just starts, and then backing it off by a few dBs. Once this has been done for all the mics individually, you do it again with them all turned up and, if you hear ringing or feedback, reduce the master level slightly.

My quick and dirty way of ringing out for small gigs is to set all the mic channel faders to 0dB, then turn up each mic trim one by one, until feedback starts, after which I back it off until the feedback just stops. I do this for each mic in turn, check again with all the mics turned up, then pull the master mix fader down by 5dB. This not only checks for feedback problems but also leaves all the faders in more or less the same position, with room to go both up and down in level.

The situation will probably get better when the audience starts filing in (bodies are very good at soaking up sound which might otherwise be reflected back!), but it's always worth leaving at least 5dB of fader headroom between your operating level and the point where feedback starts, so that you can turn things up, if needed, when the musicians start playing.

Graphic Detail

Where feedback is particularly troublesome at one or two specific frequencies, a third-octave graphic equaliser may be used to reduce the gain by a few dB at those frequencies. The process for setting these up is much as described earlier: turn up the master level until feedback starts, then try to identify the frequency at which it is taking place and turn that frequency down on the EQ. Then turn the master fader up again until a different frequency starts howling, and notch that one down too!

Graphic EQs can be useful for dealing with feedback at specific frequencies. Just turn up the main speakers until the feedback starts, identify the feedback's frequency, and turn that slider down. Repeat until the most troublesome frequencies have been identified and pulled back.Be aware, though, that the overall tonality of the system will be affected as the graphic EQ bands are far wider than the feedback spots they're trying to notch out, so use as little cutting as you can get away with. Unless you're dealing with a very specific problem, you shouldn't be cutting more than a handful of decibels from any band, and boosting is generally considered pretty unwise if you're suffering from any feedback problems!

Graphic EQs may also be used in the monitor feeds, which can be 'rung out' like the main speakers. Do these first in isolation, to check that everything is coming through OK, then check them again with the front-of-house system turned up, and fine-tune as necessary.

Automatic For The People

Feedback suppressors, such as this popular Behringer model, work by automatically detecting feedback and then re-tuning their filters to cut the relevant frequency. They're not a substitute for proper engineering, but they can obtain you a few extra decibels of level before feedback starts to set in.In recent years, there's been a lot of interest in automatic anti-feedback devices. These use filters, just as a graphic EQ does, but they're able to identify the precise feedback frequencies and then retune their own filters to exactly match those frequencies. The filters are very much narrower than those in a third-octave graphic equaliser — usually just a fraction of a semitone wide — so their effect on the overall tonality is significantly less.

The usual way of using these devices is to turn up the system gain slowly until feedback occurs, then to wait for the filter to lock on and notch it out. Then the gain can be turned up a little more until a new feedback frequency starts up, and again it will be notched out. If you do this for the first four or five main feedback frequencies, you can then set any remaining filters to 'roam', so that they can pounce on any feedback that occurs during the performance but that isn't covered by one of the fixed bands — for example, if a singer accidentally points their mic at a monitor. Such systems can claw back a few more decibels of usable headroom and also eliminate the ringing that occurs near feedback, but they're not a complete cure. They'll make life easier and give you a few more dB to play with, but that's about it.

While stereo feedback killers can be placed across the entire mix (and maybe others across the monitor mixes), there are also personal vocal pedals, such as those made by TC, that include feedback suppression, although, of course, these only provide benefits to the individual performers using them. They will, however, be effective on both the main and monitor speakers, as the filtering occurs at source.

So there you have it! While there's no such thing as a single, definitive cure for acoustic feedback, you'll find your situation much improved if you take the time to understand how and why feedback occurs, and then take these simple steps to minimise it.