Are you constantly battling to get your vocals loud enough? Does your PA system sound great one night and dissolve into howl round the next? PAUL WHITE explores the causes and cures of acoustic feedback.

Every gigging band has suffered this problem — you want more out-front vocal level or more vocals in the monitor mix, but as soon as you touch the level control, everything dissolves into howls of audio feedback. It's infuriating, isn't it? So why doesn't somebody design a system that doesn't feed back? Surely it can't be that difficult? As it happens, many people have developed ingenious systems in the hope of reducing or eliminating feedback, some with more success than others, but in order to fight feedback effectively, you need to know what causes it.

Audio feedback occurs when sound from the PA system, or from the stage monitors, finds its way back into one or more microphones. If the sound getting back to the mics via the speaker system is loud enough to produce a bigger signal than the original sound itself, then the signal will go round and round the system, rapidly building up in level until all that is audible is a loud howl or squeal. In theory then, all you have to do is to make sure as little sound gets back to the mics as possible, but this isn't always easy. Time to sidetrack to explore how microphones and loudspeakers actually behave.

In an ideal world, all microphones would have flat frequency responses as would loudspeakers, amplifiers, mixers, and so on. Rooms would also be perfect with no resonances. Back in the real world, rooms tend to 'ring' at certain frequencies, and PA components, especially mics and loudspeakers, invariably have significant bumps and dips in their frequency response characteristics. If you have a loudspeaker system with, let's say, a slight peak in its response at 1 kHz, then all signals passing through the system will be boosted slightly around this frequency, just as if you'd added boost on an EQ unit.

And wherever the highest peak is, that's where your first howl of feedback will occur when you crank up the gain. If this peak is, say 3dB higher than the average level, then feedback at this frequency will occur at a 3dB lower level than it would if the peak were not present. Similarly, if the room has a pronounced resonance at, say, 200Hz, then that's the frequency your system is most likely to start hooting at when you turn up the gain. What can be done to flatten things out?

LOUDSPEAKERS

Right back at the system planning stage, you should be looking for loudspeakers that have reasonably flat frequency responses, and this is especially true of stage monitors which are used in close proximity to vocal mics. Not only should speakers have a nominally flat response, they should also have properly controlled dispersion. Simple physics dictates that a large cone loudspeaker will produce a much narrower beam of sound at high frequencies than it will at low and mid-range frequencies, and this is what is meant by poor dispersion. To avoid this problem, PA loudspeaker systems generally comprise two or more loudspeakers, each handling the part of the audio spectrum with which they are most comfortable. A 12 or 15-inch speaker might handle the bass and mid frequencies, for example, while a small horn tweeter could handle the higher frequencies. Crossover circuitry is used to limit the range of each speaker to its own part of the audio spectrum. If properly designed, such a system can give adequately controlled directivity over the whole audio spectrum, and it can be designed to have a flat frequency response. If you do use a system that produces narrow beams of sound at certain frequencies, the chances are that you'll get a reflection from a wall or ceiling that will bounce the sound right back into the microphone — and the result is feedback.

"Never cup your hands around the basket of a mic while singing... the microphone will become less directional, making feedback more likely."

Speaker placement is also vitally important. Sound levels diminish as you move further from the source, so it stands to reason that the further away your mics are from the speakers, the more level you can use before feedback raises its ugly head. Speakers are also directional devices, so they should be aimed away from the business ends of microphones rather than at them.

I've already mentioned the dangers of reflected sound, so you should also position your speakers in such a way as to minimise unwanted reflections from venue walls or ceilings. In an ideal situation, all the front-of-house sound would be directed at your audience, and even though this isn't possible in practice, you should aim to get as close to this ideal as you can. For more about loudspeakers, check out our 'Practical PA' series beginning on page 54 of this issue.

MICROPHONE TYPES

Most vocal mics have a slight boost at around 3kHz to help with projection, but this shouldn't be enough to seriously affect your system's susceptibility to feedback. Probably more important is the microphone's directional properties — how it responds to sounds from different angles around it. Most live work is done using unidirectional mics, because they respond mainly to sound entering from the front. However, they aren't completely 'deaf' at the back or sides, and they can still pick up reflected sounds if they are pointed towards hard surfaces, such as a wall at the back of the stage.

A cardioid (meaning heart-shaped) microphone is most sensitive in front and least sensitive at the rear, and these make a good choice for stage use. However, some people prefer a hypercardioid (sometimes called supercardioid) model, because these pick up sound from an even narrower angle. Whereas the cardioid has its least sensitive spot at the rear, the hypercardioid is least sensitive at 45 degrees either side of the rear axis. Figure 1 shows both cardioid and hypercardioid patterns. Notice that the hypercardioid mic retains some sensitivity to sounds coming from the rear, but the dead spots either side of this can be used to position stage monitors, as shown in Figure 2 (overleaf). If reflected sound is a particular problem, the situation can be improved by fixing a curtain or soft drapes at the rear of the stage. It also helps to ensure that the mic in question isn't too close to the back wall of the stage area.

A QUESTION OF NUMBERS

The point at which any system feeds back isn't related directly to volume, but rather to gain — gain is simply the ratio of the original sound level to its amplified level. If a singer is naturally loud, then you need less gain to get the required level out front — and the less gain, the less likelihood there is of feedback. Someone with a weak voice will need more gain, and it's no use choosing a more sensitive mic, because increased mic sensitivity is exactly the same thing as increased gain. All you can do is try to get very close to the microphone so that the mic 'hears' a louder sound level.

"...the further away your mics are from the speakers, the more level you can use before feedback raises its ugly head."

One very important point is that you should never cup your hands around the basket of a microphone while singing. The directional characteristics of a cardioid mic are set by tiny air vents behind the capsule, and if you obscure these, the microphone will become less directional, making feedback more likely. By the same token, don't grip the microphone too close to the basket as this can have the same adverse effect.

If you use multiple open mics, the system gain is effectively higher than if you only have one open mic — which means more potential feedback trouble. A practical approach is for the mix engineer to shut down unused mics on the desk. This also prevents unwanted spill from other instruments entering the mics.

ROOM ACOUSTICS

Room acoustics have a very profound effect on the behaviour of a PA system. In some venues you can get all the level you need, whereas at others, you're continually struggling against feedback. Such problems are due to resonances caused when sound reflects back and forth between surfaces in a room, and there's little you can do about the room, other than hope your audience will be large enough to soak up some of the unwanted sound.

You may not be able to change the room, but you can control the sound you feed into that room. You can position your speakers so they're covering the audience, not spraying sound at the walls or up into the ceiling, and you can use a graphic equaliser to compensate for the room's shortcomings at least partially. For example, if the room is emphasising everything at around 200Hz, use your EQ to cut back at 200Hz to keep things under control. In effect, the room acoustics are causing an increase in gain at 200Hz, and you're cancelling this out by introducing a 200Hz dip in your equaliser curve.

"It is not advisable to boost frequencies on a graphic equaliser... the system is more likely to feed back at the boosted frequencies."

Audio professionals go to the lengths of measuring the room response using a specialised tool called a spectrum analyser, plus a calibrated microphone, and a pink noise source. (Alternatively, you could use Stephen Court & Alan Parsons' Sound Check II test CD with built-in spectrum analyser and mic, see page 92 for details. We'll be covering the use of room analysers in a future issue.) However, with a little experience, most engineers can identify the more serious room problems by ear. It is not advisable to boost frequencies on a graphic equaliser unless you are very experienced, because the system is more likely to feed back at the boosted frequencies. Just use it to cut the problem frequencies and not only will feedback be less likely, but you'll also obtain a more subjectively natural sound.

Even if you don't have a graphic equaliser on your main PA, it is worth using one on the stage monitor system — here the speakers are much closer to the mics and the propensity for feedback is high. The more bands the equaliser has, the finer degree of control you can apply, which is why most professionals use third-octave equalisers (each slider controls a section of the audio spectrum just one third of an octave wide).

OTHER OPTIONS

Various attempts have been made to reduce feedback using specialised processors, and one of the early approaches used a form of pitch-shifter which moved the whole PA signal up by just a few Hz. This wasn't enough to make the performance sound out of tune (although it did beat rather disconcertingly with the original sound!), but the idea was that whenever sound re-entered the mic from the speakers, it would be shifted up in pitch slightly to prevent the feedback from settling on a fixed frequency. This does improve the feedback threshold by a few dB, but when feedback does set in, not only does it warble like a taxiing UFO, it also comes in loud and fierce, with no warning. Because of these shortcomings, pitch-shifting systems are rarely used by today's sound engineers.

A much more effective system is a relatively recent development from Sabine. Their device comprises a box of filters which automatically lock on to and attenuate feedback. During the soundcheck, the PA level is slowly increased until feedback starts to build up, at which point the unit identifies it and blocks it with a filter. Several filters are used to counteract the main feedback frequencies, with several more roving filters poised to jump on any new feedback frequencies as they occur. This system can be thought of as a robot sound engineer and graphic equaliser, constantly on the lookout for feedback problems.

A quite different approach to keeping sound out of the mics is to use in-ear monitoring (see p60). Stage monitors are the main cause of age feedback, especially at larger venues where the main PA is some distance from the performers, so by replacing these with tiny, low powered in-ear phones, the main feedback path is eliminated.

SUMMARY

Acoustic feedback is a simple function of physics and can never be eliminated as long as there is a need to use microphones with loudspeaker systems. However, a properly designed system, carefully set up, can be significantly more resistant to feedback than a poor system that is set up without regard for the acoustics of the venue. The main requirement is for system components with reasonably flat frequency responses, loudspeakers with well-controlled dispersion, and, very importantly, the best quality stage monitor loudspeakers you can afford. Such a system, positioned to direct sound onto the audience rather than onto reflective surfaces within the room, can produce excellent results, even under quite adverse conditions. The more severe room anomalies can be compensated for by using a graphic equaliser, and for those with an adequate budget, proper room analysis equipment is recommended. Specialist feedback reduction systems such as those built by Sabine can also be very effective, but there is no substitute for the skill of the mixing engineer.

In future issues of Sound On Stage, we'll be covering those skills in more depth as well as interviewing sound professionals to see how they tackle tricky situations. As equipment becomes ever more sophisticated, there's a need for human skills to keep pace, and live sound technology is advancing at an incredible rate.