Last month, we looked at the mechanics of setting up and operating a small PA system, but a fair amount of politics, lateral thinking, and diplomacy comes into working small gigs, as any club and pub regular will tell you — and so does Paul White.

These days you can buy a small off-the-shelf PA system for around £1,500 that will cope with most small pub and club gigs, but while the system itself may be designed with optimum sound quality in mind, acoustics were almost certainly well down the list when most venues were built. What's more, the physical layout of the venue may make it impossible for you to set up your system in the optimum position. I've spent many a gig tripping over the legs of a speaker stand that had nowhere else to go! Often, you have to make the best of a bad job, but by understanding the nature of potential problems, and acting accordingly, it is often possible to improve matters dramatically.

Acoustic feedback is the number one enemy in small venues, and as explained earlier in the series, the likelihood of feedback is not directly related to how loud the system is running but to how much gain is being used. In other words, a loud singer working close to a microphone will be far less likely to encounter feedback problems than someone with a weak voice who doesn't keep close to the mic. There will always be some sound from the PA speakers that gets back into the vocal mics — even the pros can't prevent this entirely — but the secret is in preventing this from building in volume to the point where the system howls uncontrollably.

VOCAL MICROPHONES

Feedback is a function of gain, but it's also a fact of life that you need a lot of gain to help a typical singer cut through over several hundred watts of backline and drums. Using cheap vocal microphones is generally asking for trouble, as peaks in their frequency response increase the risk of certain frequencies feeding back. Furthermore, unless their directional characteristics are tightly controlled, they pick up a significant amount of sound from the sides.

Most vocal performers opt for a good quality dynamic microphone with a cardioid or hypercardioid directional characteristic, and a slight frequency peak between 3 and 5kHz to help cut through loud backing. This is known as a presence peak, and although it does — in theory — increase the risk of feedback in the 3-5kHz region of the audio spectrum, this is more than compensated for by the increase in intelligibility of the vocals. In any event, most feedback problems occur at lower frequencies than this, but it is wise to be aware of the performance differences between cardioid and hypercardioid mics, as outlined in the 'Feedback' article in issue one (November 1996).

Essentially, hypercardioid mics have the most tightly controlled pick-up pattern, but they are more sensitive to sounds coming directly from behind than cardioids. For this reason, the back of a hypercardioid mic shouldn't be aimed at a stage monitor or a PA speaker. Better to aim it at something absorbent, such as the audience. It's important to choose a microphone that has good feedback rejection characteristics, but it must also suit your voice.

It's one thing having the right mic, but a certain amount of microphone technique is necessary to get the best out of it. Obviously the loudest results are obtained with the microphone very close to the mouth, and many rock singers work with their lips almost touching the wire grille, but you also need to sing directly into the end of the mic, not across the top of it as so many club entertainers seem to do. Figure 1 shows the correct mic position — cardioid mics are designed to be most sensitive to sounds in this direction and also produce the most accurate sound when used on-axis. However, the so-called proximity effect causes a noticeable bass boost whenever the mic is used very close to the source. Although experienced performers can vary their mic distance to make deliberate tonal changes, inexperienced singers may just end up with an uneven sound.

It is also very important to hold the mic so that your hand isn't touching the grille of the basket or blocking any of the small vents directly below the basket on some mics. These vents are what create the directional characteristic of the mic, and covering them up reduces their efficiency and greatly increases the risk of feedback. A great many singers still cup their hands around their mic, but this is asking for trouble — the sound quality will deteriorate, and at the same time, the risk of feedback increases dramatically.

Unfortunately, a vocal mic doesn't just pick up the singer; it picks up sound from everywhere, especially along its axis, which means anything loud occurring directly behind the singer will also be picked up and amplified. If you try to sing directly in front of a drum kit or a guitar amp stack, not only is it harder to hear yourself, the offending sound will also be piped through the PA so that it also competes with you out front. If the engineer tries to turn up the vocals, all that happens, apart from feedback, is that the drums or guitar stack get louder.

If there is any room for manoeuvre whatsoever, ensure that there is no backline gear directly behind the singer, or too close on either side if at all possible. Also position the amplifiers as far back from the mics as possible to reap the benefits of the inverse square law (doubling the distance from the sound source reduces the power, or intensity, to half). Combo amplifiers are less troublesome than stacks in this respect as the sound tends to pass below the level of the microphone rather than aiming directly into it.

Having minimised spill from the backline by careful mic positioning, there's also the spill from the main PA speakers to worry about. On stage spill merely causes a bad sound; excessive spill from the PA causes feedback. Careful positioning of the PA speakers will minimise sound travelling directly from the speakers to the mic, but that's not usually the main cause of PA spill.

More often than not, what's picked up by the vocal mic is reflections from the back or side walls of the venue — a subject explained in some detail last month. If there is a low ceiling, reflections will be picked up from there too. Be aware that a hard wall at the rear of the stage will also have the effect of reflecting sound from the room right into the vocal mic. The closer the rear wall, the more likely feedback problems are, so in small or cramped venues, consider hanging a temporary curtain behind the singer. In general, use a heavy material and leave it partly pleated rather than stretched out. If you're using multiple speaker cabinets, also take note of last month's suggestions on speaker stacking.

Stage monitors are invaluable in allowing performers to hear what they and other band members are doing, but ensure that monitors are also well placed to prevent feedback problems. Ideally, they should be positioned at around 45 degrees to the rear of a hypercardioid mic, or directly behind a cardioid mic — these are the points at which the mics are least sensitive, see Figure 2. It also helps if the main PA speakers fall on or close to the 45 degree line, but in practice, so long as they are a few feet forward of the main vocal mics and as far to the side as possible, then that's about the best you can hope for.

Where it is practical to use two stage monitors, it is possible to wire one out of phase with the other. As long as the monitors are placed symmetrically about the mic stand, the direct sound from the monitors reaching the mics will tend to cancel out, reducing the risk of feedback. However, the singer, having the benefit of two ears, will still be able to hear the monitors quite clearly. Figure 3 shows how this might be arranged in practice.

CHEAP MONITORS

Good quality monitor speakers are essential, as a well-controlled dispersion pattern is required at all frequencies. Unfortunately, many monitor systems are designed as an afterthought, using cheap components, and in such cases, they are a recipe for feedback disaster. Obviously any attempt to use a single 12-inch driver on its own are doomed to disaster, as the high frequencies will be emitted in a narrow beam, which can easily reflect back into the microphones. However, even if a tweeter and crossover are employed, problems will still arise unless the driver/crossover/box combination exhibits a reasonably flat frequency response with no obvious peaks. Equally, the dispersion pattern is unlikely to be acceptable unless quality components and sensible design are employed.

Even with the best monitors, some equalisation is often necessary to 'notch out' troublesome frequencies where feedback is threatening to strike. Because of the physical parameters of a room and the equipment being used, feedback will always start at whatever frequency the system has the most gain. This will vary from system to system and venue to venue as it depends on the PA speakers, the monitor speakers, the mics, and the positions of all these components within the room. The shape and size of the room also has a significant influence, as do the nature and position of any reflective surfaces.

For this reason, graphic equalisers are routinely employed, not to boost parts of the audio spectrum, but to cut those parts where feedback is becoming a problem. The usual setting up technique is to increase the system gain during the soundcheck until feedback starts to build up, then find the offending frequency on the graphic EQ and apply a little cut. Unless the engineer is experienced, this can take a little trial and error, so using a model of graphic EQ with 'telltale' overload LEDs on each band will help identify problem frequencies more quickly. The system gain can then be wound up a little more until the next feedback hot spot is discovered, then this too can be pulled down in gain. How far you take this depends on your patience and experience, but there comes a point when you can't EQ any more out of the sound without making it sound unnatural. The problem is that a typical graphic equaliser attenuates far wider sections of the spectrum than the feedback frequency in question.

In large concert systems, it is common to use a graphic EQ in the main PA as well as in the monitor system, but this requires careful setting up if the overall sound isn't to be compromised. It is possible to tailor the general sound to compensate for gross room resonances, but in general, corrections should be made using cut rather than boost and the changes should be as subtle as possible. The reason for using cut only is that the human hearing system is far less sensitive to missing frequencies than it is to having extra ones added — a few dB cut at one frequency may be quite unnoticeable, but the same amount of boost at the same frequency will stand out immediately. Many powered mixers come with five-band graphic equalisers built in. Although these are useful for general tone shaping or for countering the worst anomalies of a room, they are too imprecise for feedback control or for precision room compensation.

MAIN PA SPEAKERS

As stipulated, the main speakers should be positioned in front of the main microphones and to the sides. Last month, I showed various ways of stacking multiple cabinets to control the vertical dispersion of the sound, but in general, the most important consideration is to direct as much sound as possible onto the audience, while directing as little as possible onto the rear wall. In a small venue, this is best achieved by setting up the speakers so that the tweeters are around two or three feet higher than the heads of the audience, and then angling the speakers slightly downwards so that they are aimed at an imaginary point around two-thirds of the way back in the room. This limits the amount of energy directed onto the rear wall, which leads to a clearer sound and minimises the amount of reflected sound that might otherwise contribute to feedback problems.

In a small venue, take particular notice of how much sound is being directed onto nearby walls, and imagine a mirror fixed to these walls to see which way the sound will be reflected. Ideally, as little energy as possible should reach the walls, but if this is inevitable, at least try to ensure the reflections don't come directly back to the stage.

SENSE AND SENSIBILITY

There comes a point when you've done all you can do, and you just have to make the best of what you have. The best way to tame an unruly system is to work at realistic sound levels — most audiences tend to think bands are too loud anyway. Quality will always be appreciated more than quantity. Establish a safe maximum working vocal level first, then balance the backline to this — not the other way around. If the guitar amps are always too loud, consider using smaller combos or power soaks at smaller gigs so that they can be used at a lower level without sacrificing tone. Also be aware of the fact that the sound will change when the audience start to fill up the room; as people absorb sound to some extent, their presence is likely to help control feedback rather than make it worse. It also helps to turn backing vocal mics down or completely off when not in use as system gain is affected by the number of live mics open at any one time. The more open mics you have, the more system gain there is and the more problems you'll have with feedback, not to mention spill from the backline.

If you're operating you own mix from on-stage, balance it from out front, then make as few changes as possible during the performance. Set up the mixer within arm's reach so you can address any feedback problems simply by pulling the master faders down a touch; if you use an effects footswitch, try to get one with a status LED on it as it is sometimes difficult to tell whether the reverb is on or off in some venues. I've often sung a song dry, then turned on the reverb to announce the next number!

THICK SKIN

If this all sounds like it makes a live sound engineer's life hell, you're right, but for some reason, you just keep on doing it. I operated a small PA system for a number of years, and the one thing I learnt was that if there's a problem — any sort of problem — it's all the engineer's fault. It doesn't matter that the drum kit is still fitted with the original heads and sounds like cardboard boxes — it's your fault. Then again, if you're a masochist with a rhino's skin, then maybe a live sound engineer is what you ought to be!