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Live Sound (Part 5)

How To Survive Live

Article from Recording Musician, March 1993

This month, we address the practicalities of working in a less than ideal live venue.


Live sound engineers are expected to work miracles on a daily basis without even the regulation quantity of loaves and fishes. Paul White tells you how to deal with everything from the feedback of the 5000 to the intransigent venue manager.

It's all very well to go on about the attributes of a perfectly designed, optimised PA system, but those of us who have had gigging experience know that most venues are far from ideal, both in their physical layout and in their acoustics. 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 perennial bane of the live sound engineer, especially in small venues such as pubs and clubs. 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, but the secret is in preventing this from building in volume to the point where the system howls uncontrollably.

The Microphone



Keeping feedback under control starts with the microphone. Using cheap microphones is generally asking for trouble, as peaks in the frequency response increase the risk of certain frequencies feeding back, while their directional characteristics might allow them to pick up too much sound from the sides or rear. 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 a loud backing. This is known as a presence peak, and though 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. A popular vocal mic is the Shure SM58, but there is a wide choice of suitable models currently available and it pays to try several to see which best suits your voice.

Did You Know That...

...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.

Having selected a decent microphone, pay some attention to how it is held. It goes without saying that 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. At this point, it pays to be aware of the so-called proximity effect, which causes a noticeable bass boost whenever the mic is used very close to the source. This occurs when the distance between the mic and mouth is less than a couple of centimetres or so, and though 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 important to sing directly into the end of the microphone, as shown in Figure 1. Cardioid mics are designed to be most sensitive to sounds in this direction and also produce the most accurate sound when used on-axis. It is equally 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.

Figure 1: Singing directly into a cardioid mic.


One problem encountered in live performance is that the vocal mic doesn't just pick up the singer — it picks up sound from everywhere, especially along its axis. In other words, anything loud going on directly behind the singer will also be picked up, to a greater or lesser extent, depending on how far back the rogue sound source is. I've seen many a poor club and pub singer standing right in front of a drum kit and wondering why the drums seem to get louder whenever the mix engineer turns up the vocals. If there is any room for manoeuvre whatsoever, ensure that there's no back-line gear directly behind the singer, or to a few feet either side if at all possible. If the stage is cramped and amplifiers have to go behind the singer, then stand these close to the floor if you can, so that the sound passes under the singer's mic rather than directly into it.

Of course the back-line isn't the only source of sound that's likely to find its way into the vocal mic; there's also the sound from the PA speakers themselves to consider. As we've seen, this is what causes feedback. Careful positioning of the PA speakers will minimise sound travelling directly from the speakers to the mic, but be aware that a hard wall at the rear of the stage will have the effect of reflecting sound from the room right into the vocal mic. The further you are from the wall, the less serious this problem is likely to be, but 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.

Figure 2: Monitor positions relative to vocal mic.


Stage Monitors



Stage monitors also provide a means for sound to re-enter the vocal mic; consequently, careful positioning of stage monitors is essential. Ideally, they should be positioned at around 45 degrees to the rear of the mic, as shown in Figure 2, as this is the point at which most cardioid and hypercardioid mics are least sensitive. 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 positioned 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. Plowever, 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.

Figure 3: Out-of-phase monitoring.


Good quality monitor speakers are essential, as a well-controlled dispersion pattern is required at all frequencies. Attempts to use something like a single 12" speaker 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. But even with the best monitors, some equalisation is often necessary.

Because of the physical parameters of a room and the equipment being used, feedback will always start at whichever frequency the system has the most gain. If the speakers or the mic have peaks in their response, then that is where feedback is most likely to start. The shape and size of the room also has an influence, as do the nature and position of any reflective surfaces.

For this reason, graphic equalisers are often 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. The system gain can then be wound up a little more until the next feedback hot spot is discovered; a little more work with the equaliser should bring this under control too. How far you take this depends on your patience and experience. 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.

Main 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 back along 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.

Once you've taken these precautions, all that's left is to work within realistic sound levels, and keep in mind that most audiences tend to think bands are too loud anyway. Quality wins over quantity every time when it comes to PA. Establish a safe maximum working vocal level first and then balance the back line 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 you can play 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, but as people absorb sound to some extent, their presence is likely to help control feedback rather than make it worse. Turn down backing vocal mics when not in use — the more open mics you have, the more problems you'll have with feedback. If this all sounds like it makes a live sound engineer's life hell, you're right. I did it for years and you can guarantee that if there's a problem — any sort of problem — it's all your fault. But if you're a masochist with a skin like a rhino, than maybe a live sound engineer is what you ought to be!

Live Engineering Checklist

  • Feedback control starts with the microphone. It's worth spending a bit extra on 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 a loud backing.

  • Pay attention to how the mic is held, and be aware of the proximity effect, which causes a bass boost when the mic is used very close to the source. It is also important to sing directly into the end of the microphone.

  • 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. This can reduce the mic's efficiency and increase the risk of feedback.

  • Try to ensure that there's no back-line gear directly behind the singer, or to a few feet either side if at all possible. If amplifiers must go behind the singer, stand these close to the floor so that the sound passes under the singer's mic rather than directly into it.

  • Be aware that a hard wall at the rear of the stage will reflect sound from the room right into the vocal mic — potentially causing feedback. In small or cramped venues, consider combating this by hanging a heavy, partly pleated curtain behind the singer.

  • Stage monitors should ideally be positioned at around 45 degrees to the rear of the mic. It also helps if the main PA speakers fall on or close to the 45 degree line, but if not, try to position them a few feet forward of the main vocal mics and as far to the side as possible.

  • Even with the best monitors, some EQ is often necessary; graphic equalisers can be employed to cut those parts of the audio spectrum where feedback is becoming a problem.

  • When positioning loudspeakers, try to direct as much sound as possible onto the audience and as little as possible onto the rear wall. In a small venue, set up the speakers so that the tweeters are two or three feet higher than the heads of the audience, then angle the speakers slightly downwards so that they are aimed at an imaginary point around two-thirds back along the room.


Diplomacy

A great many problems can be avoided if you can establish a constructive relationship with the venue manager, and this is especially important at the smaller pub and club gigs where there is no proper stage area. It's surprising how many people expect you to be able to set up in an odd corner with the PA speakers behind you, but it's not their fault entirely — they may not be aware of the laws of physics as they relate to acoustic feedback.

One important prerequisite is to arrive at the venue in plenty of time and identify any potential problems before you go too far with the setting up. If you can arrange your speakers so that they are not pointing directly at the bar, you are less likely to get complaints about the noise, and if you have a multi-cabinet PA system, don't use more than you need for the size of the venue. Above all, don't take a belligerent stance when the landlord tells you that you can't set up the mixing console in the middle of the floor — try to see his point of view and suggest a compromise that won't inconvenience either party too much. In these smaller venues, the same person is likely to be responsible for rebooking, so if you come over as sensible and co-operative, you're more likely to get asked back. If you do try to lay the law down to the manager, the chances are that he'll just dig in and refuse to budge at all. Being polite, and tactfully suggesting that a few simple changes might result in better sound quality is far more likely to get results.


Series - "Live Sound"

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All parts in this series:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 (Viewing) | Part 6


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Previous Article in this issue

Using Compressors

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Making New Age Music


Publisher: Recording Musician - SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


The current copyright owner/s of this content may differ from the originally published copyright notice.
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Recording Musician - Mar 1993

Donated & scanned by: Mike Gorman

Topic:

Live


Series:

Live Sound

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 (Viewing) | Part 6


Feature by Paul White

Previous article in this issue:

> Using Compressors

Next article in this issue:

> Making New Age Music


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