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Sunday, December 12, 2010
P.A. Systems for Dummies ;The absolute basics of getting a good sound as a band
An audios mixers function is to collect sounds from various sources, musical instrument, vocalist, effects etc. and bring them together to give a total stereo sound balance.
The basic sounds are collected by two methods:
1.Miking up -this entails placing microphones in front of the vocalist, the individual on stage amplifiers and around the drum kit.
2. Direct injection (usually abbreviated to D.I.) The sound is collected either prior to the on stage amplifier via a unit known as a D.I. box, or from a tapping taken from the amplifiers pre-amplifier stage. Once collected, each sound source is fed to a separate channel on the mixer. After being equalized it is positioned in the stereo spectrum by way of the pan control.
When every channel has been adjusted the end result may be an exact reproduction of the stereo image on the stage, or the sound system could be used to completely change the apparent stage positions of the vocalists and instruments. It may even modify an instruments sound or a voice via external effects units to bear no resemblance to the original. In the case of recording, the musicians would not be positioned in the studio as they would be on stage. In this instance the mixer could be used to synthesize the stereo spectrum so that it will appear to the listener that they are indeed hearing a live recording.
The variations are potentially endless, this is the beauty of audio mixing. It may be used to naturally reproduce or enhance or modify the sound in countless ways, which are limited only by the engineers imagination and ability.
Also today there are so many effects that can be used to add harmonies, reverb, etc. that only your imagination is your limit. But here, as in so many things, less is more. Don't 'overcrowd' your sound image!
Principles of setting up and operating a sound system
The mixer is best situated approximately 25-40 metres from the stage, centrally placed for the correct stereo balance. Any further back gives poor visual contact and a long delay of the sound between speakers and mixer. Remember, sound takes one millisecond to travel one foot. As there are (as the name implies) 1,000 milliseconds in a second, distance soon leads to confusion.
The mixer should be earthed at all times, the P.A. slave amplifiers should not be earthed nor should any other piece of equipment directly connected to the mixer. (All connected equipment will find an earth through the screened leads to the mixer. More than one earth may cause a hum loop). All stage equipment, i.e. guitar amps, organs, synths, electric pianos etc. should be earthed. Indeed they must be earthed by law.
Microphones should always be low impedance, 200 ohms balanced dynamic. You will find cardioid characteristic microphones (i.e. are most sensitive in one direction) are the best to use for all P.A. work to avoid crosstalk and feedback. Most studio condenser microphones are useful for recording but tend to distort when used on stage because of the high sound pressure levels involved.
Lay out the microphones on stage to correspond with the mixer fader layout, i.e. left to right on stage and the mixer. This is especially important on vocal microphones to avoid confusion. Label the mics and mark that mic on the mixing desk.
Instrument amplification, electric pianos, synths, etc., may be miked but it is preferable to directly inject them into the mixer via a D.I. box or straight from the pre-amplifier output which may be fitted to some amplifiers. Direct injected signals are always much cleaner because they avoid any distortion that may be introduced by using a microphone. Most guitar amps today have D.I. jacks on the back, though they are labelled 'speaker simulation.'
(Never under any circumstances connect an amplifiers loudspeaker output to the mixers input. The output voltage is high enough to destroy the preamp in most desks!)
Leads should be well clamped to the mic body as they are constantly wrenched. Nine times out of ten, the Shure SM 58 is used for vocals. It is crisp, clear, cheap and reliable.
Miking up a drum kit is regarded in the rock scene as the highest of the audio high arts, but in reality, it is fairly simple. The clue is to have a really great sounding drum kit. (Or as a colleague of mine put it, 'You can't polish a turd!')
Snare drum microphones should be placed above the top skin near the edge. You may need to place a second mic on the underside to pick up the snare 'rattle' in bigger gigs where every part of the drums has to be close miked.
Everybody has a different idea about miking up the bass drum. There used to be a time when special bass drum mics were sold that had especially large diaphragms to get as much bass as possible. This was because the EQ on desks was poor and gates needed lots of signal to help them to trigger. Today we know better and use ordinary dynamic mics to get as much 'impulse' as possible. Tom-Tom mics can be placed near to the heads and panned to taste.
There are two alternatives to conventional miking up for drums. Firstly, you can place contact mics directly onto the skins or on the inside of the drum's body. We do this often in the studio, particularly for the bass, using C-Ducers. The other alternative is to place triggers onto the skins and run them to a digital drum controller such as the dDrum. The advantage is that one can change sounds at the touch of a button and there is no need for any further dynamic processing.
In the good-old bad-old days, we always were told that the speakers had to have double the power handling of the amps. The result was the amps went into distortion and the peaks that came off the distortion blew the horns.
Today we have sophisticated protection circuits and better horn construction and cooling, so amps and speakers should be about as powerful as one another.
There was a time when we all practised the dark art of DIY speaker manufacture. Speaker cabinets were expensive and often loaded with cheaper speakers. This was largely because transport costs were high and retail outlets wanted 60% to 100% mark-up. Today, companies like JBL, RCF and EV sell full range systems for as little as 300 Pounds for a 450 Watt full range speaker with horn and two 15" bass drivers. That is less than the cost of building it yourself!
If you are playing a large hall or an open air venue, you should take a close look at line array systems. The speakers are hung in a long curve from the top of the stage and create a powerful pressure wave across the audience that does not suffer from the ill-effects of temperature inversion This means that the power is used to provide lots of sound for the audience and not to annoy people living five miles away.
After you have connected the components of the system, but before it is turned on, check the wiring thoroughly. This is particularly important if you are using an electronic crossover unit, as it is possible to accidentally connect the bass section to the horn drivers, which could result in permanent damage. Switch the amps on last when you power up and first when you power down. Always use the switches on the amplifiers themselves as these should have a protective circuit that prevents massive clicks damaging the speakers.
NEVER THROW ONE MASTER SWITCH FOR THE WHOLE SYSTEM.
When the equipment is on, check that all amplifiers are functioning and check the loudspeakers and horns individually. If there is a fault, the chances are that it is a cable, so always carry lots of spare cables. If this is not the case, you must isolate the fault. For instance, try connecting the speaker, which is not working to a different amplifier. If it works you should check the original amplifier with a loudspeaker and cable which you know to be working. The separate pieces of equipment can be eliminated in this way until you are left with one faulty item.
Whilst it is not always possible to carry spares of everything, it is a good idea to carry a tool kit containing a mains test screwdriver, wire cutters, strippers, soldering iron and solder, pliers, spare plugs and connectors and spare fuses. You may not be able to repair every possible fault but armed with the above you can remedy the minor ones (which most faults are), on the spot. Remember, never leave home without spare cables.
Getting a good sound is very much a question of experience. It is also very true that a poor band or musician will still sound poor when amplified. There is nothing you can do to make a bum note sound right and there is no way to keep a sloppy drummer on the beat. However, even when it is the band's fault, you as sound technician will probably get the blame!
Also remember that the hall or venue is the most important part of your sound system. It is very difficult to get a poor sound in a Roman Arena. It is close to impossible to get a good sound in a square room with a tin roof (without hanging lots of heavy drapes on the ceiling and on some of the walls).
An old, but very effective trick when dealing with a difficult hall, is to use a spectrum analyser that works in real time (i.e. the LEDs or the curve falls instantly) to see where the problem frequencies are. This is done by putting pink noise (random noise of equal energy at all frequencies) through the speakers and setting up the graphic equalizers until the analyser shows all frequencies to be more or less level. Cut the pink noise and see which frequencies take the longest to die down. These are going to be where you will have the most problems. Bring them down on the equalizer.
Large systems use a stereo electronic crossover on the mixer outputs to split the sound into two or more sections (i.e. bass, middle, treble) Each individual sound is sent to an amplifier and loudspeaker cabinet which is specially designed to handle these particular frequencies. The crossover has a separate gain control on each section.
Once you are satisfied that everything is as it should be, you can begin to position the channels in the stereo spectrum by means of the pan controls. One feature, which is standard on most larger mixers and is a great aid to live mixing, is the ability to route the input channels to subgroups. If seven microphones are placed around a drum kit and fed to seven mixer channels, if it became apparent during the show that the kit was not loud enough it can be brought up on the subgroup faders rather than the engineer trying to raise seven channel faders simultaneously. It also means that the entire group can be fed via an auxiliary send to an effect such as a compressor.
You can also use the subgroups as an easy way to feed the monitor (or foldback) mix. Just take a direct out from each group mix and feed it into one of your monitor amps. On larger systems a whole separate monitor mixing system is used using specialized mixers that can give every musician a different mix.
A cheap trick to getting a good sound with a band is to give each instrument and voice a different part of the frequency spectrum as far as possible. Try giving the upper highs and lower bass to just bass and drums. Cut the highs on backing vocals and boost the upper mids to lead vocals. Try keeping keyboards in the back and don't give lead guitars too much bass, if any. Keep effects down to a minimum and after years of practice, you too can become an audio engineer!