Shavano Music Online

    PA Systems; Equalization

    10/99 - Jens Moller - http://www.colomar.com/Shavano/pa_eq.html
    updated 5/03 and 9/06
    When you get a new piece of gear, its always to good to put it thru its paces and learn how it responds. If you just bought a P.A. system, or added to one that you already own, its good to verify how it responds to music. The goal here is to get the best sound possible and not cause equalization problems when playing live, simply because you weren't familiar with its frequency response characteristics and how that might relate to the room you are running sound for. Also be aware that performing outdoors has a wealth of sonic problems associated with it that people try to address by using their on board equalization, often without any concern as to how it affects everything else running thru the P.A. system. Know your system and its strengths/limitations.

    Scoping out the Room

    The room you will be playing in will have a great affect. Are the floors carpeted? Acoustical Ceiling Tiles? Has someone painted them (which turns them into non-Acoustical Ceiling Tiles)? Large Windows? Paneling on the walls?

    What I look for are things that cause reflections or standing waves. The reflections will tend to affect feedback. The standing waves will accentuate the bass, but only for certain notes and make the whole thing turn into a muddy sound. Too much carpeting turns a room dead - in that you tend to really boost the highs to make up for it.

    Know your P.A. Systems Limits

    If you use your P.A. system only for Vocals, You really don't need exceptional Low Frequency response, and you should focus on the frequency range that you will be reproducing (ie. Vocals). If you have Keyboards running thru the P.A. system and/or Bass Guitar and/or Drums, the P.A. will need to be able to handle a much broader frequency range - make sure that you have adequate rated power amplifiers to handle any needed low frequency response, as well as speakers that can move the volume of air needed to cover your venue.

    EQ Starting Points

    When ever I go into a new room, I start with all EQ at zero. If its a small to medium sized venue, I don't put any microphones on the drums (a sure way to mess up room dynamics is to boost an already loud instrument). Then, if its within my power, I make sure that the band doesn't play overly loud - by this I mean that there are acceptable volume levels and then there are excessive volume levels. Your P.A. has to compete with these. Things that work well at reasonable volumes cause sonic problems at excessive volumes. Its hard to know where that point is since your ears will respond to excessive volume levels by dampening and artificially reducing your perceived volume levels (which only makes people tweak the Eq more).

    Start with the mains down to no more than 20% (or 2, if you have a 0 to 10 scale for that level setting).

    If you have an Audio Cassette or CD of the band (or any other recording with similar dynamics), throw it into a tape deck/CD Player and Perform it back thru your P.A. at a low volume - loud enough to hear everything, but not so loud that you can't talk to the person next to you. Walk around the room and see what it sounds like. Adjust the Eq to make it sound like you think it should. once done, go to the P.A. and bring the mains up to approximately where you think the levels will be when the performance is going on. Then walk around again. All the room oddities will come to light here. Look for Boomy Mushy bass (it may only occur in certain places of the room) - These are Standing Waves - if you find them (large panes of glass windows seem to bring these to life), try to compensate by bringing the bass down. Do all the adjustments on your main graphic Eq and leave all the tone settings for the individual channels at 0db if possible.

    Once you get this Eq level set, you have the general room balance. It will changes as a result of having people in the room (mostly you will need to bring the mains up some and boost highs during the evening).

    Individual Channels

    Now, turn on the microphones and test them one at a time. You can play the tape/CD during this time if you want if you want a comparative feel for the sound. Set these levels, starting at 0db on all the tones. Tweak as needed for individual channels, but leave the Graphic Eq alone.

    The rule is to alter the room Eq mix only if all the single channels need exactly the same correction, otherwise, alter the individual channel. Find the source of any feedback and alter microphone positions until it either goes away, or scale back that channels treble. If you need the treble boost, and can't control the feedback, look into either Sabine or Behringer Feedback control rack-mount units - these suppress feedback via a microprocessor based tone control that instantly adjusts narrow frequency bands to eliminate the feedback in real time - see below for more on feedback.

    I avoid running any instruments thru the P.A., unless its a very large room, or the instruments are using lower powered amplifiers of their own that you want thru the main sound system. I try keep the P.A. management to vocal issues - which allows me to react quicker. Don't hesitate to wander around the room once the performance starts to get an idea of what things really sound like to the audience & go back to tweak things.

    NOTE: Many Guitar Players like to run a 10 to 40 watt guitar amplifier rather than a 100 to 200 watt amplifier. They are trying to get a specific sound and not have to play too loud to do it - a Shure SM57 (or similar) microphone placed 1 foot (30 cm) in front of the guitar amplifier speaker(s), aimed at the center of the speaker will work quite well when run thru a P.A. system. If the amplifier has more than one speaker, pick one to aim the microphone at.

    Keyboards can plug directly into the P.A. systems mixer board. Bass Guitars, and any other instrument with a wire wound pickup may not sound like you feel they should if directly connected to a P.A. system input - you may need a Direct Box for these.

    Feedback

    Its always a problem, and there are a lot of things that you can do about it. Feedback is caused by the re-amplification of a signal that is an input to an amplifier, where as the signal starts, it gets picked up again by the source (such as a microphone or any acoustic instrument with a pickup/transducer added to it) and continually re-amplified. You can hear this high frequency pitch (altho it can also happen with low frequncies too) start and get louder and louder, often in a matter of a few seconds. The high frequency feedback often forces a lot of power into your tweeters and can very quickly cause them to fail (voice coil tweeters can't take too much of this). Cutting back on the treble on any channel that is causing the feedback usually stops it, but it also alters the tone quite a bit.

    So, what do you do about it? Microphones are the number one cause of feedback, so, as a sound person, you need to study thier placement relative to all of the speakers. If you use floor monitors, make sure that the microphones a pointing away from them (if your microphones are not directional, this will make it a lot harder to do - look at the specs and see what the rejection pattern for your microphones are). If your main speakers are behind you, thats likely another cause of the problem - try to keep your speakers in front of you where there is no easy way for the microphone to be the cause of the re-amplification. This may also mean that your singers have to stay in one place, once you find out where the microphones can be. Acoustic instruments with pickups/transducers are like non-directional microphones - these can cause major feeback problems. Often putting a strap or rubber band over the strings when no one is playing them helps quite a lot, however plugging the sound holes also helps (many people use masking tape and cardboard to cover sound holes in guitars - foam rubber in the F holes of a guitar or violin will also help and not affect sound quality that much). You often have to play with the stage layout and speaker positions to address this. Sometimes if your singer holds the microphone too close to the input element, it can cause feedback - its amazing how something so seemingly minor can cause this sort of problem.

    I do sound with people who have wireless microphones that dance all over the stage - a true feedback nightmare until I bought a feedback eliminator (Models available from Sabine and Behringer) - this is a microprocessor based rack mount unit (Sabine has smaller stand alone units also) that sits between your mixer output and the power amp to either your monitors or the mains (I find that the monitors are the best location for this). It can also be used on the input of the microphone itself (but you will need more of them if you do this, and you'll need impedance matching transformers - this gets expensive). The microprocessor unit analyses the signal looking for the tell-tale feedback pattern in the signal - when it finds it, it suppresses by enabling a notch filter (like the controls on a graphic equalizer, except a very limited frequency range) that suppresses that frequncey and only that frequency. Its like turning down the treble, bit it really doesn't affect the tone very much. This all happens in milliseconds and as such, you never even notice that it happened. The best part is, the feedback is gone. It constantly monitors and corrects for feedback.

    Paul Kytzia (from the UK) writes: Beware of acoustic guitar players who insist on using radio/wireless gear - it makes your PA go berserk if you have a very finite space in which to set up. Secondly - the hidden menace - monitors ! I took EXTREME care to position the mikes with the mike chassis pointing as closely as possible down a direct line with the speaker cones of the monitors.

    Dave Robinson has some additional thoughts:

    • I tend to use the individual channel EQ on my mixer to adjust the tone of each instrument or voice, and use my final EQ in the rack only to compensate for room dynamics and feedback issues. Other than that the main EQ stays relatively flat. The biggest mistake most novices with EQ make is slipping into "Smiley Face" or "Sine wave" syndrome even before they power everything up and blame the subsequent problems on something else. In all actuality, I find an elongated frown, cutting the ultra lows and the ultra highs works in most venues.

    • This isn't really a difference, but more of highlighting an omission. I find it preferable in most instances, should a certain freq range get lost in the mix or otherwise need a little boost, it's usually better in the long run to cut the surrounding freqs back instead of boosting. 95% of the time, the sliders on my EQ's are run below the Db ± line

    Final Thoughts

    After a while, you'll get the hang room of dynamics. You'll walk into a room and immediately see things that you know will cause problems and you'll think about those things as you set up and react to them very quickly. I hate places that are mostly windows and places that have carpeting over 50% of the walls. I'm also leery of painted ceiling tiles (things become very bright as a result & feedback can be hard to control). Metal ceilings can make you sound as if you were in a deep cave (the natural reverb is fun for a while but gets tedious when you discover that all the echos cause the whole band to have major timing problems until they get used to hearing things repeat 20 times at apparently random intervals).

    You may also discover some limitations in your P.A. system relating to how you use it. Some of these may be correctable via Eq tweaks, but if any of them require major adjustments, you probably would be better off looking at correcting the problem at the source and not in the P.A. system. Common problems are:

    • Not enough power amplifier headroom for low frequency response - ie. your power amp does not have enough power for what you are trying to do, or your voltage supply cannot give you the current you need (both of these have been known to burn out horns and tweeters).
    • Speaker systems that are lacking capability at some frequency range.
    • Microphones that can't be EQ'd to be usable
    • A musical instrument run thru the P.A. swamps the system at times.

    Knowing these problems can usually help you prevent them. All can be addressed by upgrading the P.A. system, but others might be better resolved at other places. Unless you are specializing in outdoor concerts, you probably have bought your P.A. system to solve a specific need - don't try to solve other peoples problems if they are better solved by equipment upgrades in their gear.

    Questions? Comments? .

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