How To Run Small PA SystemsSound is tricksy. It's full of illusions. But if you learn to see through the illusions, running a PA system isn't that difficult. This article is about how to run sound at a small event (two or four main speakers, possibly some monitors). I don't know as much about big installations or fancy equipment. Sound power is measured in watts, watts/meter2, and decibels. Watts are good for amplifier calculations, but don't express what your ears hear very well. Decibels express hearing better, but decibels are a logarithmic scale, so the math is harder. If you were playing a CD, and your volume knob was marked in both decibels and watts, you could see how they compare. 100 decibels might be one watt, depending on your speakers. 100 decibels would be plenty loud, maybe too loud for your living room. In bar with a crowd it wouldn't be nearly loud enough. One decibel higher, for most people, is "just noticeably louder". 10 decibels more sounds more like "twice as loud". A one decibel change is approximately 26%, so 101 decibels would be 1.26 watts. But 102 decibels is not 1 +.26 +.26 = 1.51 . Instead, it is 26% higher than 1.26, so it is 1.26 * 1.26 = ~ 1.58 . This change may not look like much with just two decibels, but try it with 10 or 20 decibels:decibels watts 100 1 101 1.26 102 1.58 103 1.995 104 2.51 105 3.16 106 3.98 107 5.01 108 6.31 109 7.94 110 10 111 12.59 112 15.85 113 19.95 114 25.12 115 31.62 116 39.81 117 50.12 118 63.095 119 79.43 120 100 121 125.89 122 158.49 123 199.53 124 251.19 125 316.23 126 398.11 127 501.19 128 630.96 129 794.33 130 1000Adding 3 decibels actually requires twice the watts, and doesn't sound much louder. Adding 10 decibels (twice as loud) requires 10 times the watts! The difference between 100 and 101 decibels is a quarter of a watt. The difference between 129 and 130 decibels is more than 200 watts! Middle C is 262 Hz (Hertz, or cycles/second). The A above middle C is 440 Hz. Bass notes are above and below 100 Hz. At 100 Hertz, wavelength is 10 feet (= speed of sound / frequency = 1000 feet/second divided by 100 cycles/second). Since the wave is so large, even a large (15 inch) speaker is not wide enough to push the wave in any particular direction. So bass notes leave the speaker with a pretty even spread 360 degrees around, even behind the speaker, and vertically too. They run away to the ceiling and down around peoples ankles. Notes above middle C are much more directional. Treble notes go the direction the speaker is aimed at. At large rock concerts, they install huge arrays of speakers, which are more successful at projecting bass notes in a particular direction. They may also have fancy electronics which delay the sound going to certain speakers to keep things in phase. Since bass notes go in all directions, their energy spreads out, and their intensity drops off quickly at further distances from the speaker (at 1/r squared). The Bass energy is wasted, running off in all directions. Since midrange notes are directional, their intensity drops off more slowly. Their energy is focused where it is aimed at. So if the bass equalization is adjusted right, close to the speaker the bass will seem too high, and from far away it will seem too low. Next time you are at a local event, walk back and forth, and listen. Behind the speaker (on stage), there will be bass at almost full volume, but very little treble. So the performers may think you have the bass set too high. Important rule: It is best to position speakers high, perhaps on speaker stands, so that everyone in the audience can clearly see them. If they can see them, the sound can travel directly to their ears without being distorted and muffled by reflecting off of walls or peoples bodies. When a room is full of people talking, they are all talking in the midrange area, with some highs, and very little bass. Ears have separate hairs for receiving each frequency, so people talking will blot out midrange instruments, but nothing competes with the bass, it will be heard. Violins are loud instruments, and have a lot of highs, so they tend to cut through conversation also. Don't be afraid to turn the eq knobs (bass, midrange, treble). Turning them just a little won't have much effect. Turn them each up and down to hear when the sound starts sounding bad, then put each knob safely in the middle of the good sounding area. But be careful, turning these knobs up can cause feedback. In general, the most common bad thing to happen to live sound is for it to sound "muddy", that is, it is too loud, but not clear. This means it has too much midrange, or too much low-midrange. Many sound guys automatically put a "smile" on their graphic eq, that is, turn down the middle frequencies some, while turning up the bass and highs a little. If a person is singing directly into a microphone ( less than two inches away ), just like the speaker, there is a lot of bass energy near the mouth. You may notice this if someone is whispering in your ear, if you are not distracted by someone whispering in your ear. The bass frequencies will be over-amplified, this is called "proximity effect". When someone sings this close to the mic the soundman should turn the bass equalization knob for that channel down, probably to about 3 (out of 10). A person singing six inches or more from the microphone usually won't need the bass turned down. If a person is wearing a lapel mic, then the directed treble frequencies are projecting outwards, mostly missing the mic, while the bass frequencies are bending all directions, and reaching the mic. So again, the bass is over-amplified. Turn down the bass. Something similar happens to acoustic guitars played through microphones. Most of the bass comes out of the soundhole. Most of the highs come off of the face of the guitar, back near the bridge. Near the block of wood that mounts the neck, there is less vibration. Since the bass spreads out, if you put a mic at the 24th fret, you get nicely balanced sound, but not much volume. If you put a mic at the soundhole, you need to turn the bass down. If you put a mic the below the bridge, you get a louder balanced sound, but it may interfere with the players hands. My preference is: in the studio, mic the 24th fret. On stage, mic the soundhole or below the bridge. The convenience of an acoustic-electric guitar is undeniable. Most produce decent sound, and don't cause feedback. Many can be improved by turning eq knobs. A few just sound bad. Guitarists who use temporary magnetic pickups mounted in an acoustic guitar soundhole produce way too much midrange, and very little highs. The midrange needs turned down a lot, and the highs turned up. Usually this doesn't entirely fix the problem. Avoid these temporary magnetic pickups if possible. Miking the guitar can produce high quality sound, but may cause feedback, especially if they play softly. There is a difference between when someone picks single strings or strums all six strings. When a person strums an miked acoustic or a plugged-in acoustic-electric guitar, there tends to be too much midrange, it sounds muddy, it is best to turn the midrange or low-midrange down some. If a person picks single notes on an acoustic guitar, the midrange helps it sound "fatter", best to turn midrange up some, and perhaps turn down the high-highs. In general, it is easier to amplify loud instruments or voices, instead of quiet ones. If a person sings loud and close to the mic, you don't have to turn the gain up as much. Less gain means less feedback. Less feedback means more room to play with the EQ knobs also. Encourage singers to sing fairly loud, but not so loud as to lose control. Encourage guitarists to play fairly loud. Another interesting effect is the difference between living-room-voice and stage-voice. In USA culture, people learn to speak near their lowest frequencies, and think this is normal. You may notice that some foreign folk you meet, perhaps African or South American, speak in higher, musical tones. People in USA naturally try to sing at these low tones also. This works OK for a quiet tune in your living room, but your voice has greater power and smoothness at higher frequencies. In fact, some higher notes you may not be able to hit softly at all. Worse, when you are singing loudly on stage, you may find that you cannot sing the lowest notes loudly, notes that you could hit in the living room OK. Moral is, when choosing songs to perform, choose (somewhat higher) songs and keys appropriate for your stage-voice, not your living-room-voice. Microphones, speakers and rooms are physical things, and each one has an effect on the sound. A common myth is that we try to use microphones with a flat frequency response, and try to amplify sound exactly as it is. No. We try instead to put out the best possible sound. We look for microphones that help us accomplish this. Good microphones make a singer sound better than they actually sound. Microphones that cost more than $2000 are cherished because they sound "warm". In my opinion, Shure SM57's and SM58's microphones provide the best compromise between clear sound and feedback for any live sound situation. Other more expensive mics may provide a flatter frequency response, but feedback much sooner. The Shure SM58 is the most common live-vocal mic in the world, with good reason. It is reasonably priced, usually a little less than $100. It is very durable, surviving much abuse. It has a good sound, making most speakers and singers sound "stronger". It rolls off the bass frequencies some, limiting proximity effect. It rolls off the high frequencies, limiting feedback. It has a cardioid pickup pattern, also reducing feedback. It has a windscreen, limiting breath sounds. It is solidly built, which limits handling noise. It does not need a battery, so it is always ready to work. It is low impedance, so it eliminates most electronic interference. I do not own any Shure stock. The Shure SM57 is similar to the SM58, but it does not have the big thick windscreen. This means it picks up more high frequencies, and more wind/breath noise. It is usually used as an instrument mic, to amplify or record guitars, mandolins, drums, anything. It is better for studio recording than the SM58. It can be used for voice, especially recording voice, but works best if the singer is several inches from the mic to avoid breath noise. It is known as the Swiss-army-knife of microphones. If you want a microphone mostly for live-voice, get a SM58. If you want to record voice and instruments, get a SM57 and a three-dollar foam windscreen from Radio Shack. When you run a sound event INDOORS, your main problem is feedback, followed by balance (making sure all the instruments get heard), then tone (making sure each voice and instrument sounds good). Feedback is caused by sound that comes out of a speaker and gets into a microphone (or some amplified instruments). It goes into the microphone, then to the amplifier, then out the speaker again, this time louder, and goes into the microphone again. It starts small, but quickly builds up to a loud hum or squeal. If you are using monitor speakers, most likely the feedback is coming from the monitor speakers. You can get rid of feedback by using microphones that limit feedback, keeping a large distance between microphone and speaker, aiming speakers away from microphones, aiming microphones away from speakers, turning down the monitor volume, turning down the master volume, turning down some eq knob that is set too high. You will have less feedback problems in a situation where you are miking one to three voices (which are fairly loud) than in a situation where you are miking five instruments (some of which are fairly soft). You will not be able to give the performers as much monitor volume when you are miking instruments. To adjust balance, look at each musician on stage. Can you hear what they are singing and playing? If not, turn them up. If they are drowning out other players, turn them down. Of course, be aware that at certain times, a player may be playing very softly, to let another player have their turn. Don't try to turn them up far enough to hear them. Currently, I am mostly involved with acoustic music, which means no one has their own amplifier. Every voice and instrument is mainly amplified by the PA system, which means the sound guy can control overbearing instrumentalists, which is good. But I remember back when I did rock n roll, guitarists with large amplifiers would drown out their band mates, and perhaps even the lead singer. I don't have an easy solution, but I can empathize. Tone depends on the instrument. For voices, some need the highs turned up, to make them clearer. Some need the highs turned down because they are too piercing. Turn the knobs, see what happens. In a bluegrass band, there is typically a guitar, a bass, a fiddle, a mandolin, and a banjo. The guitar is the quietest instrument, turn its volume up, but be careful about feedback. Probably turn the bass eq down. Banjo and mandolin are loud and somewhat piercing, turn the high eq down some. Violin is also usually improved by turning the high eq down, often turned down a lot. Harmonica can be the most piercing of all. If you turn the highs way down, and the low-mids way up, and the reverb up, you will get a surprisingly good sound. The large stand-up bass doesn't put out much high frequencies, so that knob won't effect it much, but be sure to give it plenty of midrange or high-midrange, that will help it sound clearer. Indoors the walls reflect your sound energy, making everything louder and causing feedback. 100 watts can be adequate indoors. Outdoors the sound energy just keeps going, and the audience may spread out over a much wider area. 300 watts or more is good for smallish outdoor events. In all, outdoor is probably easier. Wider rooms are nicer than narrow rooms. If you have a long narrow room, with the speakers at one end, then the sound near the speakers will be too loud, and the sound at the other end too soft, and you will have feedback problems too. Lots of people, and lots of talking only makes things worse. Best to set up at least one speaker halfway down the room, and make sure the Mikes are spaced 15 feet from any speaker. Most newer sound systems have a LED light to show when they are clipping or distorting. This is when you have the volume turned up too high, and the amplifier cannot produce that much power. It tries anyway, and fails in an ugly way, distorting the sound. Turn it down. It does no good to loud and distorted, people can't hear the music. Better to be softer and clearer. Electric guitar amps have a have an amplifier and a speaker built into one box. Their speakers are purposely designed to not put out very much high-high frequencies. They usually have 10, 12, or 15 inch speakers, and no tweeter. This is because guitar distortion effects can produce way too much high-high frequencies, and the large speakers eliminate most high-highs, making it sound smoother. But if you put a voice through that guitar amp, it will loose some clarity due to the missing high-highs. Acoustic-Electric guitar amps are different. They have a tweeter in addition to the main speaker. They may sound harsh when used with a guitar distortion effect, but they are designed to amplify voice clearly, as well as the 'clean' guitar, therefore they function as a small, easy to transport and set up PA system. It is best to get one with an extra speaker box. Having two speaker boxes allows you to spread the sound more evenly in the room. Well, I hope you find this info useful. If I wasn't clear on anything, email me. And Good luck. Home |