You've got your amps, you've been practising in a garage or small rehearsal room and things sound pretty tight. Now you've got a occasion to play down at the local pub/bar. How difficult could that be? Well, if you feel that all you have to do is set up and play just as you've been practising then there is a 99% occasion of disaster. Many bands sound awful at their first gigs because their Pa isn't sorted; they find themselves playing out of time and out of tune.
Why? It's monitoring - or to be spoton a lack of it! Your singer will only be able to sing in tune if he can hear what he is singing. Your guitarists will only bend notes accurately if they can hear their own guitars. Bass and drums will only lock in if they can hear each other, and each of you will only know where you are in a song if you can hear the rest of the band. It's wholly distinct to the confines of the garage or institution space and this can categorically throw you all. You will be struggling to hear the extensive sound and may even have issue in properly hearing your part of it. No matter how good you all are as musicians, no matter how much you've practised, without good monitoring you will end up out of time, out of tune and wishing you'd stayed at home. When the on-stage sound is wrong, amplifying it will just tell the audience how wrong it is.
For bands of three to six citizen with galvanic guitars and bass, a drummer, a singer and possibly a keyboard, playing in small venues to audiences of less than 200 (a typical start up scenario) your amplification needs are threefold:
1) Back line - this includes your guitar and bass amps, your drummer
2) Monitors - these are on-stage speakers relaying "fold-back" - your own sound
3) Pa - the front of house sound which the audience hears
Notice that 1) and 2) originate your on-stage sound whilst 3) delivers the on-stage sound to the front of house. Lets take them one at a time:
Backline
You will need 30 to 50watts Rms for your guitar to match the drums. The bass will need 50 to100 watts. If you are using keyboards they will need 100 watts as a back-line instrument but if you pick to put the keyboards directly through the Pa to the front of house, bear in mind that without a mixer they will be a distraction for the vocalist. Those pieces of kit which do not have a volume operate - the drummer and the singer - now need to be considered. The vocals will absolutely need a ideas that delivers over guitars, bass and drums to a room full of noise enchanting citizen (and audiences digest the higher frequencies more efficiently than the lower ones), so Pa's are first and leading the province of the vocalist, but also for keyboards and, if you chose to microphone them, the drums.
Monitors
You need stage monitors so the singer can hear themselves and the rest of the band can hear the singer. A monitor is a speaker, often quite small and wedge shaped, which can be pointed at the singer without hiding him and ordinarily raised at the front edge by a stand (or propped up by an old brick) to project better. Add more monitors so the rest of the band can hear the singer and position one of these next to the drummer. You'll need a separate power amplifier to drive the monitors although it needn't be as suited as a Pa power amp, and if you have extra acoustic instruments you will need to put these through the monitors too. Look for monitors of 100W, you can get active ones with built in amps or go for a monitor amp with separate speakers. Understand, too, that your guitar and bass amps are, on-stage, monitors. You need to set these so that you can hear yourself and the other band members, and that they can hear you as well as themselves. This is the trickiest part and when sound wars break out on stage, often a competition over being loudest, it's the audience that suffers. Don't make the mistake of turning up your amps to impress the audience as you will just be unbalancing the band's sound. In very small venues and for generally acoustic bands, the monitors can be angled to spill sound to the audience.
You now have operate and equilibrium issues to consider. Decision one: either you equilibrium the back line to the drummer or you microphone the drums and equilibrium through the Pa. There are compromises potential when playing small venues where it may be productive to naturally microphone the kick drum.
Pa
To the singer, the Pa is what an amplifier is to a guitarist, but it's much more besides. What goes to the Pa goes through a mixer. These days a mixing desk capable of handling the whole band can be picked up for the price of a low mid-range guitar. Given that many venues, even smaller ones, now have their own Pa systems, along with a mixing desk, you will need to come to terms with these either way. At the basic level if you only have vocals going through the Pa you will only need 100W per channel and a integrate of full range speakers with stands. The speakers will have a ten, twelve or fifteen inch bass speaker and a horn to handle the high notes. You'll need to raise the horns above the audience or the citizen at the front will digest all of the treble and the rest of the room will get mush. This is why most Pa systems highlight stands, so use them! Make sure the cabinets are well to the front of the vocalist or your back-line sound could overpower the vocal mike and set up a howling feedback loop. If you've got your on-stage sound right then you won't get this problem from the back-line but if you do then you now know the answer. Turn down the back-line!