Let me respond to the compressor discussions above. Yes, compressors are used for Rock Music but they do have their place in acoustic music reinforcement, and almost exactly in the situation described by Seamus Kennedy. If the sound is set at a reasonable volumn with realistic headroom, and one performer decides to increase his volumn for "dramatic effect", it is still there; just not at the rate he expresses, and certainally not at an input which would cause the amps to blow the speakers.

You need to remember that if you have set your levels well after a realistic sound check, you most want to ensure that the noise floor is extremely low so that during the quiet sequences of a given piece of music, you do not pick up what is often called "hash" "frying bacon" or "pink noise", but in reality is the 60 cycle hum (in the US) or any of its resonant/harmonic cycles, such as 120, 240, 480, etc. or input from the lighting system or a bad ground.

(Several people have objected when I suggested a ground lift to reduce "hiss," but the voltages in the US are not the same as in Europe, and for about $9 you can by a two pole 15 amp GFI which will allow you to plug a three prong plug into a two prong adapter and then into this GFI, hence to the power source, with no danger, and a significant loss of 60 cycle hum. For you European types, they may manufacter a similar item in your voltages, as a retrofit GFI for "older homes' as this one is marketed in America.)

When I said "soft knee compression" it is clear that several individuals who objected have no idea what I was saying. Without getting into a long side-bar, it is a ratio of reduction which gives plenty of room for "dramatic effect" but the draw-down is at a lesser rate than for other forms of compression.

Remember that at the end of the day, that "dramatic effect" may be completly limited by the amp, without the sound engineer doing anything at all; kind of like crashing into a brick wall. Almost anyone has heard it, as it develops into incoherent distortion. What a compressor does is slows the approach to the brick wall without the distortion (providing it is set at a proper rate of attack, decay, and the ratio is "soft" but within limits of the amp/speakers outputs.

Hope this clears things up.