Understanding room acoustics
Part 1 - Boundary reinforcement
Continuing on from stage or floor acoustic coupling, also known as boundary reinforcement, there are a number of other boundaries in venues that you should consider, most importantly the back wall behind the stage and the side walls either side of the stage.
If these are solidly built walls (not flimsy partitions) they too will be 'seen' as boundaries by the low frequency output from your speaker cab. So if we return to that image of the sphere of sound surrounding the cabinet, picture what happens to the half-sphere created by placing the cab on the floor when we move the cab backwards so it is close to the back wall. The half-sphere is now a quarter-sphere and the sound that was previously going backwards is now going forwards and this reflected sound is reinforcing the direct sound from the cabinet. This gives us another low frequency gain of ~3dB.
Now push the cab sideways so it is the corner of the room and the quarter-sphere becomes an eighth-sphere with another ~3dB gain in output in the lows. Just as when you lift a cab off the floor the frequency below which you gain reinforcement drops, when you move a cab away from a wall the same effect occurs. This gives you an excellent way of acoustically controlling the low frequency sound from your cabinet without using EQ.
Part 2 - Boundary cancellation
So that covers how acoustics can reinforce your sound but there is another issue to consider - those horrible moments when acoustics actually cancel out part of your sound! Again this is to do with direct and reflected soundwaves colliding.
To understand this we need to firstly consider what a soundwave actually is - the best way to do this is to picture is as being like waves on the open sea, rolling up and down with peaks and troughs. When a reflected peak meets a direct peak then they combine and you get a peak that's twice as high (3dB). When a reflected peak meets a direct trough or when a reflected trough meets a direct peak you get a deep narrow cancellation, usually about 18dB deep.
So how do we know when we're going to get reinforcement and when we're going to get cancellation? Well it's all down to the distance between source (speaker cab) and the reflector (wall) and the wavelength of the sound.