Okay basically an impluse is like a complex reverb. The way it works is this:
You play a sound into the room (normally a sine wave, for reasons explained in a second). You record that sound, and put it into a deconvulver - because you know you played a sine wave at a certain frequency, it can easily be removed from the rest of the recording. What you're left with is a representation of how the room sounds - the impulse. So if you then say, record a dry drum track, you can put the impulse on it and it will sound like the drum kit was in that room.
Think of it like a stencil. You have a plain piece of white paper, then the stencil on top of it. You spray over the whole thing with black paint - the paint is you dry signal. When you pull the stencil off, you have a black shape on the paper - that's your wet signal. The stencil is the impulse - it stops all the stuff you don't want getting to the end result. You can put any kind or colour of paint you want on top of it, but you'll always get the same shape out. Obviously with sound it's more complex because rooms can add to a frequency range as well as subtract from it, but it's a similar principle.
In this case, it's used like a cabinet simulator. Although technically you're recording how the cab sounds in that room, because you close mic cabs (generally) you don't really get any room sound - instead you just get how the cone sounds. So you can then record your guitar into your amp, and straight into the desk - like a DI signal. But when you put the impulse on it, it sounds like it's being played through the cab.
Hope that helps.
I actually have a somewhat cunning idea relating to impulses - does anyone know a plug-in or something that you can use to produce a sine wave? If my plan works, it could turn out to be very very cool...
Steve
