The bottom line is for this direct sound from the IR is just EQ. It's just processed through convolution.
Impulse responses are recordings.
Impulse responses originally they were a "Bang" like that of a starter pistol
The engineers would record the starter pistol bang with a known mic in anechoic chamber to get a baseline.
When you break the bang down you'll find it's made up of a bunch of sine waves. All sound it built up from sine waves. You can pick any frequency and measure how loud the sine wave at the frequency.
Then they'd take the pistol and mic to a big hall and fire and record it there.
A few things come back, first is the direct sound from the pistol shot. The rest is reverberations/reflections.
Ignore the reverberations/reflections for now.
The direct sound is going to be like the original except the sine waves have different amplitudes. This represents the EQ the room has on the bang.
This second recording can be fed into a "convolution reverb" along with your own signal and get the results of the room eq.
It turns out that a frequency sweep of a sine wave from low to high (or high to low) can do the same thing as a bang for most IRs. A sweep has the frequency and loudness at each point in frequency in the direct sound. This when fed into a convolution reverb does the same thing as a bang. Listen to an IR and if you hear a swoop - then it's a sweep. Load it into audacity to see if it's a sweep or bang. Both work.
For the reverb part, that's reflections and resonances (modes) of the rest of the sound. There are very expensive IR that are essentially famous recording studios that you can use in your own recordings.
For a cab impulse, everybody wants some room in there too. It doesn't need to be added at the same time, it can be added on a master bus to get all the instruments in the same room.
