So it helps to understand a little bit about how a normal guitar amp works. Very simply, the signal flow is like this:
Where EQ is any bass, mid, treble controls (this is also sometimes called the “tone section”). Sometimes the EQ is in different places, depending on the exact amplifier.
The important thing here is the speaker.
After all the stuff that happens before it, if you don’t go through the speaker, it’ll sound really gross and buzzy. A typical guitar amplifier speaker is not a terribly accurate speaker, in the grand scheme of things You have to remember that even the ones today are modeled after loudspeakers from the 40s and 50s.
The cabinet the speaker is in and even the room that the cabinet (and speaker) are in affect the sound. For example, your speaker might have a really low bass response, so you can think of it like running through an EQ where it reduces specific bass frequencies.
So, you can think of the speaker, cabinet, and room as a sort of combined effect.
Together, they are the final thing shaping the sound and, in the audio world, later parts of the chain get priority on having the most effect on the overall sound.
Because the reverberations in the cabinet and room are time based, you can think of it like a very specific EQ that is changing over a (very very short) slice of time (usually on the order of milliseconds to seconds).
An Impulse Response (IR) is a special audio file that aims to capture that. When you load the IR into special software called a “convolver” and send your guitar signal through it, it is as though you were playing through that speaker.
However, IRs capture more than just the speaker. In reality, they capture the effects of the speaker, the cabinet, the room, the microphone, and the position of the microphone in relation to the speaker
What’s really cool about IRs is that you can run a guitar signal straight in with no other effects and, depending on your IR, it should sound like a really clean guitar amplifier. If you want a distorted sound, all you need is a source of distortion, which could be one of thousands of things.
More and more, we are seeing a lot of new guitarists and many are foregoing traditional guitar amps altogether. Especially with younger players who might not be as interested in recreating traditional guitar tones, they might just load up an IR and some other effect in their DAW. No “amp” required.
Also, there are actually pedals that are really just IR loaders. However, it’s important to note that actually running an IR can be pretty processor intense. It’s nothing that even a cheap laptop can’t handle, but definitely a lot for a pedal.
Also another thing to note: not all “cab simulation” is done via IRs. IRs are just a single form of cab simulation. If you see an amp that has headphone outputs “with cab simulation!” it might be IRs… but probably not, unless it explicitly says so.
Overall, IRs are largely the reason digital amp models are so good in recent years. Again, the last thing in the chain has the most potential effect on the sound, which is the speakers. For the longest time, we might’ve had absolutely perfect digital recreations of pre-amps, but awful cab sims. So the whole thing sounded “digital” and awful.
With the existing prevalence of IRs and their rapid introduction in more and more gear, we are entering a golden age of digital guitar tone.