What a cabinet impulse response actually is

Everyone selling amp software says "impulse responses" as if the phrase explains itself. It does explain itself, but only after someone walks you through the three steps once. Amp Rig is a convenient teaching example because its cabinet stage is exactly this and nothing more, so here is the walkthrough with audio at each stage.

Step one: the sweep

Put a speaker cabinet in a room, put a microphone in front of it, and play a signal through it that contains every frequency you care about. The usual choice is a sine sweep that glides from around 20 Hz to 20 kHz over several seconds. You record what the microphone hears.

What the microphone hears is not the sweep. It is the sweep plus everything the cabinet, the speaker cone, the microphone, the mic position, and the room did to it. The cone rolls off the top end hard. The cabinet resonates somewhere in the low mids. The room adds early reflections. All of it is baked into that recording, indivisibly, which turns out to be the useful part.

Step two: deconvolution

You now have the input (the sweep you sent) and the output (what came back). You want the thing in the middle — the transformation the system applied. Recovering it is deconvolution, and for a sine sweep there is a well-behaved way to do it: convolve the recording with a time-reversed, amplitude-corrected copy of the sweep, and the whole excitation collapses into a single point in time. What is left around that point is the system's response to an ideal impulse.

That result is the impulse response. It is a short audio file, typically tens to a few hundred milliseconds, and if you play it you hear a brief click with a tonal quality to it. It sounds like nothing. It contains everything.

The sweep method is used instead of literally firing a starter pistol at the cabinet because a sweep puts far more total energy into the system while staying inside its linear range, which is the difference between an IR with usable signal-to-noise and one that is mostly hiss.

Step three: convolution

Playing guitar through that cabinet is now a matter of convolving your signal with the IR. Every sample of your input scatters a scaled copy of the impulse response across the output, and the sum is your guitar as that microphone would have heard it in that room.

Done naively this is expensive — every output sample costs one multiply per IR sample. Real-time convolution uses partitioned FFT blocks instead, splitting the IR into chunks so that early chunks are processed at low latency and later chunks are handled in larger, cheaper blocks. Amp Rig does this in the kernel, once, and both the browser and plugin builds get it because they are the same code.

Why a 4x12 flatters a bad take

Here is the part that is actually useful when you are recording rather than reading.

A guitar cabinet is a fairly brutal filter. A 4x12 rolls off sharply above roughly 5 kHz and has very little below 80 Hz or so. The fizzy, glassy top end that distortion generates — the part that makes a raw amp sim sound like a wasp in a can — lands almost entirely in the region the cabinet throws away. Pick noise, string squeak, and the harsh edge of a slightly-flat bend all live up there too.

So the cabinet is not a flattering EQ curve added to good playing. It is a band limit that removes most of the frequency range where inaccuracy is audible. Play a sloppy take through a raw distortion stage and you hear every fault. Put a 4x12 after it and a good deal of that evidence is simply gone.

This is worth knowing in both directions. It means a distortion sim judged without a cabinet is being judged unfairly. It also means that if your take sounds great through a heavy cab and thin through anything else, the cab was doing more work than you were.

Bring your own

Amp Rig's IR slot takes your own .wav impulses, in the browser as well as in the plugin, and the file stays on your device. Capture your own cabinet with the method above and you can play through your actual amp in a tab on a laptop. That is a strange and good thing to be able to do.