Impulse response

For a linear, time-invariant system (a room, a guitar cabinet, an analogue filter), knowing its response to an ideal impulse is enough. Everything else follows, because any signal can be treated as a stream of scaled impulses. Convolve a dry recording with a cathedral's impulse response and it sounds as if it was played in the cathedral. That is the whole trick behind convolution reverb.

Nobody fires starter pistols or pops balloons for this any more. Since Angelo Farina's 2000 method, you play an exponential sine sweep through a speaker and compute the response from the recording, which buys a better signal-to-noise ratio and separates out the speaker's distortion. The limitation comes baked in: an impulse response captures nothing non-linear, so a valve amp's saturation can't be frozen this way.