Convolving a sound with a space's impulse response places that sound acoustically in the space

A room’s acoustic signature can be captured as an impulse response (IR): the sound the room produces when a very brief, broadband impulse — a starter pistol, sine sweep, or electronic click — is played in it. The IR is a time-domain signal containing all the room’s reflections, early echoes, and decay pattern. A reverberator is thus a filter with a long impulse response. Convolving any dry signal with this IR maps the sound into the room’s acoustic signature: by the convolution theorem, the output sounds as if the source had been played in that room — its spectrum filtered by the room’s frequency response and its time structure smeared by the room’s echo pattern, so the reverberation time of the result matches the IR’s decay. This is convolution reverb, and it can apply any real acoustic space to a recording with high accuracy. The same principle generalizes to cross-synthesis: the IR can be any signal, not just a room, so convolving two arbitrary sounds imposes one’s spectral/temporal characteristics onto the other.

Examples

Convolve a dry snare with a cathedral IR and the snare sounds as if played in the cathedral. A dry guitar convolved with the Sydney Opera House IR sounds as if played there. Convolve a Rhodes with a bell IR and it acquires bell-like resonances. IR databases (e.g. OpenAIR) provide thousands of real-space IRs; plugins like Altiverb and Reverberate 3 implement convolution reverb.

Assessment

Explain why convolving a dry recording with a room IR produces realistic reverberation, and what determines the reverberation time of the result. Given a cathedral IR with a 4-second RT60, describe what happens to the time structure and frequency content of a dry drum hit convolved with it.