Convolution reverb explained: impulse responses, and when to use one

A convolution reverb puts your sound inside a recording of a real space. Instead of building a room out of delay lines, it uses an impulse response (IR): a short recording of how a hall, studio, plate or even a cardboard box answers a single sharp sound. Ableton's manual puts it simply: a convolution reverb "uses recordings of actual spaces (called impulse responses) to create its effect" (Ableton).

If you are new to reverb controls such as decay and pre-delay, start with our reverb and delay basics guide. This one goes one level deeper.

What an impulse response actually is

Think of clapping once in a stairwell. What comes back (the first slap off the walls, then the long wash) is the stairwell's fingerprint. Record that, and you have an impulse response. Convolution is the maths that applies that fingerprint to every sample of your vocal or snare, so it sounds as if it was played in that stairwell.

The technique is older than most plug-ins suggest. Convolution "uses impulse responses to record the reverberation of physical spaces and recreate them digitally", and Sony announced the first real-time convolution reverb processor, the DRE S777, in 1999 (Wikipedia). In film production, sound engineers record impulse responses of sets and locations so that sounds added in post-production get realistic reverberation (Wikipedia).

The controls, using Hybrid Reverb as the example

Ableton's Hybrid Reverb pairs a convolution engine with an algorithmic one, which makes it a clear way to see what each does (Ableton):

• Attack, Decay and Size shape the impulse response itself. Attack and Decay set the attack and decay of its amplitude envelope, and Size changes its relative size. In plain terms, you can shorten a cathedral without finding a new recording.
• Predelay is the time before the first early reflection. The manual gives 1 ms to 25 ms as the typical range for natural results.
• Routing decides how the two engines meet. Serial "routes the output of the convolution engine into the algorithmic engine"; Parallel runs both separately; Algorithm uses only the algorithmic side.
• Blend balances convolution and algorithm in Serial or Parallel mode.

Loading your own IRs

To use your own impulse response, drag the audio file onto Hybrid Reverb's convolution waveform display. Drag a file from a folder with other audio files in it and all of them are added. One catch the manual flags: these User IRs are lost if you delete the device and add it again, so keep your IR folder somewhere you can find it.

What we think: when convolution earns its place

Convolution is best at realism and specificity. If you want a vocal to sound like it was in one particular room, or want to drop an overdub into the same space as a live recording, an IR of that space is hard to beat. It is also a sound-design tool: run a pad through an IR of a spring, a guitar body or a metal bin and you get textures an algorithm will not give you.

Algorithmic reverb is usually better when you want to tweak and move: long, modulated tails, or a reverb you automate heavily through a song. An IR is a snapshot, and stretching it far from how it was recorded tends to sound less natural than dialling a room from scratch. Our rule of thumb: use convolution for the "where" (a real room, a real plate), algorithmic for the "effect" (a lush, obviously artificial wash), and a hybrid when you want a real-sounding front with a controllable tail.

Two practical tips that hold for any convolution plug-in:

1. Record your own. A clap or a balloon pop in your kitchen or stairwell, trimmed and faded, is a usable IR and a sound nobody else has. It will not be studio-clean, and that is part of the point.
2. Filter before the reverb. As with any reverb, cutting low end going in keeps a realistic room from turning into mud under the kick and bass.

Quick summary

• An impulse response is a recording of how a space or object responds to a single sharp sound.
• Convolution applies that response to your audio, which is why it sounds so real.
• Shape the IR with attack, decay, size and pre-delay; pair it with an algorithmic reverb when you need movement.
• Use it for realism and sound design, and send to it on a return track rather than inserting it on every channel.