What Is Convolution Reverb?

We’ve all heard about convolution reverbs, but what exactly are they and how do they work?

A Reverb primer

There are five basic types of reverb used in music production.

  1. Natural reverberation; recordings made in real-world spaces like chambers and concert halls, often these spaces are designed to be acoustically pleasing
  2. Plate reverb; a suspended rolled steel plate set resonating with a set of transducers (speakers to excite the plate, pick-ups to capture the reverb)
  3. Spring reverb; from Hammond organs and guitar amplifiers to large bespoke designs; requires transducers similar to a plate reverb
  4. Algorithmic synthesis; attenuated delay loops with injections and reads at various points, beginning in the 1970s with devices like the Lexicon 224 and EMT250
  5. Convolution reverb; a sampled capture of reverb is applied to an audio stream digitally

What is convolution reverb?

Convolution is acoustic photography. It captures the sound of a room or effects processor. This capture could be the spectral imprint of an equaliser, the tone of an amplifier or the time and colour of decay in a reverberant room or effects processor.

What is an impulse response?

An impulse response is the film of acoustic photography. It is where we capture the response of a reverb or any other musical processor to an input signal over time and in frequency.

Impulse responses are gathered in many ways.

  • Recordings of short impulsive sounds like a gunshot or balloon pop which capture an impulse response directly, but are highly susceptible to colouration and background noise
  • Noise sequences ranging from a ‘pinking’ signal to more complex linear sequences with desirable properties that require special post-processing to extract the impulse response (known as deconvolution)
  • Tone sweeps running from the lowest to highest frequency one wishes to capture which also require deconvolution; many variants of the progression of this tone in time and amplitude exist

Any of these sequences can be used to capture digital or real-world sources, although some are much better suited to a given source than others.

The real art of impulse response capture is in knowing the best technique for a given source, particularly in the digital domain.

A key differentiator with LiquidSonics reverbs, and Fusion-IR in particular, is the care and attention taken to sampling the sources is second to none. Various novel techniques were developed for sampling Fusion-IR reverbs, and the results speak for themselves.

The challenge of modulation

Digital reverbs are prevalent in studios around the world and have been used on famous recordings for decades, so there is much interest in capturing them for reproduction. They represent a unique challenge because many of them are modulated.

Such modulation could be used anywhere from the delay loop length, injection and read points, or within the tiny sub-loops (known as all-passes) often used to increase reverberation density. These techniques are used extensively in most reverb designs that need to run on a device with limited computing resources. Only using a handful of reads and writes in a delay loop cannot generate a convincing reverberation due to audible repetitions and resonances that quickly emerge. The process needs thickening and blurring to suppress resonances from building up, so modulation is used to solve this problem.

Modulation in reverberant environments is not something we are traditionally used to hearing because real rooms would have no perceptible time-based variability in them unless the room is physically changing. Any movement of air or people within a room is typically imperceptible. The modulation of classic reverb devices has, however, become something we expect to hear in reverbs used for music as it is psychoacoustically very satisfying. It helps synthetic reverb blend with its source and gives a reverb tail a lush organic quality.