Convolution Reverb Explained
Home < Pro Audio < B&H Email Newsletter <

Convolution Reverb Explained

By Ken Hamberg

What does a drum or piano sound like in a stairwell?

As a rookie studio engineer and programmer in the 1980's I remember being amazed and buffeted by the daily intrusion and profusion of digital music-making machinery, from the MIDI protocol to digital "black box" reverbs and delays, sampling keyboards and drum machines. The influence on the recording process was immediate and irreversible. Sampling in particular quickly developed into an art form, resulting in far-reaching legal disputes and a boom time for manufacturers, as the sampler became a required fixture at any competitive recording facility.

One day it occurred to me that if you could sample a drum set or a piano or a French horn, it would also be very cool to sample the sound of the stairwell or wood-paneled room or iso-booth in which that instrument had been recorded. I mentioned this to several friends in the business, and the response was largely unprintable.

Apparently other people had the same idea, however, and the sampled reverb is now a reality in the New Millennium, available to virtually anyone with a computer and an ear for the esoteric in the realm of acoustic spaces.

Convolution Revolution

A Convolution Reverb consists of a recorded sample (called an Impulse Response or "IR") of an acoustic space to excitation from a signal such as a sweep tone, starter gun, or snare drum crack, and the effect on the space of that signal after it has been removed and usably transformed by the convolution processor. Convolution reverbs essentially record and process the reverberant behavior unique to a real acoustic space.

acoustic space

Convolution in everyday speech is defined as a coiled or twisted condition or appearance; in mathematics the term refers to the result of the intersection of two functions, one of which is known as a Fixed Filter Impulse Response (sound familiar?) and the second of which has been reversed, producing a third function. Convolution reverbs are so-called because the sequence of signal "events" is reversed: A space is excited by a signal and recorded, and the resulting sound or ambience of that space is then processed and used to treat and react to an entirely different signal.

The original convolution reverbs were fantastic-sounding but exorbitantly-priced hardware boxes, and as such largely an elite studio commodity. By now this technology is available as computer software at a fraction of the original cost, and may be used the same way any other DSP plug-in is used in signal processing. In short, you can sample your favorite cave, closet, or garbage can and use it as a reverb effect on a lead vocal or acoustic guitar track recorded in your software of choice. Or, you can simply select a preset from the large sampled-space libraries that accompany most convolution reverb packages.

Algorithms De-mystified

replicating acoustic behavior

The essential difference between this reverb technology and the digital reverbs we're used to as musicians and engineers lies precisely in the fact that the ambience is that of an actual acoustic space, not an artificial or ideal one created, however brilliantly, by a programmer using a series of algorithms designed to replicate acoustic behavior.

An algorithm is a set of successive mathematical or verbal instructions, a sort of recipe, compiled to accomplish a task or solve a problem, progressing logically from a beginning to a conclusion.  Algorithms are essential to computer programming, and the instruction-set manner of code writing has found its way into both software and hardware effects processors and sound modeling design.

Algorithms are also used by convolution reverb developers, but mainly for the removal of the original "trigger" signal and other analytical or clean-up tasks necessary to complete the recording process and reveal the space, not create it. Thus the remarkably realistic sound and detail from this reverb technology: It is, again, a recording of an actual and specific space, not strictly a representation or simulation of one.

Spelunkers with Microphones

How does one go about sampling a space? Well, it is a recording, and therefore involves a microphone (or one or two stereo pairs depending on the number of output channels desired), a recording device or rig, a pair of high-quality speakers, and either a playback device such as a CD player, or a waveform generator. Most engineers opt in favor of either a starter gun or a sweep tone as a means of creating some acoustic action.

sampling a space

The sine wave sweep tone has become the recommended method of exciting an Impulse Response in most spaces because of both the reliability and range of the frequency response it causes as the room reacts, and the ease of perceiving a distorted sine wave through the speakers while optimizing recording levels. The sweep tone is generated and played back through the speakers, triggering a reverberant response in the space which is then recorded. The takes or samples are then convolution-processed and edited as sound files, the sweep tone is removed from the recording, and there is your church, bathroom, or stairwell, ready for use in the mix.

Great, Dude! But can we just tweak it a little?

So, after much trial-and-error you're back in the studio with a gorgeous sample of a marble office building lobby, apply it to a lead guitar track for some serious ambience, and it doesn't work.  Are you stuck with it?  Not at all. The current crop of IR-based convolution reverb developers generally includes a large set of after-the-fact editing parameters as deep as any of the higher-end black boxes of yore, with the same algorithms and user interface deployed for programming "conventional" hardware digital reverbs. So that marble room can be modified in size, length, amount of pre-delay, equalization, and so forth while retaining its original sonic integrity and character.

An IR-based convolution reverb developer

I should also add that many users sample borrowed, rented, and even their own hardware reverbs as well, using the same IR techniques and convolution processing with excellent results, while compiling a library of classic reverb sounds from units most of us could never afford to own.

Sampled instruments and sound effects have become a required part of the music and film production process for years. Now we can create a collection of sampled acoustic spaces to put them in.

I want one! Who's got one?

Here's a list of the premier developers of (mostly) cross-platform convolution reverb plug-ins and their products, all of which are available at B&H, in no particular order of preference:

Audio Ease Altiverb 6

Audio Ease – Altiverb 6: One of the pioneering software developers of the convolution reverb. Sounds great, looks great, superbly documented and easily-operated, with a large library and deep editing capabilities. Serious CPU horsepower recommended.

Trillium Lane Labs TL Space Native Edition
Trillium Lane Labs - TL Space Native Edition: Another pioneer in the field, now distributed by Digidesign. Nice sound, nice graphics, easy-access virtual control surface brings some fun and familiarity to the editing process.

Waves - IR1:  Comes in Light and Surround versions as well. Possibly the most ambitious and sophisticated parameter control out there, with in an intuitive if busy operating environment and a large beautifully-executed sample library of both real spaces and hardware classics.

Waves IR1
McDSP Revolver

McDSP – Revolver: Mac only, unfortunately. Fast processing, surprisingly light on the CPU load, heavy on instant gratification in terms of sound quality and editing response. Advanced EQ and a 3-band crossover are featured in the parameter list.

If you have any more questions about convolution reverbs, we encourage you to contact us on the phone, online, or in person at our SuperStore in New York City.1-800-947-9923

Top Pro Audio categories:

Recording | Desktop Audio | Keyboards & Synths | ENG, EFP & Broadcast | Live Sound & PA Accessories

Please email feedback on this article, or suggestions for future topics, to