You Say You Want a Convolution. . . .

Acoustic spaces create the most natural reverb, but there’s only one “preset” — and try fitting a concert hall in your project studio. Granted, some people run mics and speakers to a tiled room (e.g., bathroom) for some decent, tight-sounding reverbs. But emulating the classic concrete room sound that was on so many great recordings, let alone other acoustic environments, is not an easy task.

Old Skool DSP

Before checking out convolution reverb, consider synthesized digital reverb, which has ruled for the past couple decades. This generally breaks down the reverb effect into two processes. The first is “early reflections,” the initial sound that happens when sound waves first bounce off of various surfaces. Then comes the reverb “tail,” which is more of a wash of sound caused by feeding these reflections into an engine that calculates and synthesizes a gazillion reflections, with their various amplitude and frequency response variations.

Many, if not most, digital reverbs are not true stereo devices; they sum stereo inputs into mono, and synthesize a stereo space. Unlike “real world” reverb, though, where you hear different reverb effects from different sound sources in a space, DSP produces a “one size fits all” reverb that subjects all sound sources — regardless of location — to the same reverb effect. While most of the time this is okay, for complex orchestral emulations, standard reverb algorithms lack precision.

Enter Convolution Reverb

Convolution reverbs are based on samples rather than synthesis (see Jargon Jockey), which produces a surprisingly realistic sound. The tradeoff has traditionally been the usual sampler vs. synthesizer issue: lack of parameter control. But just as some companies have figured out how to get “inside the sample,” convolution reverbs are getting more flexible as well.

Waves broke through with their IR-1, which allowed tailoring the sound produced by the impulse. Now most convolution reverbs are quite editable, and as easy to use and understand as standard reverbs. You may notice that changing parameters feels a little slow due to all the calculations being performed, but this isn’t a big deal.

As to the impulses, they’re created by recording a space’s reverberant characteristics after “exciting” the room with a set of sweep tones designed for impulse recording, or firing a shot from a starter pistol. The object is to use a trigger that generates sound throughout the entire audible spectrum, to allow capturing the space’s total frequency response.

The Rest Of The Story

Convolution-based processing slurps CPU power, but fortunately, host software “freeze” functions (which apply effects to a hard disk track, then “disconnects” the effects) tend to make that less of a drawback, even with slower CPUs.

Another problem is latency, which is added by the convolution process. For reverb, it’s not too serious; think of it as free pre-delay. Early convolution reverbs used to have latencies in the hundreds of milliseconds, but many now hit under 5–10ms with a fast enough CPU. If the delay is problematic, you can bounce just the processed sound to a track, and slip it forward in time.

Finally, a convolution reverb is only as good as its impulses: If someone recorded an impulse with a tinny-sounding mic, you’ll have a tinny-sounding room. You also want a reverb package that supplies a lot of impulses, so you can really discover convolution reverb’s power. However, note that you can download free impulses from www.noisevault.com, and some are quite good. The site also hosts discussions and news about convolution-based reverbs.

But whether you use a stand-alone convolution reverb, the one bundled into a host program, or even one included with an instrument like Gigasampler 3 or Kontakt 2, you’ll find convolution presents the most convincing reverb emulation yet.