Convolution reverb imposes the characteristics of a real-world reverberant space on an input signal. To create convolution reverb, an impulse response (IR) signal that is recorded in the reverberant space is convolved with the input signal. You can go well beyond the confines of reverb processing, however, by substituting other audio material for the IR signal (see Web Clip 1).
FIG. 1: Top to bottom: shaku sound-file window; hallIR sound-file window; convolution options settings.
I'll start with an example of stereo convolution reverb, and then demonstrate how easy it is to generate a much less conventional reverblike effect using a sample of a crash cymbal in place of the IR signal.
Many sample editors and other audio applications now support convolution. Here, I'll use SoundHack (www.soundhack.com), Tom Erbe's Mac OS shareware utility. SoundHack is easy to set up and use, but you should keep two things in mind. Under OS 9, your should set the memory partition to at least 16 MB before performing the examples here. Under OS X, SoundHack may not recognize your audio interface, in which case you'll need to temporarily set the Default Output to Built-in Audio in the Audio MIDI Setup utility.
You'll find my audio source sound files on the EM Web site (see Web Clip 2). For more details on convolution theory, see “Square One: Convolution Number Nine” in the June 1999 issue of EM. For a survey of convolution reverbs, see “Trading Spaces” in the October 2004 issue, available online at www.emusician.com.
For convolution reverb, I'll convolve a dry shakuhachi sample with an IR taken from a large hall. I created the IR sample by recording 2.6 seconds of reverberation from a short (roughly 100 ms) noise burst.
First, open the file shaku.aif in SoundHack and play it back by pressing the spacebar. Notice how the signal transitions from a clearly pitched narrowband spectrum to a noisier, breathier broadband spectrum.
Next, open and play the file hallIR.aif. Notice that the spectral energy is distributed fairly evenly across a broad frequency range, much like pink noise. That is desirable in a convolution reverb IR signal.
Convolution analyzes and multiplies the spectra of two signals. This process reinforces frequencies that are common to both signals, while attenuating frequencies that are not. There will generally be many common frequencies when a broadband IR spectrum is convolved with another signal's spectrum, and that will tend to preserve the attributes of the second signal.
Getting the Hack of It
Convolving in SoundHack is simple. First, activate the hallIR.aif window and select Convolve from the Hack menu. When the Convolve With Impulse Response window opens, select the Normalize checkbox option and leave all other options at their default values. Next, click the Pick Impulse button and, using the file selection dialog box, select and open shaku.aif. You should now have three windows visible (see Fig. 1).
Click the Process button to start convolution processing, and select a name and location for the output file in the Save dialog box. Click Save, and in a few seconds the convolved sound file will open. Press the spacebar to play it back.
Next, I'll use a cymbal crash as an IR instead of the hall recording — an effect I call CymbalVerb. First listen to the sound file called crash.aif and notice how similar it is to the hall IR, containing an initial impulse followed by a long decay. Its spectrum is also similar but confined to a narrower frequency band, much like highpass-filtered noise.
Repeat the previous convolution steps, substituting crash.aif for hallIR.aif. The result is on the borderline between a very strange reverb (shakuhachi played inside a cymbal) and a hybrid instrumental sample.
Try using other instrumental sounds as IRs. Virtually any signal can function as the impulse response of some space, even if that space is imaginary.
John Duesenberry would like to hear about your convolution successes. You can email him firstname.lastname@example.org.