A standard digital reverb uses a set of algorithms to generate a reverb tail from an audio input. An old school spring reverb sends your audio along a spring in a metal box causing sound reflections, and then adds those sounds to your dry audio. A plate reverb is a similar principle, but runs audio through a suspended sheet of metal. There are a few other variations, of course, but let’s talk about the type that causes the most confusion: convolution reverb.
Convolution reverb uses an actual recording done in a specific space as a basis for adding reverb to your input signal. This recording is called an ‘impulse response’ recording, often shortened to IR. The IR will usually be a short sharp bang or sine wave sweep, the idea being to have a recording that demonstrates how frequencies across a broad range behave in this space. The point is to accurately model a specific space’s reverb signature. For producers, this allows you to use a convolution reverb plug-in to place your instrument or sound in any specific space as long as you have an IR file for that space.
There are an astonishing amount of IR files to be found online. Want to put your piano in an abandoned Berlin power station? No problem. How about the Batcave from the original ‘60s Batman series? Yep. What about beneath a glacier? Easy. Of course, more conventional spaces are readily available too if you just want to put a guitar player in something trivial like a room, but where’s the fun in that?
Unsurprisingly, convolution reverb is used a lot in film production, with sound engineers often recording impulse responses of film sets and locations so that the sound FX wizards can accurately add sounds in post production – or, as is so unfortunately common these days, adding realism to scenes created entirely with CGI and green screens.
Now that we’ve covered the practical and intended use of convolution reverb, let’s talk about ways to abuse it. As you can imagine, convolution reverb was a fairly taxing task for a 1990s era computer to do on the fly when this technology first came about, but fortunately, it’s 2018 now. In the past, convolution reverb software has had limitations on what can be used as an IR file – specific file type, bit rate, length, etc. Now quite a few convolution plug-ins will accept any old audio file, with the length only limited by your computer’s RAM.
If you’re a bit of a sonic explorer, there’s a lot of fun to be had here. Try putting all sorts of audio samples in the IR slot; instruments, vocals – hell, put an entire song in there if your computer can handle it. Things will get weird.
Image via John Hult.