The History of Reverb

One of the great things about capturing a musical performance, is how unique different spaces can sound. Capturing the sound of a lapsteel guitar in a wooden room deadened with carpets sounds entirely different to the same performance recorded in a parkiing garage, or a 2000 seat theare. There's an inherent warmth to certain environments that enhance not only the sound of a performance, but in extreme cases in very echoey places, the performance itself as a performer may improvise on top of an echoed phrase.

If you're plugging an instrument directly into a mixer, a recording is going to sound dry and almost other-worldly as there is no context for where that sound is located. Other than the sound of the room the recording plays back in, it's very unnatural to the ears. It's contrived. Fake.

Early attempts to combat this are for instance spring and plate reverbs. Use a transducer to induct the natural reverberant quality of a metal material and amplify it. It's a unique sound. Not quite natural, but natural-ish in a way. Tape echoes similarly created a sense of space mechanically using a short tape loop to echo and decay that has natural sounding qualities to it, even if we know inherently that it's not a natural space we're hearing.

Early analog delays used primitive BBD circuits to delay sound. Again, a distinctive sound as due to the tech involved, each subsequently repeated echo would lose resolution and definition. Later, digital delays improved the bandwidth quality to a degree, but early digital tech suffered from low sample bit depth that resulted in a certain grainy sound that ironically has become fairly fashionable again these days.

Most reverb units work by way of a series of these delay lines that are integrated through some manner of algorithm. In essence they are an attempt to emulate the various echoing qualites of some defined space. Sound echoes quite differently in a small room say, than a large hall or auditorium. The different component echoes that bounce around and wash out in some circumstances result in what we come to think of as the 'sound' of a space. What kind of materials are used in the space? Concrete? Wood? Steel? Different materials resonate and reflect sound differently and can be perceived as more or less warm, or cold.

As tech improved, so too did the general quality of digital reverb units. At this point, if realism was your goal, it was more about the quality/design of the individual algorithms than the quality of components. Some companies like Lexicon and Eventide became legendary for their algorithms, many of which are now ported over to plugins.

But what if there was an entirely new approach to simulating the sound of a space? Not simply better quality tech, but an entirely new approach? We're about to enter the world of convolution reverb where a space can be sampled as opposed to synthesized from the ground up. Here's how it works.

Place microphones around a space in the same places you'd put them if you were capturing a performance there. Play a short burst of pink noise near where the recording source would be. Record the noise and subsequent reverberations. Put that entire recording into a convolving algorithm and then voila, you've just created what's called an Impulse Response (IR). Now, when the de-convolving software is used as a plugin, everything playing through it will sound as though it was being played in the space that you just sampled.

Ok, so what kind of sorcery is this and how does it work?

In short, pink noise is sonically flat and comprises signal at all audible frequencies at equal volume as we perceive them. Any resulting reverberation recorded is a perfect representation of any signal that could be created in that location and is stored in the file.

Sony created the first hardware convolutiion reverb in 1999, and AudioEase created the first software plugin (Altiverb) in 2001. There are plenty of Convolution plugins available these days including some that are freeware/shareware. Many of them include the software needed to create your own IRs as well. Interestingly, IRs are now used not just for reverb, but also for cabinet simulation in guitar amps and related plugins.