We describe convolution reverbs and how to use them, including sources for inexpensive digital recorders for sampling the space you want to capture, free reverb plugins and free libraries of pre-recorded samples from exotic spaces around the globe.
Convoluted Explanation
All this is made possible by the magic of convolution reverbs. Convolution reverbs are a unique form of sound treatment. They take an “impulse response” or IR, a sample of recorded sound in a real life location, then the IR is processed by the reverb plugin and the reverberation qualities of the space analysed. After that any sound played into the reverb sounds like it was recorded at that original location. Neat, huh? You are not recording the cave; you are capturing the precise way the cave shapes any sound. For example, you could sample a cave, a cathedral, a bedroom, a tunnel, a stadium, etc., then use that captured reverb in your computer to play your guitar through.
It sounds complicated, but it really isn’t. The first step is to sample your IR in your location of choice.
IR Recording
IR samples from a space use what sound geeks call “transients,” that is to say loud, sudden noises like a clapperboard or handclap which pass quickly. Record your transient by some means (see below) and save it as a WAV file. Edit the recording using Audacity or a similar wave editing program so that the sound starts at the beginning of the recording, and you don’t cut of any of the sound trailing off at the end. You can use a starting pistol to make your transient noise in the space, but of course we can’t recommend you start firing off a starting pistol in public places. A clapperboard like those used in filmmaking is a much safer option which won’t get you shot by local law enforcement.
You can just do a handclap, but handclaps are not very consistent as your hands don’t necessarily clap together the same way or the same loudness every time. It’s okay to use if you don’t have anything else.
Add to Your Reverb
Once you have your impulse response, you can load it into any of the free or modestly-priced convolution reverbs out there. Here is a list to get your started, but searching for convolution reverbs on Google will provide more. In this example, we made a handclap in a normal dining room in a residence in South Wales, UK. The sound was recorded with an iPhone internal microphone which provides a decent digital recording but is quite noisy so provides an interesting rather than high quality reverb effect.
In this case, the DAW software used was Reaper on a Mac, and the reverb was the free AU plugin called LAconvolver. Load the IR into the reverb and you get a natural-sounding room reverb.
Now there are two ways you can use a reverb sound, wet or mixed. Obviously the dry signal is the untreated audio. Wet is just the effect coming out of the reverb. A mix of wet and dry simulates a microphone being closer or further away from the source of the sound, e.g. a voice or guitar. The more space around the microphone, the more background reverb you get. Try different combinations of wet and dry for the best effect for the space your are trying to simulate.
Better Recording
Of course, the higher the quality of the digital recording of the IR sample, the higher quality of the reverb effect you get out the other end. It will also represent a closer replica of the space you sampled originally.
An inexpensive digital field portable recorder to provide high quality IR samples, like the Zoom H1, should cost you less than $100. Zoom, Tascam and Sony all make inexpensive devices which will do the job admirably.
IR Libraries
To save time or to try this out before you spring for a digital recorder, there are a number of online IR libraries for free where you can get recordings from all over the world. Here are a few to get you started, but once again search Google for “impulse response” for more. If you have any questions about sound design or convolution reverbs, please let us know in the comments below.
