Recording a convolution reverb
Capturing a space's reverb using convolution
In audio signal processing, convolution reverb is a process for digitally simulating the reverberation of a physical or virtual space. It is based on the mathematical convolution operation, and uses a pre-recorded audio sample of the impulse response of the space being modelled. To apply the reverberation effect, the impulse-response recording is first stored in a digital signal-processing system. This is then convolved with the incoming audio signal to be processed.
The primary goal of a convolution reverb is to sample real spaces, in order to simulate the acoustics of the sampled space.
Using a small Genelec monitor I played a sine sweep (12 sec, 30 sec, 60 sec) in the 6-storey stairwell that is used as a fire escape at London College of Communication. The tall enclosed nature of this space combined with hard concrete surfaces and lots of stairs create a unique and spacious reverb. 2 Fostex digital recorders were used, one to play back the sine sweep and one to record in both mono and stereo.
I used a similar setup to record the Atrium at LCC, this time using a pair of Tannoy Reveals and recording only in stereo. This type of recording really requires a high powered full-range monitor to properly excite the space at every frequency. At this level bursting a balloon to create the instant impulse (a sine sweep is deconvolved by comparing it to a dry sweep of the same length) is just as effective, though the Genelec produced better results in the larger stairwell space.
I also recorded several impulses for experimental convolution: striking the metal handrail, a handclap, stomping feet, various shouts and some overtone singing.
These impulses were then loaded into Voxengo Pristine Reverb for signal processing of other sounds.
You can download the pack of LCC stairwell impulses here, and Atrium impulses here
