Many of the sounds in the electronic part of Gobo were created using a technique called convolution. Convolution is most commonly used as a reverberation technique, in which a given sound is convolved with the “impulse response” of a real acoustical space (like concert hall, or cathedral, for example) in order to simulate the effect that particular space would have on it. However, convolution can also be used as a cross-synthesis technique. In this case, two audio signals are convolved with one another, which has the result of multiplying their two spectra. Whatever parts of the audio spectrum the two sounds share will be retained or accentuated, while the parts they do not share will be reduced or eliminated. Interestingly, if one of the sounds is very short and with a distinctive spectrum, while the other is long and spectrally rich (i.e., noisy), it has the effect of retaining the rhythmic characteristics of the longer sound, but with the spectrum of the shorter sound.
In Gobo, I made extensive use of this cross-synthesis technique in constructing the electronic part. The longer sounds are various recordings I made of dragging a large piece of plastic (the translucent cover of my kitchen’s old fluorescent light fixture) across the cement floor of my garage. I convolved these recordings with various short musical sounds (a note on an English Horn, a bassoon, a contrabass, some chords played by a string section, etc.). Since the scraping sounds of the plastic on the cement are very noisy (i.e., spectrally rich), convolving them with anything tends to result in keeping almost the entire spectrum of the musical sounds, but extending them in interesting ways. Here are some examples:
Bassoon staccato note:
English Horn staccato note:
Long Scraping and Resonant Bang:
Convolution with Bassoon note:
Convolution with English Horn note:
Contrabasses Low Short C:
Convolved with long scraping sound:
String Chord:
Convolved with long slow scraping sound: