Figure 2.8: Explosion Source and Sporchestration
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By including peaks across the entire audible frequency range, SPORCH may also match noisy sounds with percussion instruments. The peaks in this case roughly correspond to the amount of energy in the spectrum at those points (though this is not how they are actually calculated). The program tends to not match pitched instruments with non-pitched sound sources if a noisy sound can match a larger number of smaller peaks spread across the spectrum. Depending on the source, pitched instruments/pitch spectra subtracted from noisy spectra often create large negative peaks as well.
The following figure shows an orchestration of an explosion. The algorithm was allowed to choose between snare drums, bass drums and crash cymbals (samples of drum and cymbal rolls were added to the database). Both the percussion instruments and the source were analyzed with the alternate peaks representation described above. The result was two bass drums playing fff and two snare drums playing at p. The spectral envelope of the result matches that of the source fairly well and the aural results are similar as might be expected (minus the attack of the explosion).