Analysis and digital enhancement of “brassiness”.

Abstract

The distinctive “brassy” timbre of instruments such as the trumpet and trombone has been variously attributed to spectral brightening of the mouthpiece excitation and to nonlinear wave-propagation effects, primarily shock waves, whose existence and importance are the subject of ongoing investigation. In the research presented here, spectral brightening at higher amplitudes—even at levels insufficient to produce shock waves—is explained analytically in terms of the known amplitude-dependent propagation speed of sound. This spectral-enrichment effect is experimentally demonstrated by acoustic measurements, analytically quantified in terms of phase modulation and the generation of harmonically related sidebands, and perceptually confirmed by digital simulation. Efficient discrete-time methods for implementing the desired signal-dependent delay or phase modulation, as well as shock formation, are described. This “brassification” effect can be used for synthesis of brass-like sounds or to increase the perceived brassiness of live or recorded instrument sounds.