Envelope Functions for Sound Spectra of Pipe Organ Ranks and the Influence of Pitch on Tonal Timbre
Frank Hergert, Nicolas Höper · Proceedings of meetings on acoustics · 2022
Long-term average sound spectra (LTAS) of organ pipes from different flue and reed ranks have been studied with the intention to obtain pitch-dependent LTAS models for the constant part of the pipe sound. The measured LTAS of each rank of pipes cover up to nine octaves within the tonal range C0…C9. The envelope functions are piecewise linearly parametrized and serve as empirical approximations for the sound pressure levels of the harmonic partials. They allow for extrapolating the pitch range of most ranks and thus to explore these spectra beyond their physically existing compass. Two parameters for tonal timbre were calculated from the spectra: the first is a weighted average slope of the envelope function; the second is the spectral centroid of the sound spectrum. Each rank of pipes depicts as characteristic curve (from bass to treble) in a two-dimensional timbre chart of these two parameters, in which the different families of organ tone clearly separate from each other. The use of such timbre charts could assist in the work of pipe voicing by means of an electronic voicing device, which extracts the LTAS from the measured pipe sound and calculates the timbre chart from it.