Directivity and the Acoustic Spectra of Brass Wind Instruments

Abstract

A single-point harmonic analysis of the pressure wave radiating from a brass instrument is inadequate as a physical expression of the instrument's acoustic output. This is partly due to the dependence of the analysis upon the relative position of the receiver. A complete statement would require a rather laborious expression of the analysis as a function of position. Probably an analysis of the total power output is the simplest single expression which is an improvement over the point analysis. Measurements of the directional radiation characteristics of several instruments in an outdoor location were obtained by driving the horn electroacoustically and plotting the sound pressure field. By integration of the intensity over a sphere, a ratio is obtained between the total power of a cornet and the power of a non-directional source having the same pressure amplitude as on the cornet's axis. A curve of this ratio is used to convert from an axial pressure wave analysis to the total power analysis which is a more representative property. Also when experimental changes are made in any part of the instrument except the bell, such a conversion curve expedites a study of the effect of the change on the power output analysis.