This thesis focuses on physical measurements of the sound radiated by stringed musical instruments. The radiation efficiency, defined as the ratio of acoustical power output to mechanical power input, was measured to study the acoustical behaviour of the instruments between 80 Hz and 2000 Hz. The research used spherical-harmonic decomposition to determine the power output from monopole and dipole sources. On classical guitars, monopole power produced by the low-frequency resonance triplet provided the greatest contribution to the power output below 300 Hz. At higher frequencies, where the body modes have more complex shapes, dipole sources dominated the total power output. As the dipole contribution to the power output increases, the radiation efficiency of the instrument decreases.
The research demonstrated that the resonance frequencies of the body modes of the instruments do not correspond with either a large or small value of radiation efficiency. Instead it is the mode shape that determines the radiation efficiency. Modes with similar-sized anti-nodal areas, of opposite phase, were found to be less efficient than modes which had unequal-sized anti-nodal areas. Measurements of the in-plane velocity of these modes, made with a 3D scanning laser vibrometer, showed that the less-efficient modes had greater values of in-plane velocity. The largest values of radiation efficiency for classical guitars occurred between 200 Hz and 600 Hz. The upper frequency limit of this range was determined by the resonance frequency of a particular mode. This was confirmed by experiments on a purpose-built guitar in which the cross-grain stiffness could be adjusted. Experiments on classical guitars, steel-string guitars and violins produced characteristically different radiation efficiencies.
Ian Perry http://ethos.bl.uk
Sound radiation measurements on guitars and other stringed musical instruments