Knut Guettler
Norwegian State Acad. of Music, Post Box 5190, Majorstua, 0302 Oslo, Norway
The resonant bow seems to some extent capable of modifying the power spectrum of the bowed string, even during a steady-state Helmholtz motion, where only one slipping interval occurs during a fundamental period. Longitudinal resonances in the bow hair, strongly coupled to the transverse vibrations of the stick, are being excited by the changes of the frictional force which occur during each individual period. In general, these velocity fluctuations---superimposed on the steady bowing velocity---show small amplitudes compared to those of the string under the bow. In certain frequency regions however, dependent on the bow/bridge distance, impedances, etc., their energy content is sometimes great enough to cause noticeable modification of the string velocity. Such spectral changes would at any rate be small, but may still bear some acoustic significance. The present analysis is based on measurements on a violin bowed by a computer-controlled bowing machine, and supported by computer simulations. [Thanks are extended to Dr. Anders Askenfelt for sharing his experience in measuring bows, and to The Swedish Royal Institute of Technology for the use of their bowing machine and other laboratory facilities.]