Dynamic mechanical properties of violin wood

Abstract

Values of complex shear compliance (J* = J′ − iJ″) and modulus (G* = 1/J*) have been measured for spruce and maple over a continuous frequency range from 2–10 000 Hz, and at temperatures from 15 to 40 °C. The wood strip samples of European spruce and Norway maple were supplied by Carleen M. Hutchins in connection with her investigations of the effect of various acoustical parameters of wood on plate tuning and the tone qualities of finished violins. Measurements were made in an automated electromagnetic transducer apparatus [E. R. Fitzgerald and R. E. Fitzgerald, Polymer Bull. 18, 167–174 (1987)] in which samples are sheared while clamped between stainless steel blocks. Values of the shear parameters vary with frequency, but also with the grain orientation, moisture content, and the perpendicular, clamping force on the sample faces while they are vibrated in shear. The mechanical spectra differ, but, in general, several sharp, microstructural compliance modes are superimposed on broad retardation, background spectra [E. R. Fitzgerald, J. Acoust. Soc. Am. 33, 1305–1314 (1961)]. Typical are the results for a spruce sample at 22 °C, sheared cross grain, for which values of elastic compliance (J*) decrease from 2.74 to 0.306×10 9 cm2/dyn (2.74 to 0.306×10−2 MPa−1) as the frequency increases from 10 to 10 000 Hz; the loss compliance (J″) rises to a broad maximum of 1.65×10−9 cm2/dyn (1.65×10−2 MPa−1) at 4000 Hz. Values of the shear sound velocity and attenuation, together with the mechanical loss tangent (J′/J′ = G″/G′), are also calculated for the samples.