Acoustical Effects of Violin Varnish

Acoustical effects of violin varnish—those of mass, stiffness, and internal friction—are examined qualitatively and quantitatively. Dynamic measurements of Young's modulus, real and imaginary, are made for films of typical oil varnishes on a substrate of cross-grain spruce through aging periods of 1 yr or more and expressed in the practically useful terms of modulus over density. After 112 yr, the hard varnish had a real modulus per unit density of 2.5 × 1010 cm2/sec2 and Q about 23; after 1 yr, the values for the soft varnish were 1.6 × 1010 and 8. A high-grade floor varnish had intermediate values. The imaginary component of the soft one was substantially independent of frequency from 400 to 5600 cps. Exact calculations of added loss and tuning shift, made for a “pseudofiddle” comprising rectangular top and back of orthotropic materials simulating spruce and maple, are believed roughly representative of the violin. Added loss is more apt to be damaging than is detuning of plates. The loss comprises two components: (1) a reduction in sound caused by the reactive parameters of varnish, i.e. mass and stiffness, and effective irrespective of resonance; and (2) one caused by internal friction and greatest at a resonance. With an intermediate varnish, a coat of 0.0127 gm/cm2 (thickness about 0.005 in.) causes “flat” loss of about 1 dB and resonance loss about 3 dB; this is excessive. Varnish loss in the top plate is about three times that in the back; unless the spruce is overly resonant, its varnish should be thin. Top is more subject to detuning than back. Loss due to varnish is best controlled by weighing, with final adjustment of wood thickness after varnishing.