“A great violin is alive; its very shape embodies its maker's intentions, and its wood stores the history, or the soul, of its successive owners. I never play without feeling that I have released or, alas, violated spirits.” So wrote the violinist Yehudi Menuhin, in his book Unfinished Journey. Would he say the same of the balsa-wood violin created by Chris Waltham (Am. J. Phys. 77, 30–35; 2009)?

Credit: ISTOCKPHOTO

Considered by many to be the 'perfect instrument', the violin of today shares the basic principles of its design with the first such instruments, which appeared in the mid-sixteenth century. The design was perfected over the next two centuries, most notably by the master luthiers of Cremona in Italy — the Stradivari, Guarneri and Amati families; little has changed since then.

Modern science, however, has revealed much about how the Cremonese masterpieces produce their superb sound. In the 1960s and 1970s, John C. Schelleng — who, after his retirement from Bell Laboratories as director of radio research, pursued his interest in violin acoustics — published a number of results on “the physics of the bowed string”, including scaling relations that describe how the vibration of a violin's top and back panels depend on their geometry and material properties. Using these scaling rules, it's possible to explore the use of woods other than the traditional choices of spruce and maple in the construction of a violin. Still, Waltham's choice of balsa wood might seem unlikely, known as it is for the making of model aircraft rather than musical instruments.

It is actually not the first time that balsa wood has been used for violin-making, but those instruments also had innovative body shapes. Waltham, however, has stuck with the classic Stradivari design and is therefore able to verify Schelleng's scaling rules directly.

The key effect, when the top and back panels are replaced with balsa wood, comes from an altered mechanical impedance (a direct analogue to the a.c. electrical impedance). The mechanical impedance scales with the ratio between the density of the material and the speed of sound in the material, and its value is particularly low for balsa wood. This translates into a high volume of sound. The balsa violin is “distinctly loud”, Waltham reports, and “a more appropriate instrument for a jig played in a noisy bar than for a soft lament.”