Varnish affects the sound of a violin

http://www.eurekalert.org/pub_releases/2016-03/sflf-vat031716.php

PUBLIC RELEASE: 17-MAR-2016
Varnish affects the sound of a violin
In Stradivari's footsteps
SWISS FEDERAL LABORATORIES FOR MATERIALS SCIENCE AND TECHNOLOGY (EMPA)

Leonardo da Vinci's oil painting "Mona Lisa" looks rather gloomy with its dark green and brown colours. It is a little-known fact that the maestro painted the picture in bright colours - so the subject appears bathed in the light of a sunny day. What caused this difference in colour? Da Vinci covered his oil painting with varnish to protect it. The varnish acts like a filter, causing the picture to become darker than the artist originally painted it.

It has the same effect on the spruce used to make violins and other stringed instruments. In its raw state, it is white with a touch of gold. The wood lacks durability, so violin makers protect it from environmental damage using a varnish. It is this protective coating that gives the violin its elegant dark brown colour.

Violin makers pass their varnish recipes down from one generation to the next. It is rare for anyone outside the family to find out how exactly the mixture of resins and oils is formulated. The chemistry of varnishes has been scientifically researched for a long time. In contrast, little attention has been paid to the physical properties of varnishes - wrongly in the opinion of Marjan Gilani from Empa's Applied Wood Research Laboratory. The scientist and her colleagues suspected that varnish affects the vibrational properties and therefore the sound characteristics of violins, and they began to investigate this relationship. The first stage of the research was concluded with the publication of a scientific paper in the journal Applied Physics A. The findings support the Empa researchers' hypothesis.

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In the 40 samples in total that were cut along the grain, the varnish increased the sound insulation of the heartwood and young wood and reduced its stiffness compared to the raw state. This may be desirable because, up to a certain point, the more elastic the wood of the violin, the warmer and softer the sound, particularly in the case of high notes, which can sometimes be perceived as unpleasant. However, this change is accompanied by a loss of clarity and sharpness of the violin sound, which was evident in the laboratory as a reduction in sound propagation. The findings were different for the samples cut across the grain. With these, both the sound insulation and the stiffness increased. Similarly, the sound propagation values also increased. An audience in a concert hall would therefore perceive clear, sharp sounds with strong acoustic presence if the instrument was made in the same way as the samples cut across the grain.

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