Musical acoustics
Tapping into the sound of violins

Violins made by instrument makers in northern Italy up to the early 19^th century are world-famous for their sound. Violins from that epoch have been thoroughly analysed by modern methods in order to understand — and replicate — their superior acoustic qualities. Specifically, the shapes and sizes of old Italian violins have been studied in detail, which helps to comprehend, to some extent, the original design of these instruments. However, because not all violin-making technical know-how was consistently passed on to apprentices, it is not entirely clear today how exactly the renowned designs of the ‘old’ Italian violin builders were conceived. Now, Eriko Aiba from the University of Electro-Communications and colleagues report that frequency maps derived from the sounds obtained by tapping a violin’s top plate are valuable acoustic characteristics, and suggest that such tapping may have been a key technique used by the famous Italian violin makers from centuries ago.

The scientists compared violins from three categories: two old Italian instruments (a violin made in the 1590s by Gasparo da Salò from Brescia and a violin made in 1647 by Nicolò Amati from Cremona), two post-19^th-century Italian instruments (one made in 1916 in the Mantua region and one made in 2016 in the Cremona region), and two instruments made by a Japanese instrument builder in 2016. The latter two were manufactured making use of the tapping technique with the aim of reproducing the characteristics of old violins. Each of the violin tops was placed on a soft cushion. A laser grid pattern of lines spaced 1 cm apart was projected onto the top; tapping was done (either by a finger or by a metal rod) at about 500 grid points for each top. The sounds produced by tapping were recorded by microphones installed near the violin top.

The researchers then calculated the so-called centroid frequency for each of the grid points on the violin tops; the centroid frequency can be thought of as a representative value of a frequency spectrum. Plotting these frequencies as ‘heat maps’ then made it possible to compare the acoustics of the 3 violin pairs. Within each of the three categories, and especially for the two pairs of Italian violins, the frequency maps were strikingly similar. The two old Italian violins display a high centroid frequency around the chin rest area, which can be attributed to repeated coating with varnish, whereas the later Italian violins have a generally asymmetric frequency distribution. The results also showed that the contemporary Japanese violins, the top thickness of which was adjusted during construction by analysing tapping sounds, had frequency distributions similar to those of the old Italian violins.

Aiba and colleagues conclude that their results “support the possibility that tapping was used in the violin manufacturing process in northern Italy by the early 19^th century when measuring instruments had poor precision, although further evidence of old violin samples is needed to substantiate this hypothesis”.

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