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Tapping sounds were measured for the varnished boards and the uncoated ones of Sitka spruce wood used for musical instrument soundboards, and the acoustical effect of varnishing was investigated from their sound power spectra. The Young's modulus and the internal friction were measured for these boards, and the equations for the case of a coated board were derived theoretically. From these experiments and theories, the important mechanisms were clarified. With varnishing, the sound power level decreased at low-frequencies ( 3kHz) and overall. This difference in frequency characteristic variations was due to the difference in the variations of Young's modulus (ER) and internal friction (QR-1) in the direction, R, perpendicular to the grain. The level drop at low-frequencies was due to the shifting upwards of the lowest mode frequency with increasing ER. For wood having a low ER value the power level increased at frequencies above about 300Hz because of the larger increase of ER, whereas for wood having a high ER value it decreased at high-frequencies because of the larger increase of QR, -1. The varnishing is beneficial acoustically in the former but harmful in the latter regardless of varnish thickness.

Effects of varnishing on acoustical characteristics of wood used for musical instrument soundboards.