Absorption of gilding in concert halls

A few renowned concert halls have walls, statues, and ceiling surfaces covered at least partially with gilding. Despite its appearance, such material is not actually gold. Instead, it is composition gold, which is a very thin leaf of copper-tin-zinc alloy, finished with shellac lacquering. In order to investigate the effect of treatments on room acoustics, we asked an artisan, a Master Gilder, to prepare gypsum cylinders that were pre-cut to diameter of 29 mm for impedance tube measurement. The top surfaces of these specimens were treated either with composition gold, real 24 carat gold leafs without shellac, or linseed oil paint. The absorption coefficients of all these cylinders were measured with an impedance tube. Each specimen was measured on both sides to provide a direct comparison between different surface treatments to untreated gypsum. This paper describes the measurement process, the practical issues encountered in fitting the specimens tightly for measuring, and finally the results of the measurements. The results show that the absorption coefficient of the composition gold is 0.02 over a wide frequency range, and it absorbs approximately 50% less sound energy than bare gypsum.