Luthiers for centuries have agreed that the wood on the top of an instrument affects its tone far more than any other part of the instrument. Spruce is the most widely used top wood for mandolins, and for my current build, I have three different spruce species: Lutz, Englemann (left side of the picture), and Sitka plus one Coastal Redwood set.
The ideal top wood is light weight, but stiff, which is a bit of a contradiction since in general wood density and stiffness vary together. The top wood in my collection varies from 378 for the lightest piece of Englemann to 480 kg/m3 for the heaviest Sitka.
Measuring stiffness presents more challenges. I first placed each board over a 35 cm span then measured how much a 50 lb weight made it bend. I got a good workout lifting the weight and determined that the Sitka was stiffest and the redwood was the least stiff.
To further measure stiffness, I tapped each piece of wood and recorded the resulting frequency. Stiffer wood produces higher frequencies and allows me to calculate the dynamic stiffness of the wood, as happens when an instrument is played. Plotting stiffness against density, most samples followed the trend of denser = stiffer. The clear loser was the redwood, which had an average density, but very little stiffness.
To further distinguish samples, I calculated the sound radiation coefficient (SRC) of each sample. SCR uses stiffness and density to estimate how efficiently a piece of wood radiates sound energy. The winners were the Englemann and Lutz with SRC values from 13.6-14.5. The Sitka samples varied from 12.5-13.5, and the Redwood came in last at 8.2.
Conclusion: I’ll be using Englemann and Lutz for most of these mandolins to produce instruments with outstanding volume and projection, but the woody mellowness of softer species. I’ll also use a couple of the best Sitka sets, whose stiffness should produce instruments with the percussive “bark” sought after for bluegrass rhythm playing. If anyone needs any Redwood, I have a free set!