Found a Swedish paper in English about wood, "Structure and Properties of Scandinavian Timber". Easy and interesting reading and one thing that I didn't know before, the density of spruce is biggest when the width of the annual rings is about 2 mm, tighter or wider annual rings have less density! Also, the hardness in spruce is almost 3 times harder in the direction of the stem compared with the radial and tangential directions. Pine is a bit harder (will try pine for my plugs). You can red the PDF article here.
Here is the interesting graph of the density and width of the annual rings.
I make the high point of the graph 1.2mm, which since I am not metrically inclined I converted to around 21 grain lines per inch, which is very tight gain in my experience. Neat information. If only I knew what it meant...
In the text 2 mm is mentioned, but the graph show something else. No mention how the graph was calculated in the text.
High density is probably not a good thing, light and stiff is the way to go. Gibson made many good sounding instruments in the 1930ies with very wide annual rings in the top (mandolins and guitars). Wide annual rings is not what a buyer expects from an expensive instrument, but maybe good for the sound!
Ah, nice article (the PDF) from a general point of view. Density and strength are (generally) accepted as being related - higher density equals higher strength. What this means in the construction of tops for acoustic guitars is complex - as there are crossover points with ratios of strength to weight and stiffness etc that change the way that a top responds. Not to mention the size, bracing and top thickness, grain runout variables etc.
Simple interpretation (as in growth ring numbers) is akin to reading tea leaves or throwing chicken bones as far as I can figure. The guys that do this for a serious living such as the big quality manufacturers and the top end boutique dudes know the ways to work all these factors together to come up with good tone every time (once again generally speaking) with often varying numbers of growth rings or a spread across the sound board. Perhaps someone who does work in this area (I don't ) can give us a bit of insight to wet the appetite.
I agree about the simple interpretation of growth rings. But if the graph is correct maybe the chance to find the best guitar tops is highest for wood with either really tight or wider than 2 mm annual rings. Or maybe the highest density 2 mm annual rings makes for the best tops? I don't know :-)
Yep, that's the issue also bearing in mind that not all Spruce comes from Scandinavia and Spruce changes its characteristics along altitude, temperature and humidity lines and then overlays length of growing season and available uptake of water and available nutrients. On top of these things mix in time of harvest and subsequent splitting/sawing drying and conditioning.
Its possible to generalise on how to choose a top but I take my hat off to those who consistently deliver the goods when presented with the full spectrum of variables - and that's just for Spruce.