The top of the instrument is responsible for 75% of the quality of the sound. Spruce is evaluated for selection based on the following four criterion:
Split: If you can plane the wood in both directions without tearing out, then the split is perfect. The further away you get from a perfect split, the less likely it is that a given piece should be considered for use.
Spacing: Nice spacing is on the order of one grain line every 1 to 1.3mm. The more a piece of wood varies from this, the less desirable it is. It is acceptable to have a progression of grain spacing from narrow toward the center to more broadly spaced toward the edge of the plate.
Contrast: The winter grain lines should be SHARP-although, keep in mind that this contrast develops with age.
Density: TMB likes a heavy, dense top. They tend to equate with better sound (all else being equal). Lighter wood (at least in terms of tops) is indicative of softer, spongier wood, which generally, is not conducive to good sound production.
Generally, the wider the grain lines the less varnish required. Wider grain lines provide more visual contrast, ergo fewer coats of varnish can be used to provide the final visual complement to the instrument.
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