Block made with Florida limestone normally has a density per cubic foot of 120 to 124#. This puts it in the upper end of the C90 classification of Medium Weight block (Table 2 - C90-14).
NCMA TEK Note 13-01C gives an STC rating of 47-48 for an 8" hollow unit with a density of between 115 and 125 #/CF.
Links to additional documentation:
Jerry Painter, FASTM
Don Beers, PE, GC
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