What is sound absorption coefficient?
It is a measure of the absorptive property of a material. Ideally, the fraction of the random incident sound power absorbed (i.e. not reflected) by the medium. Sound absorption is measured in units of ‘metric sabin.’ It is equivalent of 1 m2 of a perfectly absorbing surface (like an open window).
 

Then what is sound transmission class (STC)?
It is a measure of sound insulation property of a partition measured in accordance with ASTM classification E413 and provides an estimate of the performance of a partition for sound insulation problems.