For good acoustic performance, more is required than simply listing STC ratings in a partition schedule. For acoustic ratings, the best resource without consulting an acoustic expert, is the Gypsum Association GA 600. This document can be viewed or purchased at http://www.gypsum.org/download.html. This document lists STC ratings for various assemblies based on tests according to ASTM E90 or ASTM E336. And GA 600 offers advice on sound isolation construction.
Sound ratings are affected by mass, flexibility, and openings. The greater the mass of a wall the more sound resistant it will be. Unbalanced assemblies (two layers of gypsum board on one face and one layer on the other face) also improve performance. Sound impacting one face will not impart sympathetic vibration in the opposite face. Stud spacing and thickness can make a difference. Sound control batt insulation should be included in every sound control partition. Resilient furring appears in many STC rated assemblies to help dissipate sound energy before transmitting the sound through the assemblies.
According to ASTM C919 - Standard Practice for Use of Sealants in Acoustical Applications, the most important aspect of sound control is sealing the openings, including the partition perimeters. ASTM C919 includes a graph of the STC loss caused by holes in a rated partition. The loss is significant, even with very small holes, so acoustic sealant to close all gaps is critical to the partition performance.
ASTM C919 includes a chart that shows significant improvement in sound control by adding acoustic sealant. For the example partition, unsealed perimeters give an STC 29. Adding a single bead of sealant on one face improves the STC rating to 49. A second bead of sealant on the opposite face improves the rating to 53.
Remember to specify sound control batt insulation and acoustic sealant for sound control partitions. Detail the partitions to show acoustic sealant closing all terminations and penetrations.
David Stutzman