In a recent build, we used several designs found in GA-600 (Fire Resistance and Sound Control Design Manual). We selected designs shown to have STC ratings of 50 to 54. However, it was necessary to have the completed partitions field-tested, and the test results came up roughly five points lower than the STC ratings published in GA-600. How is this possible?

Sound Transmission Class ratings reflect results obtained under tightly controlled laboratory conditions where two rooms of identical size and acoustical properties are divided by the assembly being tested. The test essentially measures the difference between the sound pressure level in the room where the sound is generated and the adjacent room. The generated sound is carefully controlled and measured at specific 1/3-octave frequencies, and the measurements are weighted to ensure a proper comparison of the different frequencies that pass through the partition separating the two test areas. The resulting STC ratings are primarily intended to be used to evaluate the sound-attenuating properties of different building systems or assemblies as fairly as possible.

By definition, a field test is performed in real-world conditions. Field tests reflect actual conditions, not ideal conditions. Consequently, the test results captured in the field may not exactly fit those captured under laboratory conditions.

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