Sound Transmission Loss (STL) represents the amount of sound, in decibels (dB), that is isolated by a material or partition in a particular octave or 1/3 octave frequency band. Example: 1/2″ drywall has an STL at 125 Hz of 15 dB.
Comparing material or partition performances for applications like recording studio isolation and sound proofing should involve comparing the STLs of each in the different bands, as opposed to just the more generic STC ratings of a material. If both materials or partitions are measured in accordance with the STL/STC standard, ASTM E90, then the comparisons being made will be “apples to apples.” It should be noted that real-world performance is not going to provide the same level of STL that is achievable in the laboratory. However, the relative performance of one material or partition versus another typically holds true in real-world construction. i.e., if the lab measures one partition better than another, it should hold true for a real partition built in your studio. Even though an actual field test of a concrete wall might reveal a field STC (FSTC) that is 5 points lower than the lab test, it is still better – relatively speaking – than a simple, single-leaf, uninsulated drywall partition in the same configuration.