STC

STC stands for "Sound Transmission Class".    This is a number that is assigned to BOTH a product in a laboratory setting, and to a treatment once the product is actually installed in a field application.   The higher the number, the greater the product or treatment's ability to block, isolate and contain noise.   

In a laboratory, if an original sound source emits a sound pressure level reading of 100 dB before a product is tested, and the same sound source emits a reading of 74 after the product is installed, the product is assigned an STC rating of 26.  Now move that product into a field application, and add the STC values of drywall, disconnected framing techniques, and other materials layered into a common wall treatment, for instance, and your assembly rating could total 55-60 for an overall STC rating.   Understand that STC ratings are primarily designed to measure the effect of the entire "treatment", and that a product rating is designed only to help indicate its potential effectiveness.   Much of your result will depend on how you layer products into your application.

Sound barrier products are tested at 125, 250, 500, 1000, 2000 and 4000 Hertz ranges.    The performance of the product at each corresponding interval is recorded and published as a test result.    The STC rating is determined by averaging the performance at the middle four of these frequency bands.    This figure then allows us to compare and select the right product for your treatment.    Careful consideration should be given to match the STC rating to the specific frequency band of your noise source.

Note that an STC rating of 26 does not mean you will trigger a 26 dB drop.     Remember that the STC rating is produced in a laboratory setting.  All variables are under the control of the tester.   When the same product is moved out to the field for an installation, results will deteriorate due to leakage, structural vibration, flanking paths, poor installation, and a host of other issues that will allow noise to bleed out and decay your results.    The true value of the STC is for comparative ratings, helping you to identify which product might serve you best in your application based on the frequency of your sound source.   But your final STC value is of the entire assembly, and that's where your attention should be centered.

For more information on Sound Transmission Coefficients, visit our course on the STC here at the NetWell Academy.