STC & Rw are laboratory test results and both mean the same as they relate to measuring sound, and are the terms used for acoustic ratings.
The STC abbreviation means Sound Transmission Class, it is the measurement of internal sound transmission, eg high frequencies. The number rating that is used to determine how much a material, eg glass window will resist sound travelling through the air (airborne sound transfer).
Rw abbreviation means weighted sound reduction index which uses airborne sound and is a scientific calculation, this is better for rating the attenuation of external low frequency noise, eg outside traffic travelling through a material, eg glass window.
This means a commercial window could have a STC rating of 30 but can be rated as 25 dB (decibel) for noise.
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