ASTM or ISO for transmission loss testing in Thailand?

Thai manufacturers routinely sell into both a North American supply chain and a European or regional one, and get asked for STC by one customer and Rw by the next. The temptation is to measure once and convert. Do not.

STC (ASTM E413) and Rw (ISO 717-1) are both single-number ratings derived by fitting a reference contour to a measured transmission loss curve, but the contours differ, the frequency ranges differ, and the deficiency rules differ. Two constructions with identical STC can have Rw values a decibel or more apart, and the difference is not constant — it depends on the shape of your particular curve.

The good news is that the underlying measurement is largely the same. A single laboratory test producing a one-third-octave transmission loss curve can usually be rated both ways, provided the frequency range measured covers both standards' requirements. Ask for that at the quotation stage and you will pay for one test and receive two ratings.

What you cannot do is take a published STC from a datasheet and convert it to Rw with a rule of thumb, then put that number in a European submittal. If it is ever challenged, there is no measurement behind it.