The second part is following the latest version of ASTM E477. This is particularly important since the calculation methods have changed over the years. E477 is the standard test method for measuring acoustical and airflow performance of duct liner materials and prefabricated silencers.
The first ASTM standard was written in 1973 (E477-73). Between 1973 and 1995 reverberation room requirements were solely based on the methods of ASTM standard E90, which is used to determine transmission loss of acoustical barriers. In the pre-1996 versions of the E477 standard, the test duct requirements were vague regarding allowable construction, "... duct system should be of adequate mass." The problem with this is that lighter ductwork allows more sound waves to "break out" of the test duct and therefore gives the appearance of higher insertion loss values for the silencer.