Reconsidering the '8-dB rule' in ASTM Impact and Airborne Ratings

ASTM classification E413 defines a method for calculating single-number ratings of airborne sound insulation such as Sound Transmission Class (STC), while ASTM E989 similarly defines a method for calculating single-number ratings of impact sound insulation such as Impact Insulation Class (IIC). These ratings are the basis for building code requirements and design standards in North America. The calculation methods include a rule which limits the deficiency in any third-octave band to 8 dB. This "8-dB rule" has origins in concerns about deep dips in transmission loss spectra, but these concerns have been shown to be unwarranted. The history and consequences of the rule are investigated, including the statistical effects using calculations on a large body of laboratory and field acoustical tests. It is shown that the 8-dB rule has no scientific basis and causes misleading results, and the authors argue that the 8-dB rule should be removed from the standards.