The ISO (International Organization for Standardization) airborne sound rating that is approximately equivalent to STC is called the weighted sound reduction index and has the symbol Rw. This rating is described in ISO 717-1. The STC number for an assembly is usually about the same as the Rw number.
New rating systems have been introduced in ISO 717-1 that are meant to deal with low frequency sound such as might be created by a home music system. These ratings give the difference in A-weighted sound level for pink noise attenuated by the transmission loss spectrum. They give a good indication of the effectiveness of a wall or floor at attenuating music.
The ratings can be calculated down to a frequencies of 50 or 100 Hz. On these pages these ratings are denoted ALD50 and ALD100. For more information on these ratings, see research report RR169 on the IRC's web site.
STC, Rw, ALD50 and ALD100 are all intended to rank the effectiveness of walls, floors and other building elements as barriers to airborne sound. The numeric values are not always the same. In particular, ALD50, which includes low frequency sound is often much smaller than the other ratings but it may give a better indication of occupant satisfaction. Unfortunately, there is no clear link between these ratings and the reactions of people to the sound coming from their neighbours.