A survey is made of past and present systems for rating the sound‐insulation value of building walls and floors. It is observed that in most countries ratings based on “average” transmission losses have been replaced by systems that compare transmission‐loss characteristics with a standard contour. An example of this approach is the new sound‐transmission class contained in ASTM E90‐61T. This is discussed from various theoretical and practical viewpoints, and it is concluded that the sound‐transmission class is a useful rating system for common architectural problems.