Walls and ceilings are rated according to their sound-transfer performance. The key rating is called an “STC,” short for Sound Transmission Class. This rates the barrier’s ability to block or muffle voices and similar sounds; the higher the number, the more effectively the barrier buffers sound.

A less frequently used rating, the “IIC” (Impact Insulation Class), rates a wall or floor’s transference of impact noises, such as bumps and knocks. An IIC rating of 50 or higher is considered acceptable.

For residential partition walls, recommended STCs depend on the particular type of room: For bedrooms, an STC of 40 is considered good, 45–55 is very good, and above 55 is excellent. Living rooms should be a couple of points higher; bathrooms and kitchens up to 5 points higher. Conventionally built partition walls have poor STC ratings of 15–35.