According to the mass law for homogeneous building materials, such as glass, wood, and concrete, the TL and sound transmission class rating (abbreviated STC) increase by about 5 for each doubling of surface weight (in pounds per square foot). STC is a single-number rating of TL performance for a construction element tested over a standard frequency range. The higher the STC, the more efficient the construction is for reducing sound transmission. (See pages 199-201 for further details on the STC rating method.)

STC data for a variety of building constructions listed below have considerable scatter about the theoretical mass law curve. Nevertheless, the trend clearly indicates that heavier materials provide better sound isolation. This is the fundamental principle of sound isolation for architectural acoustics.

Building Constructions (number indicates data point on graph)

Glass:

1. 1/8-in-thick monolithic float glass

2. 1/4-in-thick monolithic float glass

3. 1/4- + 1/8-in-thick double-layer glass window with 2-in airspace between panes

   Doors:

4. 1 3/4-in-thick hollow-core wood door, no gasketing

5. 1 3/4-in-thick solid-core wood door, with gaskets and drop seal

   Walls:

6. 2 by 4 wood studs with 1/2-in-thick gypsum board layer on both sides

7. 2 by 4 staggered wood studs with 1/2-in-thick gypsum board layer on both sides

8. 4 1/2-in-thick brick

9. Two wythes of 4 1/2-in-thick brick separated by 2-in airspace with metal ties between layers

   Floors/Ceilings:

10. 4-in-thick reinforced concrete slab

11. 6-in-thick reinforced concrete slab

12. 18-in steel joists with 1 5/8-in-thick concrete on 5/8-in-thick plywood with carpet and pad on floor side, and 5/8-in-thick gypsum board...