To prevent the passage of sound requires a solid barrier that is impervious to air. Common barrier materials are gypsum board, plywood, concrete etc. Soft porous materials, like rock or glass fibre on their own do not prevent the passage of sound. They are, however, beneficial when placed inside properly designed cavity walls or floors.
STC (sound transmission class) is measured in a laboratory according to ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions. Specimens are constructed in the opening between two special reverberation rooms constructed so that the only significant sound path is through the specimen. Transmission losses (TL) are measured at frequencies from 125 Hz to 4000 Hz and STC is calculated according to ASTM E413, Classification for Rating Sound Insulation.
In a laboratory, specimens to be measured are placed between two rooms that are isolated from each other so there is no transmission of vibration. Often the rooms are supported on springs.The sketch shows an idealized facility for measuring wall sound transmission. Such facilities are also used for evaluating windows, doors and other elements that are built into walls. A facility for testing floors has one room above the other.
Note that in a laboratory test, the only path between rooms is through the wall or floor being tested. This is often not true in buildings.
Noise is generated in one room and the sound pressure levels in decibels are measured in both at several different frequencies. The bandwidth for the measurement is one-third of an octave. The difference in levels is corrected to account for the acoustical properties of the receiving room.
The quantity that results is the transmission loss in decibels (dB). This represents the loss in sound power due to transmission through the specimen. The higher the transmission loss, the less sound passes through the wall.
The ASTM standard governing these tests is ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions.
The dip in the curve for gypsum board in the figure is caused by the stiffness of the board. The stiffer or thicker the material, the lower the frequency where this dip occurs. Thin or limp materials do not show such dips in the usual frequency range. The dip for the concrete is not obvious but occurs around 100 Hz. Plywood and oriented strandboard (OSB) have different stiffness properties depending on the axis for bending. This means the coincidence dip is broad and not well defined.
Plots of sound transmission loss data are complex and are usually reduced to single number ratings. In North America the sound transmission class, STC is the usual rating. This is calculated from transmission loss data according to ASTM E413 Classification for Rating Sound Insulation. This chart shows an example of the fitting process.
The STC contour extends from 125 to 4000 Hz. The measured 1/3 octave band transmission losses are fitted to the contour as shown, subject to the following rules:
As for TL, the higher the STC, the better the sound insulation.