Sound waves are reflected when they hit a hard surface. Acoustic insulation material works by preventing reflected sound waves from entering or leaving a room, or from traveling from one place to another, such as between apartments in a building.
Sound Transmission Class (STC) ratings indicate how effective a certain sound insulation material is at blocking sound. The higher the STC number, the better the material blocks sound transmission.
For instance, acoustic insulation material with a STC rating of 25 would allow normal speech to be understood through a wall while material with a STC rating of 60 would render most sounds inaudible.
Acoustic insulation material should not be confused with sound absorption material. They are treated separately in sound control design.