Carpet Sound Absorption
Carpet is an outstanding sound absorptive material. No other acoustical material performs the dual role of a floor covering and a versatile acoustical aid. When properly selected, carpet absorbs airborne noise as efficiently as many specialized acoustical materials. However, it is important to understand the acoustical values of particular carpet constructions and the combinations of specific carpet cushions. This information will assist you in selecting the appropriate combination for a specific purpose.
Sound absorption coefficients, the fraction of incident sound energy that is absorbed by a material, usually vary strongly with frequency. A noise reduction coefficient (NRC) is used to grade the effectiveness of a material employed for sound control.
Small samples can be measured by the impedance tube method, while larger specimens can be measured by the reverberation room method. Reverberation room coefficients are usually provided as a single number, NRC. This number is the average of the coefficients at 250, 500, 1000, and 2000 Hz (from low- to high-pitched sounds).
Transmission through walls, floors and other barriers is much greater for low-frequency sounds than for high-frequency sounds. Sound transmission is measured between two reverberation rooms for at least 16 standard frequency bands.
For convenience in comparison of different constructions, the sound transmission class (STC) rating condenses sound transmission information into a single number according to ASTM E-413. STC is fairly accurate for human speech; however, for low-frequency sound, such as a motor, fan or even music with strong bass, the perceived sound may be greater than that indicated by STC.