Sound Absorption

Sound absorption works by having sound energy dissipate as it is converted into thermal energy. Common household items such as carpeting, heavy drapes and upholstered furniture will absorb sound. Notice how a room echoes when it is empty, as sound waves reflect off the bare walls.

Sound absorption differs from sound deadening in that with the former, only the characteristics of the sound are changed to achieve less echo or reverberation. Sound absorbing materials and sound deadening materials are generally different, but both are required to properly achieve comfortable levels of soundproofing from noises emanating from the inside and outside of the home.

For better acoustics and noise absorption, an inexpensive and environmentally sound choice is to use noise-reducing acoustical ceiling tiles. Silencio is commonly used in basements, finished attics and home offices to stop echo and reverberation. Indeed, ceiling tiles can be a surprisingly effective addition to any room and have enjoyed resurgence in popularity of late. Step-by-step instructions for installing ceilings tiles and panels can be found here.

Noise Reduction Coefficient (NRC) measures the sound absorption qualities of a material. It is calculated by averaging the material's sound absorption coefficients at frequencies of 250, 500, 1000, and 2000 Hz. An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption. It is expressed to the nearest multiple of 0.05.

NRC ratings are given to exposed materials such as carpets, ceiling tiles, gypsum board, hardwood floor, ceramic tiles, etc. The NRC of a wall is 99% dependent on the finishing surface. Therefore, NRC ratings for materials such as insulation and soundproofing panels are useless and not commonly used. NRC ratings should not solely be used to determine a product's effectiveness in absorbing sound.

The frequencies listed above are excellent in measuring the effects of speech, but they are a poor measure for music and other low frequency noises. Further, lab tests with stringent controls are generally ineffective in measuring real-world applications. Also be careful to read the fine print for any soundproofing materials you purchase. A manufacturer may list an NRC rating of .80 for carpet, but only if installed over fiberglass insulation. The effective NRC rating may otherwise be only 0.30.

With these caveats mentioned, the table below indicates NRC ratings for common building materials.