Sound Absorber FAQs are presented to help you understand the different types of products available and their effectiveness. A variety of materials are used for sound absorption, each with different claims of effectiveness. This application note provides some background on sound absorption materials, describes from a technical standpoint how it works, examines some of the claims, and recommends best practices for using these materials for soundproofing. What is a sound absorber?-Sound Absorber FAQs A sound absorber is any material that inhibits the reflection of sound and can transform or dissipate acoustical energy. Sound absorbers can take various forms, from “everyday” materials found in homes and offices, such as carpets, drapes, wall coverings and acoustical ceilings, to more specialized materials found in commercial and industrial applications, such as acoustical panels, fibers, foams and fabrics. Generally, the specialized sound absorption materials will have a greater ability to absorb sound, as measured by the material’s Noise Reduction Coefficient, or NRC (see our application note on on Acoustical Terms and Concepts Used in Soundproofing). How do sound absorbers work?-Sound Absorber FAQs Sound absorbers are typically foam sheets or fiber mats that attenuate sound by converting acoustical energy into thermal energy. If the surface of a material is nonporous, such as smooth concrete, acoustical energy will reflect off of the surface with little loss of acoustical energy. If the surface is porous, such as a fiber batting, acoustical energy will penetrate the surface and scatter among the pores and interact and reflect off of the fibers. Scattering among the fibers and pores results in frictional losses resulting in conversion of the acoustical energy into heat and the attenuation of noise. As the pore size decreases, less acoustical energy is transferred into the material and more is reflected back into the environment. As the pore size increases, more acoustical energy can penetrate into the material, however, if the pore size is too large, acoustical energy will pass through the material with very little attenuation. The ideal sound absorber material must therefore have an optimum pore size and density to most efficiently convert acoustical energy into thermal energy. Are foams effective sound absorbers?-Sound Absorber FAQs Foams are typically polyurethane materials with a large number of gas bubbles or voids, and are classified in either the “closed-cell” or “open-cell” varieties. Closed-cell foams have material around each individual bubble or pore, that do not permit airborne sound to pass between pores. As a result, closed-cell foams are not effective sound absorbers because the acoustical energy reflects off of the outer surface of the pores. These materials, do, however, make for good vibration absorbers. Open-cell foams contain pores that interconnect without barriers between them, and are more commonly used for sound absorption. Generally, open-cell foams with small pore sizes make for more effective sound absorbers than those with larger pore sizes. Many manufacturers of open-cell foams configure the sheets with an “egg carton” facing, which are marginally better at diffusing sound, however, the depth of the egg cartons is not sufficient to attenuate low frequency noise. Some manufacturers use a smooth facing on the surface of the foam, and these should be avoided since the smooth surface may reflect sound. What about spray-on foams?-Sound Absorber FAQs Spray-on polyurethane foams are typically used for thermal insulation to fill gaps in joints and cracks in walls. These foams are effective (to a certain extent) in blocking noise flanking paths, however, they are not effective for use as sound absorber materials. This is due to the fact that the foams will typically expand and harden, and will not have adequate compliance to attenuate sound. Furthermore, if used between walls, the foams will displace the air gap between the walls and provide a “short circuit” path of relatively stiff material that will more readily transmit sound from one side of the wall to the other. Other types of spray-on foams are used on large exposed surfaces, such as metal ceilings in commercial and industrial spaces, which are not effective sound absorbers, although they have other properties that may be beneficial for soundproofing. These foams typically contain particles or materials that provide a rough texture to the surface, that tends to diffuse acoustical energy – that is, planar acoustical waves do not reflect off of the surface in a planar manner, and spread out in several directions. Although they do not reduce the overall acoustical energy, there will be a slight benefit in reducing the amount of acoustical reverberation within the room. The foam also provides mass and damping to the surfaces they coat, deadening potential structural vibrations.