Sound Absorbers - What Are They?
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?
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 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?
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?
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.
Why is mineral wool a better sound absorber than either fiberglass or foams?
Sonic-Shield can teach you about our Sonic-Fiber Faced Mineral Wool Facing options
Fiber mats are typically made of fiberglass or mineral wool, and are comprised of continuous filaments that trap air and sound between them. Sound penetrates the fibers, causing them to vibrate, and friction between the fibers dissipates the acoustic energy. If the fibers are too densely packed, sound will reflect off of the surface of the mat and sound will not be attenuated. If the fibers are too loosely packed, sound will pass through the mat with very little sound attenuation. Important considerations that determine the effectiveness of sound absorption of fiber mats include the diameter and mass of the fiber.
Fiberglass can be manufactured with diameters as small as 6 microns, whereas mineral wool can be manufactured at 3 microns. The smaller diameter allows for a more efficient and dense packing of the fibers, resulting in a smaller effective pore size. The smaller pore size of mineral wool will therefore provide greater sound attenuation than either fiberglass mats or open cell foams, particularly at higher frequencies. A further benefit is the fact that the mineral wool fibers have a greater mass density than the fiberglass or foam. This added mass makes it much more effective at attenuating lower frequency noise. Throughout the audible frequency range, mineral wool sound absorbers will therefore provide superior sound attenuation than either fiberglass mats or foam sheets.
How much sound absorption should be used?
The required thickness of the sound absorption material should be determined through consideration of the frequency content of the noise together with the NRC of the material. The overall NRC of a material is the average of the absorption coefficients obtained in tests at the 250, 500, 1000 and 2000 Hz octave bands, although test data is commonly provided in the 125 Hz and 4000 Hz octave bands. In many soundproofing applications (e.g., industrial equipment, interior acoustics), the frequency content of noise is generally rich in acoustic energy below 250 Hz, where the NRC of the materials is reduced. In such cases, it will be necessary to compensate by increasing the thickness of the sound absorption material. For example, at the 125 Hz octave band, two inches of 8 pcf (pounds per cubic foot) mineral wool has an absorption coefficient of 0.35, but four inches of 4 pcf mineral wool has an absorption coefficient of 1.01. This is due to the fact that the larger effective pore size of the less dense mineral wool is adequate to absorb acoustic energy at the lower frequencies coupled with the fact that the longer wavelength at the lower frequencies will benefit by having to travel a longer distance within the sound absorption material.
How should sound absorption be installed?
This, of course, depends on the application in which the sound absorption material is being used. For industrial applications, we normally recommend that the absorber be placed as close to the source of the noise as possible to prevent the wide dispersion of the noise into the room. For applications such as listening rooms, theaters and auditoriums, where it is necessary to reduce the amount of reverberant noise within the space, we recommend that the sound absorption material be placed over “acoustically hard” surfaces, such as concrete, brick or metal walls and ceilings. Depending on the thickness of the absorber and the frequencies that need to be attenuated, placement of the absorber directly onto the wall or ceiling surface is not recommended, since the acoustic wave velocity at these boundaries is at a minimum. In such cases we recommend that the absorber be offset a distance from the surface for maximum effectivess.
Sonic-Shield has an engineering staff with advanced technical capabilities who can assist our residential, commercial and industrial customers in the selection and installation of soundproofing materials and products to solve their noise and vibration problems. Our Sonic-Fiber is a mineral wool-based product that is available in sheet form and is incorporated in many of our standard noise attenuation panel products.