Sound absorbers porous

Other fibrous and porous materials used for sound-absorbing treatments include wood, cellulose, and metal fibers foamed gypsum or Pordand cement combined with other materials and sintered metals. Wood fibers can be combined with binders and dame-retardent chemicals. Metal fibers and sintered metals can be manufactured with finely controlled physical properties. They usually are made for appHcations involving severe chemical or physical environments, although some sintered metal materials have found their way into architectural appHcations. Prior to concerns regarding its carcinogenic properties, asbestos fiber had been used extensively in spray-on acoustical treatments. 

Fibrous and Foamed Materials. Most sound-absorbing materials are fibrous or porous and are easily penetrated by sound waves. Air particles excited by sound eneigy move rapidly to and fro within the material and mb against the fibers or porous material. The frictional forces developed dissipate some of the sound eneigy by converting it into heat. 

Room Absorption One can place sound-absorbing materials on the surfaces of a room or surfaces of objects within it to reduce sound levels. That replaces hard, smooth reflecting surfaces with porous, sound energy-absorbing materials. 

Silica aerogel granules, being highly porous sohds, have the potential to be a sound absorbent material. Sound absorbers are usually characterized by surface impedance and absorption coefficient. A one-dimensional plane wave in the tube is assumed to be (Sung Soo et al. 2008). The total acoustic pressures about... 

Weft-knitted and warp-knitted spacer fabrics can be used as sound absorbers to reduce the noise level in building, automotive and other places (Liu and Hu, 2010). Figure 6.28 shows a weft-knitted spacer fabric (a) of multifilament spacer yams and a warp-knitted spacer fabric (b) of monofilament spacer yams for sound absorption. Whereas the weft-knitted spacer fabric exhibits the typical sound absorption behaviour of porous absorbers, the warp-knitted spacer fabric exhibits the typical sound absorption behaviour of microperforated panel (MPP) absorbers. Figure 6.29 shows the noise absorption coefficients of the two spacer fabrics without and with air-back cavity. The combinations of weft-knitted and warp-knitted spacer fabrics can significantly improve their sound absorbability, as shown in Figure 6.30. 

Bliss, D. B., Study of Bulk Reacting Porous Sound Absorbers and a New Boundary Condition for Thin Porous Layers, Journal of the Acoustical Society of America Vol. 71(3), March 1982, pp. 533-545. 

It is often difficult to obtain absorption at low frequencies with porous textile absorbers because the required thickness of the material is large and the sound absorbing layers are often placed at room boundaries where the absorbers are inefficient due to the low particle velocity. Resonant absorbers might be a solution. There are two common forms of resonant absorbers manbrane/panel absorbers and Helmholtz absorbers. A membrane/panel absorber is a sheet of vinyl or plywood, which is free to vibrate while for a Helmholtz absorber, the mass is a plug of air in the opening of a perforated sheet. The spring in both cases is provided by air enclosed in the cavity. Best performance can be obtained by placing a porous textile absorbent in the neck of the Helmholtz resonator or just behind the membrane in the panel absorber. The resonant frequency of this type of absorber can be tuned to the frequency of interest. 

Propagation of sound in porous media modeling sound absorbing materials / J.-F. Allard, 1993. 

As sound passes through a porous material, energy is lost by friction within the material. The material is usually attached to various surfaces in a room. The absorber will have the highest efficiency when positioned where the air molecules are moving the fastest (and hence more energy... 

Most porous materials absorb sound, and those materials specially made for this purpose include porous foams and fiberglass. However, ordinary materials such as carpets and drapes are also effective, and can be used in building spaces to reduce reverberant sound buildup and noise. 

One of the drawbacks of the two-microphone transfer function method is that the absorption coefficient determined may not be a true representation of the material s characteristic. In the case of a porous material, such as silica aerogels, the reflected wave from the rigid wall could contribute to a rise in the absorbed energy by the material. To account for this uncertainty, the four-microphone impedance tube setup is usually used to determine the transmission loss (TL) and absorption coefficient (Feng 2013). In the absence of additional microphones downstream of the specimen, a sound meter could be used instead to measure the TL of the specimen under test. However, the sound meter picks up discrete transmitted signals at periodic interval, which could result in a mismatch with the generated signals from the source. 

Most porous materials absorb incident and airborne sound waves well. A small change in pressure perturbations can generate loud noise, which dissipates into heat as it travels through the tortuous path in these materials. On the other hand. 

In apartment buildings, noise transmitted through walls is alvrays problematic the porous insulation typically used to retard heat flow does not effectively absorb transmitted sounds. Effective absorption of transmitted noise is best achieved with massive materials (such as brick), but this is at variance with modern construction practice using lightweight materials. Living spaces located near busy freeways should be designed to reduce the transmitted noise to no more than 60 dB. 

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