Acoustic-absorbing material

The reveals of a double-glazed window should be lined with acoustically absorbent material to damp the sound within the cavity. The width of the cavity should not be less than 150 mm. 

In the initial selection of an acoustic absorbing material for an underwater application, the first considerations are often the density, and the complex dynamic shear modulus. These quantities can be measured in the laboratory,requiring only small sample sizes and hence are useful as a guide to material development. 

Acoustic-absorbing material measurement of acoustic properties, 248 selection for underwater application, 248 Acoustic attenuation mechanisms mode conversion, 182-187 redirection of sound, 182 scattering of sound, 185-194... 

Enclosures which give an attenuation of between 10 and 30dBA are the most satisfactory solution since they will control both the direct field and reverberant field noise components. In enclosing any source, the provision of adequate ventilation, access and maintenance facilities must be considered. A typical enclosure construction is shown in Figure 20. W. The main features are an outer heavy wall with an inner lining of an acoustically absorbent material to minimise reverberant build-up inside the enclosure. An inner mesh or perforate panel may be used to minimise mechanical damage. 

Absorptive, where sound is absorbed by an acoustical absorbent material. 

In situations where there is a high degree of reflection of sound waves, i.e. the building is acoustically hard, the reverberant component can dominate the noise field over a large part of the work area. The introduction of an acoustically absorbent material in the form of wall treatment and/or functional absorbers at ceiling height as shown in Figure 20.14 will reduce the reverberant component by up to 10 dBA , but will not reduce the noise radiated directly by the source. 

Restrictions on shedding of fibers in air by acoustically absorbing materials Ehmination of uninspectable spaces in which vermin may hide Requirements for secure anchoring of heavy equipment... 

In a practical sense, enclosure designs often require some accommodation. For ease of maintenance, enclosures can be constmcted to rise upward and away from the machinery by overhead cranes. Access openings can be provided by tunnels lined with acoustically absorbent material (in effect, mnfflers). Access for controls can be designed with hinged covers that lift easily, or the controls can be relocated. Some enclosure panels can be lifted automatically at the correct point in the machine cycle to provide access. Ducts can be provided with small ventilating fans to prodnce a controlled cooling airstream and, if combined with filters, can allow a controlled environment for some operations. 

An enclosure is a prefabricated housing that absorbs the sound energy of the equipment. Its effectiveness is increased if acoustical absorbing material is added to its interior. A totally leak-tight enclosure may reduce exposure by 10—15 dB(A). 

The sound-absorbing properties of acoustical materials also are influenced by the manner in which the materials are mounted. Standard mounting methods for use in laboratory testing are specified in ASTM E795-92 (2). Unless noted otherwise, pubflshed data for acoustic ceiling materials are for Mounting Type E-400, for which the material being tested is suspended 400 mm below a hard surface. 

Metal Pan Assemblies. These units consist of tiles and panels formed from perforated aluminum or steel with pads of fiber glass or mineral wool inserted into the pans to provide the sound absorption. They are used primarily for ceilings in a similar manner to acoustical tiles and panels. The pads are sometimes sealed in plastic film to prevent absorption of moisture, dirt, and odors. The perforated metal is relatively sound transparent and functions as the finished ceiling and the support for the sound-absorbing material. The perforated metal by itself has no acoustical value. 

Acoustical Louvers. Acoustical louvers are used in building mechanical systems when exterior walls are penetrated for fresh air intake, exhaust, or rehef air, in situations where the impact of HVAC noise is of concern in the surrounding environment. The louvers consist of a series of hoUow sheet metal blades. The bottom faces of the louver blades are perforated and the blades are filled with fibrous sound-absorbing material. Typical acoustical louvers are 20 cm (8 in.) to 30 cm (12 in.) in depth. The amount of insertion loss they provide is limited. 

Acoustic muff (muffler) Sound-absorbing material that is placed around a noisy item in a plant. 

Acoustic pod Sound-absorbing material inserted in ductwork to absorb sound. 

Applications of ultrasonic techniques to solid-gas systems rely on the fact that velocity and attenuation of US-waves in porous materials is closely related to pore size, porosity, tortuosity, permeability and flux resistivity. Thus, the flux resistivity of acoustic absorbents oan be related to US attenuation [118,119], while the velocity of slow longitudinal US is related to pore tortuosity and diffusion, and transport properties, of other porous materials [120]. Ultrasound attenuation is very sensitive to the presence of an external agent suoh as moisture in the pore space [121] and has been used to monitor wetting and drying prooesses [122] on the other hand, US velocity has been used to measure the elastic coefficients of different types of paper and correlate them with properties such as tensile breaking strength, compressive strength, etc. [123]. 

Absorption coefficient ratio of sound absorbing effectiveness, at a specific frequency, of a unit area of acoustical absorbent to a unit area of perfectly absorptive material. 

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