Acoustic absorber

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. 

Figure 3-35 AOTF Crystal showing the RF source, the acoustic transducer, the Acoustic absorbed, the Zero order ray and the two 1st order rays. (Reproduced with permission from Ref. 103.)...

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... 

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]. 

Acoustic absorbance and insulation are accomplished by dispersion of sound waves in the open cells. 

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. 

Ultrasound through airborne conduction does not appear to pose a significant health hazard to humans. However, exposure to the associated high volumes of audible sound can produce a variety of effects, including fatigue, headaches, nausea, and tinnitus. When ultrasonic equipment is operated in the laboratory, the apparatus must be enclosed in a 2-cm-thick wooden box or in a box lined with acoustically absorbing foam or tiles to substantially reduce... 

One of the most difficult adhesive problems involves the underwater sonar systems. Elastomers (29) (Neoprene rubbers) are adhered to metals with adhesives. The applications include cable sheath, vibration isolator and damper, acoustic absorber, sound reflector, sonar-dome window, and transducer boot. A poor bond at the Neoprene-steel interface could easily lead to saltwater penetration and corrosion, which eventually causes the failure of the sonar dome. 

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. 

Airborne noise applications include acoustically transparent open-cell polyurethane foam (148) for use as a microphone windscreen, and acoustically absorbing polyurethane foam (149) for noise abatement. Polyester-based foam has superior mechanical properties and acoustical absorption, but poor humid aging. Polyether-based foam has better humidity resistance and is inexpensive, but has less absorption. 

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. 

Cox, T.J., D Antonio, R, 2009. Acoustic Absorbers and Diffusers Theory, Design and Application, second ed. Taylor and Erancis, London and New York. 

Materials with specific properties that have been studied include acoustical absorbing and vibration damping materials. Various conventional elastomers, polyurethanes, and interpenetrating network polymers have been studied for such acoustical applications. In addition, polymers are being used in the design of new piezoelectric composite materials containing a ceramic phase. 

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). 

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