Sound absorbers panel

CeUular urea—formaldehyde and phenoHc resin foams have been used to some extent in interior sound-absorbing panels and, in Europe, expanded polystyrene has been used in the design of sound-absorbing doors (233). In general, cost, dammabUity, and cleaning difficulties have prevented significant penetration of the acoustical tile market. The low percent of redection of sound waves from plastic foam surfaces has led to their use in anechoic chambers (216). 

Enclose noisy machines in sound-absorbing panels. 

Resonant Sound Absorbers. Two other types of sound-absorbing treatments, resonant panel absorbers and resonant cavity absorbers (Helmholtz resonators), are used in special appHcations, usually to absorb low frequency sounds in a narrow range of frequencies. Resonant panel absorbers consist of thin plywood or other membrane-like materials installed over a sealed airspace. These absorbers are tuned to specific frequencies, which are a function of the mass of the membrane and the depth of the airspace behind it. Resonant cavity absorbers consist of a volume of air with a restricted aperture to the sound field. They are tuned to specific frequencies, which are a function of the volume of the cavity and the size and geometry of the aperture. 

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. 

Lightweight mats of fibres, which can be die-cut and thermally moulded, can be applied behind any interior trim panel, door panels, headliners or package shelves as superior sound absorbers. 

IP Other Parts The brackets, sound-absorbing materials, electronics, and other parts attached to the instrument panel are made from a variety of materials. For items such as brackets and sound-absorbing materials, it is best to employ materials that are compatible with the IP substrate. For electronics and other complicated small parts, it is best to design them to be easily removed during recycling. 

A possible solution for barriers and shields is to place a small panel across the top of the shield or barrier at an angle perpendicular to the sound source to stop the vertical movement of sound. Another solution is to apply sound-absorbing material to the ceiling to reduce sound reflected downward. 

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. 

Machinery enclosures Walls of acoustic enclosures should consist of heavy but flexible panels lined internally with sound-absorbing material. Adequate ventilation, demountability of the enclosure for repairs, and arrangements for operating and/or observing the machine to be provided. If hot gas fans are acoustically enclosed, the surface of the fan casing should additionally be heat-insulated. For information on design and performance of enclosures for noise control see VDI2711. 

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. 

This section considers first the behavior of fiat, unlined panels, then describes the physical reasons why corrugated, unlined panels have a different acoustic response to fiat panels. The effects of sound-absorbent Unings on flat and corrugated panels are then considered. 

When there are special acoustic requirements, the generator can be installed in a sound-absorbing enclosure consisting of a steel frame with panels of perforated steel sheets with sotmd-absorbing mineral wool in-fill. 

Because of their cellular or open-matrix construction, most insulants have an inherent ability to absorb sound, act as panel dampers and reduce noise breakout from plant by their ability to be a flexible or discontinuous link between an acoustically active surface and the outer cladding. This secondary aspect of thermal insulation specification will gain more prominence when the UK adopts the EC Directive 86/1888, Protection of workers from the risks related to exposure to noise at work . 

Sound pressure decreases inversely with distance from a source. In a free held, sound moves from a source in a spherical pattern. However, most sources and environments are not free fields, so sound may project unevenly. It may strike objects, panels, walls, and ceilings and be reflected. It may become trapped and absorbed in holes in materials and lose energy. It may reflect and create standing waves that... 

Enclosures which give an attenuation of between 10 and SOdBA 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 3.5.10. The main features are an outer Tieavy wall with an iimer lining of an acoustically absorbent material to minimise reverberant sound build-up inside the enclosure. An inner mesh or perforate panel may be used to minimise mechanical damage. 

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