Acoustics acoustic enclosures

Finally, noise is also becoming of increasing concern, particularly on sites adjacent to urban areas. Acoustic enclosures, supplied by the manufacturer, will normally reduce noise levels to 85 dB, but if further reduction is required, attention must be paid to the material used to construct the buildings and enclosures low noise levels can be achieved only at a cost. 

Feedwater pumps will not normally constitute a noise problem unless the area is particularly sensitive. Two alternatives are then available. One is to install reduced-speed pumps and the other is to site the pumps in a separate acoustic enclosure within the boiler house. Oil-circulating pumps are usually low speed and, as such, do not cause a noise problem. 

Gas boosters may be fitted with motor enclosures or designed to operate at lower rotating speeds. Alternatively, they may be housed in an acoustic enclosure within the boiler house. The booster and drive unit can be supplied with anti-vibration mountings to isolate it from the floor or steelwork and flexible bellows fitted to the gas inlet and outlet connections. 

That the authorities have refused unreasonably to accept compliance with alternative requirements or that the requirements of the notice are otherwise unreasonable in character or extent, or are unnecessary. This defense is self-explanatory. The local authorities are only permitted to ask for works that will abate the noise nuisance. Other works (perhaps to comply with legislation) should not be specified in the notice. They may, however, be contained in a letter separate from the notice. An example of this would be where the fitting of acoustic enclosures to food-manufacturing machines breached food hygiene requirements. Readily cleanable enclosures may be a requirement of the Food Hygiene Regulations, but it should not be contained in a Section 58 Control of Pollution Act notice. 

The degree of attenuation at the critical frequency can be very large, but this type of silencer has a very narrow bandwidth. This device may be suitable when the machine being dealt with emits sound predominantly of a single wavelength. Lining the chamber with absorbers can expand the absorber bandwidth of a Helmholtz resonator, but this has the effect of reducing the efficiency. The perforated absorber, which forms the basis of many acoustic enclosures and silencers, is a development of the resonator principle. 

Turbines and gas compressors are normally provided as a complete assembly in an acoustical enclosure. Because the equipment is enclosed and handles gas supplies, it is a prime candidate for the possibility of a gas explosion and fire. 

A one hour fire wall or "substantial space" should be provided between the turbine and gas compressor. The utilization of explosion blow-out panels in the acoustical enclosure will also limit damage from an explosion. Although strengthen panels could be provided to protect against shrapnel ejection, the cost installation coupled with the low probability of such an occurrence and the low personnel exposure periods, generally render this a non-cost effective safety improvement. 

Where ventilation is installed to effect a reduction in the area electrical classification, the rate should be no less than 12 air changes per hour for an adequately ventilated area in accordance with API RP 500. Areas containing ignition sources, such as control and switchgear buildings, gas turbine acoustical enclosures, and power generators, should be pressurized in accordance with NFPA 496, if located in an electrically classified area. 

Vibrations that are caused by equipment may be transmitted via the corresponding cables. To dampen out vibrations of this sort, cables can be equipped with heavy masses, e.g., metal parts. For sensitive experiments, the placement of the entire AFM scan unit inside acoustic enclosures may be also advisable. 

The cooled gases from the plant were burned at ground level within an acoustically lined enclosure at the base of a 120-ft. nigh chimney stack. 

Place a barrier on the energy source. Example Place machine guards or utilize acoustic enclosures... 

Unless specified otherwise in applicable standards, the sound level should not exceed 85 dB at 1 m from casing/surface and 1.5 m above the equipment base. The control of sound to meet this requirement shall be the sole responsibility of the vendor. Suitably designed silencers or acoustic enclosures should be provided by the vendor at no additional cost to purchaser. 

Noise level (when operating at rated capacity) this should be below 70 dB at about 2 m from the blower. In case of higher noise, an acoustic enclosure will be required, which will add to the cost. 

Noise level at full load operation should be less than 60 dB. Otherwise a separate acoustic enclosure as per statutory instmctions will be required. 

Table 20.3. Sound reduction indices for materials commonly used for acoustic enclosures...

The two major methods of personnel protection are the provision of a quiet room or peace haven, and the wearing of ear-muffs or ear-plugs. The peace haven is similar in construction to an acoustic enclosure, and is used to keep the noise out. Ear-muffs or ear-plugs should be regarded only as the final resort to noise control. Their selection should be made with care having regard for the noise source, the environment and comfort of the wearer. 

Noisy machinery is sometimes placed inside a small containment building or acoustic enclosure. Acoustic enclosures may be either of the close fitting or walk-in (large) type. The use of acoustic enclosures is generally not a preferred solution to noise problems because they create a confined space and have the potential for a buildup of a flammable or explosive cloud should the equipment leak. Also, the containment structure is likely to make maintenance and inspection more difficult (a confined space entry will be required), and it will increase the size of the equipment s footprint. 

Mills Acoustic enclosure of the mill shell obstructs heat dissipation and makes inspection difficult, i. e., is not satisfactory. Boltless liner plates on rubber backing, or rubber liners, do not sufficiently reduce noise emission. A commonly employed and generally satisfactory solution is as follows Whole grinding plant with all its noise sources (mill, drive, air separator, conveyors, elevators, filters, etc.) accommodated in a closed sound-insulated building with central control room for operators and with additional ventilation (see Funke, 1969 and 1973, Techn. Merkblatt, Bundesverband der Deutschen Kalkindustrie, 1975). 

Fuel oil pumps Pumps installed in closed sound-insulated room or in an acoustic enclosure with insulated openings for the passage of pipes and controls (see VDI Code 2711). 

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. 

Gas pressure reducing stations and pipelines Closed sound-insulated buildings or acoustic enclosures. Sound-insulating lagging around pipelines. Sound attenuators in the pipes. Acoustic covers over valves. 

Rotary kiln drives Acoustic screening walls or acoustic enclosures with soundabsorbing linings, thermally insulated against kiln heat (if necessary) and with additional ventilation. 

Drilling machines (rock drills in quarry) Acoustic enclosure of drive units and hydraulic equipment, as on compressors used on construction sites. Noise-suppression covers and silencers for exhaust air outlets of hammer drills. Sound-insulated control cabs (see Funke, 1973). 

Noisy machines should be accommodated in different rooms from quieter ones. Special acoustic enclosures for machinery with particularly high noise emission levels. 

Miller, R. K., and W. V. Montone, Handbook of Acoustical Enclosures and Barriers, Fairmont Press, Atlanta, 1978. 

The structure-borne noise generation and transmission of stiffened and interconnected structures under random loads is presented. The method is based on the transfer matrix for the stuc-tural response and on the modal decomposition for the interior acoustic field. The acoustic enclosure is taken to be rectangular in shape of which portion of the boundaries are elastic while the remaining surface is acoustically rigid. Numerical results are presented for a variety of acousto-structural problems. 

Consider a rectangular acoustic space occupying a volume V = abd zis shown in Fig. 1. The interior surface of the enclosure is assumed to be covered with absorptive materials for which the impedance characteristics are specified. Noise is generated in the acoustic enclosure through the vibration of the flexible portions of the side-walls, the partitions or the sound sources located in the interior. The perturbation pressure p within the enclosure satisfies the linearized acoustic wave equation... 

For the calculation of noise transmission into the enclosure, a simplified model shown in Fig. 4 has been chosen. A cantilever box beam is attached to two stiffeners of the discretely stiffened sidewall. A random point couple is acting at the free end of the cantilever beam. The walls at z 0 and z = d of the acoustic enclosure are treated with absorbtive materials which are represented by the point impedance model as... 

The sound pressure levels are given in Fig. 5 for an acoustic enclosure with small amount of absorption at the walls (Za — oo), and for the case where the walls are treated with porous materials for which the point impedance is given in equations (36)-(38). For this case, inputs to the stiffened panel are four point forces acting on the two intermediate stringers. The spectral densities of those four forces are assumed to be the same and equal to 0.S4 jHz over the frequency range 0... 

Turbines and gas compressors are commonly provided as a complete assembly by vendors in an acoustical enclosure. Because the equipment is enclosed and handles gas supplies, it is a prime candidate for the possibility of a gas explosion and fire. The most obvious source of a gas accumulation is a fuel leak. Other rare losses have occurred due to lubrication failures, causing the equipment to overheat, with subsequent metal fatigue and disintegration. Once disintegration occurs, heat release from the combustion chamber will occur along with shrapnel and small projectiles that will be thrown from the unit from the inertia momentum of the rotating device. 

EEMUA 161 A Guide to the Selection and Assessment of Silencers and Acoustic Enclosures... 

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