42.7 General Considerations

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In order to carry out the design of noise-control measures for a particular problem, we must consider not

only the fundamental acoustical properties of the material as discussed before, but also such practical

aspects of the problem as (1) gas flow velocities, (2) temperature of gas, (3) moisture exposure, and (4)

head losses for gas-flow. The client depends heavily on the expertise of the designer to realize adequate

protection of the noise-control equipment under operating conditions.

42.7.1 Surface Treatment with Lining of Acoustic Material

Fibrous material in the market has some form of resin binder. Comparatively long fiber flocculent and

comparatively short fiber are available. The packaging density of flocculent is about 60 to 100 kg/m3. It is

necessary to cover with perforated thin metal or wire netting so that an arbitrary shape may be

maintained in the absorbing material. The perforated metal does not take into account the numerical

aperture, hole shape, hole diameter, and metal thickness. From the acoustic viewpoint, a suitable

numerical aperture is given in Table 42.5.

42.7.2 Gas Flow Velocity

Noise control problems often involve the use of an acoustical material in high-velocity gas-flow such

as those found in the exhaust of engines or ventilating systems. Deterioration of the acoustical

FIGURE 42.16 A single-plenum chamber showing the

nomenclature used in Equation 42.26.

TABLE 42.5 Perforated Metal for Treatment of Absorbing

Material (Gas Flow Velocity is 25 m/sec or Less)

Perforation rate: 30 to 50%

Hole diameter: 5 to 10 mm

Hole shape: round, plus, slit and interminglement

Metal: iron, stainless steel (used in case of the corrosive gas)

Design of Absorption 42-15

© 2005 by Taylor & Francis Group, LLC

material due to high-velocity gas flowing past it

can be a serious problem. In addition, turbulence

in the gas flow subjects the materials to vibration

and can cause further deterioration. One solution

to this problem is to install the acoustical

material behind some form of protective facing,

which will vary in complexity depending on the

gas velocity.

A limited amount of information on this

subject is available through field experience, as

shown in Figure 42.17 [12]. However, the

parameters of the treatment structure are not

well established, for example, those concerning

perforated metal, wire net, absorbing material, and

gas flow. Multiple layers are used under conditions

of flow velocity exceeding 25 m/sec, and the

associated performance analysis can become rather

complex.

42.7.3 Gas Temperature

In many noise-control problems, temperature is a

very important factor. Sometimes high-temperature

ducts that are radiating noise, for example, in diesel engines, and induced draft fans, must be

wrapped. With a proper choice, it is possible to combine thermal and acoustical insulations using one

single material. Under extremely high temperatures, the tensile strength of materials tends to decrease,

and the material may be subjected to thermal shock.

Examples of absorbing materials that are currently available for use where temperature is an important

consideration are given in Table 42.6.

42.7.4 Dust and Water Exposure

The holes of perforated metal can be blocked if a dust treatment is not carried out, and the sound

absorption performance will deteriorate with adhesion to the surface of the absorbing material. Methods

of dust accumulation and removal may be designed into cavity type mufflers used on the sound

absorption equipment.

A fan of a cooling tower, for example, experiences a considerable amount of moisture. Precautions

must be taken so that water droplets are not deposited on the sound absorbing material. The underside

of the equipment should be treated with rust prevention material. Figure 42.18 shows the degradation

FIGURE 42.17 Protective surface for absorbing

material subjected to high-velocity gas flow.

TABLE 42.6 Fibrous Materials of Use in Hot Gas Flows

Materials Maximum Allowable

Temperature (8C)

Glass fibers with binder 320 , 360

Glass wool 960 , 1060

Mineral wool felts 1160

Mineral wool 1660

Asbestos fibers 760

Alumina-silica 1900

42-16 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

of the acoustic characteristic of absorbing

material due to moisture, using the normal

incidence absorption coefficient [13].

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