In This Issue
Noise Engineering
September 1, 2007 Volume 37 Issue 3

Engineering Controls for Reducing Workplace Noise

Saturday, September 1, 2007

Author: Robert D. Bruce

When purchasing equipment, industry leaders often fail to take into account the risk to hearing.

Millions of workers in the United States are exposed to sound levels that are likely to cause permanent hearing loss, even though many of them wear hearing-protection devices. Many people do not realize that these devices and hearing-protection programs are not the preferred way of protecting hearing. The preferred way, often called “engineering controls,” is to reduce the noise of machinery or introduce a noise control element between machinery and workers.

Engineering controls are preferred for many reasons, including permanence, effectiveness with or without worker/supervisor compliance, less absenteeism, easier communication, lower worker compensation costs, and reduced legal costs. In fact, engineering controls are the protection method of choice according to the Occupational Safety and Health Administration (OSHA).

This paper reviews the use of engineering controls for existing noise sources in American workplaces. Many of these controls could be integrated into machinery by original equipment manufacturers, but, for non-engineering reasons, they have been eliminated from the machinery design.
Since the late 1940s, scientists and engineers have been working on ways to control noise from machinery. In the 1970s, the emphasis was on engineering controls in the workplace, but since then the focus has shifted because OSHA has not enforced the requirement for engineering controls and because industry leaders have failed to take into account the risk to hearing when purchasing equipment.

Occupational Noise-Exposure Regulation
The OSHA Regulation (often called Standard) 29 CFR Occupational Noise Exposure-1910.95 is excerpted below:

When employees are subjected to sound exceeding those listed in Table G-16, feasible administrative or engineering controls shall be utilized. If such controls fail to reduce sound levels within the levels of Table G-16, personal protective equipment shall be provided and used to reduce sound levels to within the levels of the table.

If the variations in noise level involve maxima at intervals of 1 second or less, it is to be considered continuous.

Permissible Noise Exposures (1)

Duraction per day, hoursSound level dBA slow response
1/4 or less115
    Footnote(1) When the daily noise exposure is composed of two or more periods of noise exposure of different levels, their combined effect should be considered, rather than the individual effect of each. If the sum of the following fractions: C(1)/T(1) + C(2)/T(2)+…+C(n)/T(n) exceeds unity, then, the mixed exposure should be considered to exceed the limit value. Cn indicates the total time of exposure at a specified noise level, and T(n) indicates the total time of exposure permitted at that level. Exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure level.

    Note that the regulation calls for engineering controls to be used first to reduce sound levels to within the limits specified in Table G-16, and, only if the controls do not succeed in bringing the sound levels down are hearing-protection devices to be used in addition.

    The requirement is that “feasible administrative or engineering controls” be tried. Simply explained, “administrative control” means removing workers from noise exposure by rotating them from noisy to quieter areas. “Feasible” means what it always means—that it can be done. However, sometimes the cost of a noise control treatment is cited as a reason it is “not feasible” for a particular company to install it.
    A number of very simple engineering controls can often be implemented with great success:
    • proper maintenance (e.g., fixing steam leaks)
    • modified operating procedures (e.g., relocating an operator and equipment controls to a quieter position)
    • relocation of noisy vents away from workers
    • replacement of equipment (e.g., buying a quieter version of the product)
    • modified room treatment (e.g., introducing sound absorption in the space between equipment and worker to reduce noise in the distant reverberant field)
    • relocation of equipment (e.g., putting noisy equipment in areas that are often unoccupied)
    • proper operating speed (e.g., running equipment at lower speed to reduce noise)
    Despite the effectiveness and ease of taking these simple actions, they are often ignored.

    Noise Sources Involving Fluid Flow
    Noise sources that involve fluid flow include fans, compressors, engines, pumps, and valves. The most frequent problem is sound from the discharge, but engineering solutions (e.g., lined ducts; dissipative and reactive silencers; and special-purpose silencers) are available for both intake and discharge noise.

    Ducts can be lined with sound-absorbing material, such as fiber glass or mineral wool. Typical thicknesses are 1 to 4 inches. Thicker materials are used for low-frequency noise.

    Dissipative silencers involve using sound-absorbing materials, such as mineral wool or fiber glass, to attenuate noise. A simple dissipative silencer would be a series of parallel baffles running lengthwise from a noise source that requires airflow. The absorptive material would likely be covered with glass-fiber cloth to reduce erosion caused by airflow, and perforated or expanded metal might be added to protect against contact damage. The silencer could be improved by using longer baffles and decreasing the space between them.

    Reactive silencers operate on the basis of mismatching acoustic impedance. Whenever a sound wave meets a change in the acoustic impedance, some of its energy is reflected back to the source or back and forth within the silencer. An example of a reactive muffler is shown in Figure 1.

    FIGURE 1 Diagram of reactive mufflers showing expansion chambers.

    Recently, a special-purpose silencer was developed based on the principles of a Helmholtz resonator (a small neck and a larger cavity, such as a bottle). As sound waves pass over the opening of the neck, a small portion of gas at the neck of the bottle begins to oscillate back and forth. The frequency at which this “slug” of gas oscillates is a function of mass-spring resonance—the slug in the neck acts as the mass, and the volume of gas in the cavity acts as the spring. This resonance is a function of the diameter of the neck, the density of the gas, the length of the neck, the volume of the cavity, and the speed of sound in the gas.
    The resonance frequency in Hz of a simple Helmholtz resonator can be calculated from the following expression:

    f = ________1____
    2p (M C)0.5

    M =rl/A ( = the gas density (kg/m3)), l = the “effective” length of the neck (m), and A = the cross-sectional area of the neck (m2). C = V/c2 (V = the volume of the cavity (m3), = the density (kg/m3), and c = the speed of sound (m/s) in the gas).

    The ends of the tube influence the resonance frequency by increasing the effective length (l) of the neck. Note that the resonance frequency is independent of density. The resonance frequency of a typical 500 ml water bottle, about 200 Hz, can be excited by blowing across the top of the bottle.

    One special-purpose silencer, invented by Liu (2003) of Dresser-Rand, uses the Helmholtz concept to reduce centrifugal compressor noise, which often includes a strong tonal component at the blade-passing frequency of the compressor. The resonators for the Dresser-Rand Duct Resonator Array (DRA) are shown in Figure 2.

    FIGURE 2 Dresser-Rand Duct Resonator Array (per patent 6,550.574 B2).

    The numerous holes in the resonators, acting as masses in parallel, raise the resonance frequency. The cavity behind these holes acts as the volume for the resonator. The DRA, which consists of numerous resonators positioned at the diffuser of the compressor, can also be located in a pipe spool in discharge piping. The DRA reduces the A-weighted sound level by at least 10 dB—which is similar to “halving” the loudness of the sound.

    Significant noise radiating from piping in refineries and from forced-draft and induced-draft fan ducts can be reduced simply with layers of treatment around the piping (Figure 3). Typically, the first layer is 2 to 4 inches of 6 to 8 lbs/ft3 glass fiber or mineral wool wrapped around the pipe. Next a mass-loaded vinyl or lead sheeting weighing 1 to 2 lbs/ft2 is wrapped around the glass fiber or mineral wool. Finally, a weatherproof covering is added. This type of treatment can provide 10 to 20 dBA of noise reduction, depending on the details of the installation.

    FIGURE 3 Pipe Logging.

    Radiated Noise from Machinery Housings
    Airborne noise can radiate from any surface. In a piano, for example, the keys strike the hammers, which hit the strings. Although the strings do not produce much sound by themselves, they are attached to the soundboard, which is very large compared to the strings and radiates the sound. In general, the larger the vibrating panel, the more sound is radiated from the surface.

    Another example is a metal parts bin into which metal parts are dropped. If the bin is made of perforated, rather than solid metal, the radiating area, and thus the radiated sound, is reduced. Of course, materials with high internal damping radiate less noise. If the bin were made of rubber (with high internal damping) rather than metal (with low internal damping), the radiated sound would be reduced accordingly.

    Sometimes machinery is housed inside an enclosure provided by the original equipment manufacturer. If there is any possibility that the resonance frequencies of the enclosure panels will be excited, it is desirable that the housing be treated with a damping material. If the machine inside the enclosure causes significant vibration of the enclosure housing and structure, then the panels should be “vibration-isolated” from the structure. In addition, it may be useful for the machinery enclosure to be mounted on vibration isolators to minimize the amount of vibration transmitted to the floor.

    Machinery Shields
    An acoustical shield may be inserted between the worker and a noisy section of a machine. An acoustical shield, often mounted on the machine, can provide 8 to 10 dB of noise reduction under the following circumstances: the worker is near the noisy operation; the smallest dimension of the shield is at least three times the wavelength of the dominant noise; and the ceiling above the machine is covered with sound-absorptive material.

    Shields can be manufactured from safety glass, quarter-inch thick clear plastic, metal, or wood. Criteria for selecting materials include durability, cost, the need for visual observation of the operation, and the need for physical access to the operation. If possible, oil-resistant, cleanable, sound-absorptive materials should be incorporated into the machine side of the shield.

    Handles and casters can be provided to facilitate moving of the shield, and hinged sections can be incorporated into the design to provide physical access to the machine. Neoprene can be used to minimize acoustical leaks through joints or hinges.

    When shields are used to replace less acoustically efficient machine guards, the shield should be fitted carefully to cover all noise leaks and then vibration-isolated to keep the shield from vibrating with the machine and thus radiating sound.

    FIGURE 4 Geometry for determining sound attenuation by a noise barrier (d is the straight line distance from source to receiver and A+B is the shortest path length of wave travel over the wall between source and receiver).

    Any solid wall that blocks the line of sight between a noise source and an observer will reduce the noise level at the location of the observer. The noise reduction depends on the frequency of the noise, the distance between the source and the wall, the distance between the receptor and the wall, and the height of the wall. Low-frequency sound, which has wavelengths comparable to the size of the wall, diffracts around the ends and over the top. Thus, low-frequency sounds are less attenuated than high-frequency sounds. Typically, low-frequency sounds are attenuated by less than 5 dB, whereas high-frequency sounds can be attenuated by as much as 20 dB.

    The highest practical value for barrier attenuation is 24 dB. If the noise source is inside a room, then the barrier effect may be reduced, depending on the room absorption and the location of the receivers relative to the barrier wall.

    Most formulas for calculating the attenuation provided by a barrier assume that the wall is infinitely long. One typical calculation procedure uses the difference between a direct path from the source to the receiver and the path over the barrier (Figure 4). This difference is called the “path length difference.”

    Attenuation = 10 log (3 + 0.12 f P), where f = the frequency in Hz, and P is the path difference in meters (A+Bd).

    Partial Enclosures
    A partial enclosure is a series of walls around a machine with the top left open. This treatment can be effective inside a plant for positions near a wall. However, some of the noise radiates out the open top and contributes to the reverberant sound in the room. Reflections from the ceiling increase the sound level at greater distances from the barrier (Figure 5).

    FIGURE 5 Source-barrier-reciever geometry for an indoor noise source.

    For a 10 dB reduction, the angle shown in the figure must be greater than 30?, and the ceiling must either be sound absorptive or 1.5 times the distance from the source to the receiver. A sound-absorptive ceiling can reduce reflected sounds, thereby increasing the effectiveness of the barrier. For maximum effectiveness, the sound-absorptive ceiling should extend out to the location of the receivers, and the inside of the barrier walls should be sound absorptive.

    Total Enclosures
    A total enclosure with a closed top will provide better noise reduction than a partial enclosure. However, enclosures usually need openings to provide access for personnel (e.g., visual, maintenance, operator usage); access to materials (e.g., raw materials, product, scrap removal); and ventilation.

    Leaks around doors, windows, and hatches greatly reduce the acoustical effectiveness of enclosures. Closed-cell elastomeric weather-stripping with a pressure-sensitive adhesive can be used to prevent sound leakage from around doors, windows, and hatches. Special acoustical gaskets are available, as well as magnetic-strip gaskets similar to those used on refrigerator doors.

    If workers need to be able to see inside an enclosure, lighting may be required. If workers evaluate the performance of the machinery by its sound, it may be necessary to retrain them or to place a rugged microphone inside the enclosure that sends a signal to a small adjustable loudspeaker at the worker position. Occasionally, it is possible to develop processors that incorporate workers’ knowledge to automatically adjust the machinery for optimal performance.

    Openings for raw materials, products, and scrap-flow can be tunnels lined with sound-absorptive material. The noise reduction will depend on the length and cross-section of the tunnel, as well as the thickness of the sound-absorptive material.

    Ventilation is required for all total enclosures and for some partial enclosures. The amount of air required for cooling is a function of the heat generated within the enclosure, as shown in the following equation:

    Q = 1.76 W/∆T

    Q = the flow of cooling air in cubic feet per meter at sea level, W = the watts of heat generated, and T = the temperature rise permitted above the ambient temperature (?F). Ventilation openings can be acoustically lined ducts, elbows, or mufflers, depending on the severity of the problem. Machinery with special heat-sensitive equipment may require special cooling.

    Neither the enclosure panels nor the enclosure structure should be in contact with any part of the machinery. If the enclosure is mounted on the machinery, it should be vibration-isolated from the machinery. All holes in the enclosure for electricity, oil, water, steam, air, or hydraulic power must open into a junction box packed and sealed with at least 3 inches of glass fiber.

    A convenient design can be built on an angle-iron frame to which the enclosure panels can be attached with quarter-turn captive screws. The enclosure should be as small as possible without touching the machinery. Noise reductions of 20 dB can be obtained with careful attention to design and construction.

    FIGURE 6 Enclosure panels secured to frame by quarter-turn fasteners.

    Figure 6 shows the connection of the panels to the angle iron frame. Damping and sound-absorption material are attached to the interior of the panel.
    The enclosure should also be vibration-isolated from the floor (Figure 7). If the machine vibrates, it may also be important to isolate it using steel springs or elastomers, depending on the circumstances.

    FIGURE 7 Vibration isolation and toe covering.

    If the enclosure panels are metal, their resonances can be distributed uniformly in frequency if the panel is reinforced by bolted-on angle irons (bolting adds damping). These stiffeners should be placed to divide the panel into smaller areas, no two of which are the same size and shape. Frames for doors, windows, and hatches can also be used as stiffeners.

    Windows are an acoustical weak link in enclosures. Generally, if the A-weighted sound level must be reduced by more than 20 dB, double-glazing will be necessary. The inside layer, which must withstand rough handling and cleaning to remove oil, grease, and dirt, should be safety glass. All panes should be set into soft elastomer gaskets. Room-temperature-setting silicone rubbers are useful. The window(s) should be placed carefully to provide the necessary information to the operator. In extreme cases, closed-circuit video monitoring can be used.

    If the dimensions of the enclosure result in resonance, the enclosure can be driven to high levels of vibration and become a new radiater of these sound components. When the enclosure is driven to high levels of vibration, the sound-pressure level outside the enclosure can be higher than it was before the enclosure was installed. If the noise source occupies a sufficient fraction of the room volume, this effect can be significantly reduced by using absorbent lining on the interior surfaces of the enclosure and by stiffening the panels and lining them with damping materials.

    As these brief descriptions show, engineering controls are available for use in industries today! As Liu of Dresser-Rand has shown, the development of innovative noise control treatments provides opportunities for applying basic physics and engineering. Like any other engineering problem, noise control requires detailed work—first to identify the source, then to determine the most effective noise control technique, and then to determine the most cost-effective solution. However, like any other purchase, quality costs.

    Note: Numerous books and publications are available for reference. For formulae and specific applications see Berger et al., 2000; Bies and Hansen, 2003; Bruce and Bommer, 2004; and Jensen et al., 1978.

    29 CFR 1910.95 (Code of Federal Regulations Title 29, Part 1910).
    Berger, E.H., L.H. Royster, J.D. Royster, D.P. Driscoll, and M. Lane. 2000. The Noise Manual. Fairfax, Va.: American Industrial Hygiene Association.
    Bies, D.A., and C.H. Hansen. 2003. Engineering Noise Control: Theory and Practice. London, U.K.: Taylor & Francis.
    Bruce, R.D., and A.S. Bommer. 2004. The Occupational Noise Book. NIOSH Contract No. 0000037091. Cincinnati, Ohio: National Institute of Occupational Safety and Health.
    Jensen, P., C.R. Jokel, and L.N. Miller. 1978. Industrial Noise Control Manual. NIOSH Contract No. 210-76-0149. Cincinnati, Ohio: National Institute of Occupational Safety and Health.
    Liu, Z. 2003. Acoustic Liner and a Fluid Pressurizing Device and Method Utilizing Same. U.S. Patent No. US 6,550,574 B2, April 2, 2003.
    About the Author:Robert D. Bruce is principal engineer, Collaboration in Science and Technology Inc., Houston, Texas.