Noel W. Hart, Kimberly Riegel, and Robert D. Bruce, CSTI acoustics
If quiet equipment cannot be purchased, engineering noise controls should be implemented, especially for people with time-weighted average (TWA) exposures greater than 90 dBA. Engineering noise controls are much more effective than either administrative controls or personal hearing protection for several reasons: They are always active and in place, have the same quieting effect for visiting contractors as for regular workers, and require less effort from management to ensure the treatments are working properly.
Engineering based noise control carries a higher up-front cost than hearing protection, but the benefits often far outweigh the costs. Safety improves, because all forms of communication, verbal and electronic, improve in quieter environments. Studies have even shown that worker morale and productivity improve with quieter workplaces (Driscoll & Royster, 2000). With all of the financial and ethical benefits offered by engineering noise controls, it is a wonder why so many companies still rely primarily on hearing protection.
Different types of engineering controls exist, each with strengths and drawbacks. A professional noise control engineer can assist companies in selecting and designing the optimum treatment for any particular application. Enclosures are probably the oldest form of noise control, very useful when implemented correctly.
Enclosures can range in size from whole buildings to small, rigid skins wrapped around noisy pumps. There are full or partial enclosures. Figure 1 shows a full enclosure for a propane powered generator. Enclosures work by inserting a solid mass between the equipment and the outside. It is important to ensure the enclosure has a high enough transmission loss (TL) to meet the desired noise criteria. TL values are measured in dB. As simple as this concept is, much work must be put into properly designing an enclosure to ensure sound attenuation is within the desired range, ease of access for maintenance addressed, and proper cooling and ventilation implemented. The most common problems when designing enclosures are:
Sound absorbing materials absorb rather than reflect sound. Sound energy increases in confined spaces due to a large number of repeating reflections. The length of time it takes for the sound energy in the room to cease after the source has stopped is known as reverberation time (Harris, 1998). If some or all of these reflections are absorbed, the sound level inside the room will decrease, and this is why all noise enclosures should have internal sound absorption.
Most sound absorptive materials are porous, such as sheets of mineral wool, fiberglass batting, ceiling tiles, or porous foam materials, but hard surface absorbers such as the micro perforated panels first described by Daa-You Maa (1975) are slowly entering the market. Absorption is quantified by a material’s absorption coefficient (a), a number between 0 and 1. Materials with a values equal to 1 are totally absorptive, and materials with values equal to 0 are totally reflective. Due to imperfections in testing methods, it is possible to obtain measured a values greater than 1.
A barrier wall works by occluding line of site between the source and receiver. They are extremely common in highway noise control and along borders of industrial facilities. Figure 2 shows a barrier wall of a compressor station. Barrier walls work extremely well for high frequencies above about 500 Hz, but due to the nature of low frequency sound and its wavelength, become very poor controls for frequencies below 500 Hz. A barrier is effective because it lengthens the path from source to receiver, causing the sound to travel over the barrier. The closer the barrier is to the source or the receiver, the more effective it will be. A barrier is least effective when placed equidistant from source and receiver.
When designing a barrier wall, it is important to consider its construction materials. Whatever the barrier is constructed of must have ample mass to ensure the TL of the wall does not limit its noise control potential. Care must be taken to eliminate overhanging structures or vegetation from offering reflection points around or over the wall. Any holes or passages through the wall will reduce its effectiveness and should be accounted for in the design process. Because a barrier wall only reduces sound on the opposite side of the wall, often times quieting the source will provide noise control with a higher degree of confidence.
Silencers and Lagging
Piping and other flow-induced noise sources can be quieted by the addition of a silencer or a lagging treatment. Both treatments are measured in terms of their insertion loss (IL) in dB. A silencer is a device or section that is installed inline and reduces the noise output at every point downstream. Silencers can be used to quiet internal combustion engines, HVAC systems, natural gas compressors and even pneumatic tools with little to no impact on performance.
Two types of silencers exist, reactive and dissipative. Dissipative silencers work by adding sound absorptive material to the interior of the pipe or duct. The absorption works as described earlier to reduce the sound buildup. Reactive silencers work on principles of pressure differentials to cancel out some of the noise.
Pneumatic tool silencers are available in several forms to quiet machinery. Air exhaust silencers can be installed on tools with loud exhaust air, such as jack hammers. Dissipative silencers or quieter, alternative gun tips with multiple flow rates can be installed in line for blown air guns to significantly reduce their noise (Driscoll & Royster, 2000).
Lagging treatments surround ducts and pipes to form a rudimentary enclosure. Consisting of an absorptive inner layer and heavy outer layer called a mass layer, high noise reduction can be achieved due to a combination of decoupling and transmission loss effects. Figure 3 shows a typical acoustic lagging treatment. Lagging treatments can inadvertently increase the noise if done improperly without a resilient enough absorption layer to decouple the pipe from the hard mass layer. Because lagging treatments are often already specified for thermal reasons, minor modifications can ensure they also have acoustic benefits as well.
Many machines become much louder with only a slight increase in speed. For these cases it may often be more convenient in the long run to operate the machine for longer periods at lower speeds. Doing so will not only reduce the noise exposures of nearby employees, but it may also extend the life of the machine.
Punch machines can create extremely loud, impulsive sounds. The noise can be quieted by simply adjusting the distance of the stock material from the punch, or by adjusting the angle of incidence so the punch is exerting a greater force over a smaller area for an extended period of time. This time increase is usually on the order of a few milliseconds.
Vibration Isolation and Damping
Structural and machine vibrations can radiate into the air, causing airborne noise. Two approaches can combat this problem, vibration isolation and damping.
Vibration isolators reduce the energy transmitted from equipment to its attached structure. They are tipically made of springs, rubber, or sometimes a combination of both. They come in numerous sizes, deflections and maximum loads. It is very important to specify the correct parameters for vibration isolators, or vibration may not be reduced and may even be amplified.
Lightweight panels vibrate with large displacements, creating equally large amounts of noise. Damping treatments can be applied to these panels to reduce displacement and the radiated noise (Bies & Hansen, 2009). Damping treatments commonly consist of trowelled or sprayed on mastic, mass loaded vinyl, or asphalt. They can be applied mechanically, with adhesives or screws, or sprayed. Some damping devices can even be welded into place.
A wide variety of engineering noise controls exist, many with specific purposes. All of them have the ability to reduce the overall noise level inside and around industrial facilities, making the work environment not only safer but also more productive and enjoyable. Engineering noise controls serve as a permanent fix to noise issues and one that provides potentially safer, cheaper, and more productive work environments.
Harris, C.M. (1998). “Sound in Enclosed Spaces.” Handbook of Acoustical Measurements and Noise Control. 3rd edition, Ed. C.M. Harris, Acoustical Society of America: 4.1-4.18
Maa, D. (1975). “Potentials of micro perforated absorbers”. Journal of the Acoustical Society of America 104 (5): 2868–2866.
Bies, D.A. & Hansen, C.H. (2009). Engineering Noise Control: Theory and Practice. Taylor and Francis: New York.
Driscoll, D.P. & Royster, L.H. (2000). “Noise Control Engineering.” The Noise Manual, 5th edition. AIHA Press: Fairfax, VA: 279-378.
A Consultant for CSTI acoustics in Houston, Noel Hart specializes in industrial noise control modeling, testing and hearing conservation, as well as architectural and litigation acoustics. He holds a Master’s degree from Rensselaer Polytechnic Institute, concentrating in acoustics