Occupational Soundscapes – Part 12: Source Noise Control

     Limiting the amount of sound energy emanating from any given source is the best way to protect hearing and maintain effective communication.  This is reflected in the first noise control priority statement (see Part 11):  “Prevent emission of sound by a source.”  Control of noise at its source can take many forms, depending on the nature of the generator.
     In this installment of the “Occupational Soundscapes” series, various types of sources are discussed.  Representative examples of source treatments are provided to demonstrate the types of analysis that may be required.  Multiple treatments may be needed to tame a soundscape, bringing noise to a safe, manageable level.

     Much of the discussion of noise control relates to retrofitting existing equipment and facilities; that is, reactive management of soundscapes.  Addressing sound in the design phase, to develop the quietest soundscape possible, is always preferable to retrofitting noise control measures after equipment is in service.  However, unforeseeable changes can occur within a machine’s service life, requiring reactive strategies.
     Similarly, the discussion is biased toward the lower rungs of the hierarchy of controls.  It is assumed, in many cases, that all opportunities for elimination and substitution have already been exploited and, therefore, only engineering and administrative controls and PPE require discussion.  In this discussion, examples of source elimination and substitution are presented agnostically, applicable to the design phase or to retrofit while in service.  Initial designs can limit the retrofit options available; creative adaptations may be needed.
PPE is the primary topic of a future installment and few administrative controls (i.e. behavior changes) effect the emission of noise.  This presentation, therefore, mostly involves engineering controls.

Maintenance
     Regular maintenance of equipment is conducted to meet productivity and reliability targets.  A thorough maintenance program also aids noise control; some activities support multiple objectives.  Examples of this type of activity include:

• Lubricating moving parts.
• Tensioning drive belts and chains.
• Aligning drive components, such as gears, pulleys, or sprockets.
• Tightening loose fasteners, particularly on guards and enclosure panels.
• Securely latching all access panels.
• Repairing damaged seals or other leaks in enclosures.
• Sealing leaks in compressed air or other pressurized system.
• Replacing components worn beyond design specifications.
• Adjusting operating speeds and pressures to optimum levels.
• Verifying that all specified noise control measures are installed and functioning properly.

     A useful rule of thumb for machine noise is that the level of sound emitted should remain within 2 dBA of its ideal condition.  Maintaining a stable noise level facilitates troubleshooting other issues that arise within a machine, as discussed in “Troubleshooting is a Six-Sense Activity” (4Dec2019).

Vibration
     When vibration is induced by operation of a machine, it can be radiated from surfaces as sound.  In some cases, the energy can travel surprising distances before becoming air-borne; locating a source in these situations can be challenging.
     The primary source of vibration is imbalance; according to some sources, it is responsible for half or more of machinery vibration issues.  Imbalance typically refers to rotating components; however, reciprocating components generate similar cyclical forces that can manifest in vibration.  Misalignment, looseness, and collision are also significant contributors to machine vibration and noise.
     Balancing, aligning, and tightening of components are routine maintenance tasks, as discussed above.  If these actions do not prevent collisions, the design and operating parameters of equipment should be reviewed to ensure it is being operated as intended, that it has not been modified, and that it is within its expected service life.
     Some collisions are inherent to processes and equipment in use; for example, an actuator that extends and retracts to hard stops.  In this case, the collision and resulting vibration should be damped.  Common solutions include the placement of elastomer pads or dashpots at the hard stops and reducing the speed or acceleration of the actuator.
     Other collisions, or impact noise sources, include the feeding of parts into or out of a machine.  These sources include material hoppers, collection bins, and sheet stackers.  To reduce the noise generated, the height from which material falls should be minimized.  Methods of achieving minimal drop height include the use of adjustable-height equipment, fall interrupters, and transition slides; examples of these are shown in Exhibit 1.  If parts are handled manually, an administrative control can be implemented, requiring that parts be placed, rather than dropped or tossed.  Higher variability of noise levels can be expected for manual operations, with or without an administrative control in place.

     Damping at the machine level can be achieved by placing a soft layer in mounting fastener joints (i.e. a “rubber washer”) or placing the entire machine on an elastomer mat.  When a machine’s panels are significant radiators of sound, the panels can be stiffened by adding braces or curvature.  A panel’s resonant characteristics can also be changed by increasing its total mass or dividing its surface into smaller areas, as shown in Exhibit 2.  Adding damping material, such as mastic, increases the effectiveness of these methods.  The panel can also be replaced with a constrained-layer (“sandwich”) panel, a preferred approach if the operating environment is likely to damage exposed damping material.
     When damping is insufficient to prevent transmission of vibration to adjacent areas, the source must be isolated.  Isolation is often achieved by using a combination of methods.  Spring-mounting of equipment is often combined with dampers for improved vibration control.  The thickness of concrete floors beneath machines that generate, or are sensitive to, vibration is often increased to improve damping characteristics.  Depending on the composition of the soil beneath the concrete slab, it may be necessary to extend pilings deep into the ground to achieve isolation.  The reinforced area is typically separated from the adjacent floor by an elastic joint.

Hydraulic Noise
     Special treatments may be needed when noise is caused by fluid flow; this type is called hydraulic or hydrodynamic noise when referring to incompressible-fluid systems (e.g. oil, water).  The primary contributor to hydraulic noise is cavitation.  Cavitation occurs when the pressure of the fluid drops sufficiently for vapor bubbles to form, only to collapse when pressure rebounds.  Cavitation can cause physical damage to a piping system in addition to noise.  Pipe “shuddering” and noise can be reduced with improved mountings, such as those shown in Exhibit 3.

     Transmission of hydraulic noise can be reduced by wrapping pipes in acoustical insulation, known as “lagging.”  Two common configurations of pipe lagging are shown in Exhibit 4.  Reductions up to 10 dBA can be achieved with a single-layer application, shown in the upper portion of the figure.  The double-layer configuration shown in the lower portion can yield sound level reductions of 15 – 20 dBA.
     Another method of limiting sound transmission in pipes involves the use of flexible connections or sections of flexible hose between runs of solid pipe.  This method is particularly helpful near direction changes that can cause turbulence in the fluid or create impingement forces that shake the pipe.  A pictorial guideline for the application of this method is provided in Exhibit 5.

     Addressing the root cause of cavitation requires the selection of appropriate system components and operating parameters.  Cavitation is induced when fluid experiences a rapid, large pressure drop, as commonly occurs in control valves.  If a valve with sufficiently low pressure drop cannot be installed, the line pressure can be reduced to minimize cavitation.  An insert that can be installed upstream to reduce valve inlet pressure is depicted in Exhibit 6.  Fluid that undergoes a series of small pressure drops is much less susceptible to cavitation than when subjected to a single large pressure drop.
     Avoiding rapid changes in flow rate also reduces noise and risk of physical damage to a piping system.  A rapid reduction in flow rate (i.e. fluid deceleration) can cause a pressure spike, shock wave, and associated noise.  This phenomenon is known as “water hammer” and can be quite destructive.  Water hammer can be prevented with gentle transitions of fluid flows or the installation of accumulators to alleviate excess pressure.  Gradual changes in cross-section and direction (e.g. large radius bends) minimize contributions to cavitation and water hammer.

Aerodynamic Noise
     When noise is generated by compressible fluid flow (e.g. air, steam, industrial gases), it is called aerodynamic noise.  Aerodynamic noise may be present in open (e.g. fan) or closed systems operating at high pressure (e.g. compressed air) or low (e.g. HVAC).
     The conditions and countermeasures described above for hydraulic noise are very similar to high-pressure (closed) compressible-fluid systems.  One that is common and familiar to most is a distributed compressed air system.  Leaks often form in distribution piping, wasting energy, causing equipment malfunctions, and adding substantially to noise levels.  Sealing leaks in pressurized systems is an important component of a maintenance program, as mentioned above.
     Compressed air is also exhausted to atmosphere intentionally when used to operate handheld tools, automated equipment, etc.  Whereas leaks in a distribution system may be high above personnel, where piping is routed near the rafters, equipment exhausts are typically quite near operators, requiring measures to limit sound exposure.  A dispersive, or diffuser, muffler is typically installed to reduce the emitted sound.  Slotted and porous types are common; examples are shown in Exhibit 7.  The terms muffler, silencer, and suppressor are often used interchangeably; readers will be spared a discussion of the terms’ nuance.
     If a muffler cannot be installed, due to backpressure concerns, for example, the exhaust should be routed away from personnel, exploiting the inverse square law (see Part 3), to the extent possible, to reduce exposure.  Aerodynamic noise is generated in proportion to flow rate (i.e. fluid velocity), pressure drop, and turbulence.  Operating equipment at the lowest possible pressure minimizes these contributors to aerodynamic noise.

     Though use compressed air blow-offs is increasingly discouraged, the practice remains common in many facilities.  Unrestricted exhaust of compressed air by open-ended tubing, for example, should be eliminated by installing nozzles designed to reduce noise while maintaining the working capability of the air flow.
     Reactive mufflers are best suited for “pulsing” gas flows, typical of internal combustion engine exhaust.  The internal geometry of a reactive muffler reflects sound waves to dissipate energy and create destructive interference of subsequent incoming waves.  Examples of reactive mufflers that use expansion chambers and Helmholtz resonators are depicted in Exhibit 8.
     In contrast to liquid-medium systems, abrupt changes in ductwork can be advantageous to noise control.  Bends, branches, and changes in cross-section can induce reflections and eliminate line-of-sight transmission paths, providing additional attenuation of higher frequencies.
     Another feature that can be used is a sound-absorbing plenum, such as that depicted in Exhibit 9.  Flexible joints can also be inserted in duct runs to interrupt the transmission path created by long sections of sheet metal.  These can be made of an elastic material, canvas, or similar fabric.  Despite any advantage offered in noise control, these features must be accounted for in system design calculations to ensure appropriate downstream flow characteristics.

     If additional noise control is needed, it is typically achieved using dissipative mufflers, in which absorbent material converts sound energy to thermal energy.  Ducts must be designed to accommodate the absorbent material while maintaining sufficient volume to achieve desired flow characteristics.  At a constant flow rate, reducing the cross-sectional area of a duct increases fluid velocity, potentially reducing effectiveness of the muffler.  Exhibit 10 presents possible configurations of sound absorbents in ducts based on the frequency attenuation required.

     The primary source of noise in a ventilation system is typically a distribution fan.  Since noise is emitted omnidirectionally, the fan should be isolated from both inlet and outlet ductwork, as well as its mounting surface.
     Placement of the fan relative to bends, control vanes, or other features is also important.  Operating too near a turbulence-inducing duct feature can be detrimental to performance with respect to air flow and noise generation.  Considerations for specifying a ductwork fan include:

• A fan is quietest when running at peak efficiency; it should be sized to operate at its optimum speed when providing the required air flow.
• Large, slow fans generate less noise than smaller, faster ones.
• Centrifugal fans are quieter than propeller fans.  Centrifugal fans with forward-curved blades tend to operate at lower speeds, with less noise, for a given flow rate than those with other blade styles.
• Tube-axial fans tend to be quieter than centrifugal fans, providing high-volume flow at low pressure.
• Directional fan blades are more efficient than perpendicular (radial) blades.

Exhibit 11 provides a comparison of fan and blade types.

     Unducted fans are typically of the propeller type used for general ventilation of large areas.  In this application, too, a large, low-speed fan is preferred.  The benefits of a large, slow fan can be experienced in many factories and warehouses where they have been installed overhead.  Their existence is often imperceptible until stifling, stale air causes one to look up and realize that these effective air movers have been switched off!

     It has been noted that turbulence in fluid flows contributes to sound generation.  When turbulent flow is required for the fluid to perform its intended function, such as heat transfer, sound emitted as a consequence is called irreducible noise.  This is one of many trade-offs that may be encountered in system design.  Other examples include:

• The quietest type of fan or pump may not be compatible with the medium, contamination, or other environmental or operational characteristic it must accommodate.
• An increased cross-sectional area of a duct may be required to accommodate the addition of sound-absorption material; this, in turn, effects the design of the space within which it must be housed.
• A quieter power source may not deliver an equivalent level of performance; e.g. a battery-electric tool produces less torque than a pneumatic tool, requiring a larger tool and battery to achieve acceptable performance.
• A cost vs. performance trade-off is inherent to every design decision where configuration or feature options must be evaluated.

     The trade-offs mentioned, and many more, are defined by requirements and constraints that are unique to specific projects; there are innumerable potential combinations of noise sources and system configurations.  Any presentation of noise control options, such as that attempted in this series, must, therefore, be generalized.  Only the analysis of a specific system in a specific environment can ensure an optimal solution.

Design and Procurement
     Prevention of noise occurs, primarily, in the design phase of products and equipment.  Purchasing custom-built machinery affords a high level of involvement in its design and specification development.  Standard tools, household appliances, and other “off-the-shelf” products provide no opportunity to influence performance characteristics; purchasing these items is a “take what you get” proposition.
     Fortunately, increasing awareness of noise and hearing protection issues has prompted manufacturers to begin treating sound as an important characteristic of products.  Though not yet universal, the inclusion of sound level information in product documentation is becoming more common.  Publication of such information suggests that a manufacturer is committed to noise-reducing design practices.  When this information is not published, independent testing may be needed to select appropriate equipment to meet all performance requirements.
     A number of standards and guidelines have been promulgated, by various organizations, to facilitate design and purchasing decisions regarding noise, some more narrowly focused than others.  Among them are SAE International’s automotive-related standards; ASHRAE is the authority on HVAC noise.  A more-generalized set of standards, developed by the Acoustical Society of America (ASA), can be found in the ANSI catalog.

     The “Buy Quiet” movement began at NASA to provide guidelines for purchasing equipment that is not only quiet, but safe, efficient, and cost-effective.  Its focal point is a defined process that guides the collection of performance requirements, research of available alternatives, and verification of in-situ performance.
     The Buy Quiet Roadmap includes specific guidance on the procurement process, plus references to product noise databases, regulations, and equipment-specific standards.  The Roadmap also includes several worksheets that can be used to calculate costs, evaluate trade-offs, and document other steps of the process.  There is even a built-in tutorial to assist new users of the Roadmap.


     Opportunities to reduce noise, thereby protecting hearing and easing communication, range from obvious to easily-overlooked.  This range includes production machinery, material handling equipment, printers and copiers in offices, hand dryers in restrooms, and so much more.
     The success of a noise control program depends largely on an organization’s commitment to it.  In its noise control guide, OSHA declares that “[i]n the field of noise control, where there’s a will, there’s a way.”  Integral to this will is a continuous improvement mindset; until the risk of hearing loss, interference with communication, and other effects on performance and well-being are eliminated, noise control and reduction must remain a priority.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).

References
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[Link] “Voluntary National and International Noise Standards for Products and Machines.”  Robert D. Hellweg Jr.  The Bridge; Summer 2021.
[Link] “Resources for Noise Control Engineering.”  George C. Maling Jr.  The Bridge; Summer 2021.
[Link] “Controlling Noise at Work.”  (UK) Health and Safety Executive (L108- 3ed); 2021.
[Link] The Noise Manual, 6ed.  D.K. Meinke, E.H. Berger, R.L. Neitzel, D.P. Driscoll, and K. Bright, eds.  The American Industrial Hygiene Association (AIHA); 2022.
[Link] “Hierarchy of Controls.”  NIOSH; January 17, 2023.
[Link] “Noise control.”  Wikipedia.
[Link] “Technical Guide for:  Noise Control – Engineering Controls, Work Practices, & Administrative Controls.”  Georgia Tech; May 2023.
[Link] Handbook for Industrial Noise Control.  National Aeronautics and Space Administration; 1981.
[Link] “Beyond Blades: Types of Centrifugal Fans and Their Unique Applications.”  The Mechanical Engineer; May 8, 2021.
[Link] “Comparing Axial Fans and Centrifugal Fans.”  Ryan Smoot.  Digikey.com; March 22, 2022.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
[email protected]