9.3 Mechanical equipment noise control in buildings

Mechanical equipment noise in buildings can be a major source of discomfort for occupants. HVAC systems, elevators, plumbing, and electrical equipment all contribute to the acoustic environment. Understanding these sources is crucial for effective noise control.

Strategies for managing mechanical noise include source reduction, duct silencers, and vibration isolation. Proper design, equipment selection, and installation of sound-absorbing materials can significantly improve the acoustic quality of buildings. Regular maintenance and evaluation ensure long-term effectiveness of noise control measures.

Mechanical Equipment Noise Sources

HVAC Systems

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• Air handlers, ductwork, and diffusers are major contributors to mechanical noise in buildings
  • Noise is generated due to the movement of air and vibration of components
  • Examples include fans, motors, and turbulence in ducts
• Selecting quieter equipment with lower sound power levels can reduce noise generation at the source
  • Design systems with lower air velocities to minimize noise
  • Use duct silencers (dissipative or reactive) to attenuate noise in the ductwork

Other Common Sources

• Elevators generate noise through the operation of motors, gears, and cables
  • Opening and closing of doors and movement of the cab also contribute to noise
  • Proper maintenance and lubrication can help reduce elevator noise
• Plumbing systems (pipes, valves, and pumps) can create noise through the flow of water and vibration
  • Use resilient mounting and isolation to minimize vibration transmission
  • Install flexible connections between pipes and equipment to reduce noise
• Generators, transformers, and other electrical equipment produce low-frequency humming or buzzing sounds
  • Noise is caused by the oscillation of magnetic fields
  • Locate equipment away from sensitive areas and use sound-absorbing enclosures
• Mechanical rooms housing boilers, chillers, and pumps can be significant noise sources
  • Noise is concentrated due to the confinement of machinery in a limited space
  • Implement sound-absorbing treatments and vibration isolation in mechanical rooms

Noise Control Strategies for HVAC Systems

Source Noise Reduction

• Select quieter equipment with lower sound power levels
  • Consult manufacturer data and compare noise ratings when specifying equipment
  • Opt for variable speed drives (VSDs) to allow for lower operating speeds and reduced noise
• Design systems with lower air velocities to minimize turbulence and flow-generated noise
  • Increase duct cross-sectional area to accommodate lower velocities
  • Use smooth transitions and gradual bends in ductwork to reduce turbulence

Noise Attenuation in Ductwork

• Install duct silencers to attenuate noise as it travels through the system
  • Dissipative silencers use sound-absorbing materials (fiberglass or foam) to absorb noise
  • Reactive silencers employ tuned chambers and perforated baffles to reflect and cancel noise
• Design ductwork with sufficient cross-sectional area and smooth interior surfaces
  • Larger ducts allow for lower air velocities and reduced turbulence noise
  • Avoid sharp edges, abrupt changes in direction, and unnecessary obstructions

Vibration Isolation and Noise Reduction

Vibration Isolation Techniques

• Select appropriate vibration isolators based on equipment weight, speed, and desired isolation efficiency
  • Spring mounts provide effective isolation for heavy equipment and low frequencies
  • Neoprene or rubber pads are suitable for lighter equipment and higher frequencies
• Design isolation systems with a low natural frequency (at least 1/3 octave below equipment frequency)
  • Ensures minimal resonance and amplification of vibration
  • Consult manufacturer data and calculations to determine proper isolator selection
• Use elastomeric or spring hangers for suspended equipment (ductwork or piping)
  • Decouple the equipment from the building structure to reduce vibration transmission
  • Maintain sufficient flexibility to accommodate movement and thermal expansion
• Install flexible connectors at the interface between mechanical equipment and ductwork/piping
  • Neoprene or rubber connectors allow for movement and prevent vibration transmission
  • Ensure connectors are properly aligned and do not introduce additional stresses

Noise Reduction Treatments

• Design floating floors or isolated concrete inertia bases for mechanical rooms
  • Provides a stable and isolated foundation for equipment
  • Use a combination of isolators, resilient layers, and air gaps to minimize vibration transmission
• Specify sound-absorbing materials with appropriate thickness, density, and placement
  • Perforated metal or mineral fiber absorbers can effectively reduce reverberant noise levels
  • Apply absorbers to walls, ceilings, and strategic locations within mechanical spaces
• Enclose noisy equipment in sound-absorbing or sound-blocking enclosures
  • Prefabricated or custom-built enclosures can contain noise and prevent its propagation
  • Ensure enclosures have sufficient ventilation and access for maintenance

Effectiveness of Noise Control Measures

Measurement and Evaluation

• Conduct pre- and post-treatment noise measurements using sound level meters or spectrum analyzers
  • Quantify the reduction in noise levels achieved by the implemented control measures
  • Compare results to applicable standards (ASHRAE, NC, RC, or dBA criteria) for compliance
• Perform vibration measurements using accelerometers to assess vibration isolation effectiveness
  • Identify any remaining transmission paths or resonances that require further attention
  • Verify that vibration levels are within acceptable limits for occupant comfort and equipment protection
• Use sound intensity or sound power measurements to identify dominant noise sources
  • Evaluate the performance of noise barriers, enclosures, or silencers
  • Pinpoint specific areas or components that contribute significantly to the overall noise levels

Subjective Evaluation and Optimization

• Conduct occupant surveys or listening tests to gather feedback on perceived noise control effectiveness
  • Assess occupant comfort and identify any remaining noise concerns
  • Use subjective data to guide further refinements or additional noise control measures
• Perform iterative testing and adjustment of noise control treatments
  • Fine-tune the performance of silencers, absorbers, and isolators based on measurement results
  • Optimize the placement and configuration of treatments to maximize noise reduction
• Document the effectiveness of noise control measures for future reference and maintenance
  • Create a comprehensive report outlining the implemented measures and their outcomes
  • Establish a baseline for ongoing monitoring and evaluation of noise control performance