What NC level should I design to for a standard open-plan office?

For open-plan offices, the typical target is NC-35 to NC-40. NC-35 provides a quiet background that supports speech intelligibility without feeling oppressively silent. NC-40 is acceptable for general office work but may cause some speech intelligibility issues in larger open-plan areas. For private offices and conference rooms, target NC-25 to NC-30. For executive offices, boardrooms, and teleconference rooms with speech privacy and speech intelligibility requirements, target NC-20 to NC-25 (RC-25N or better). For healthcare patient rooms, ASHRAE/FGI 2022 recommends NC-35 maximum. The ASHRAE Handbook HVAC Applications Table 49-1 provides recommended NC and RC criteria for various space types.

How do I reduce noise from a VAV box in an adjacent conference room?

VAV box noise problems in sensitive spaces should be addressed in this order: (1) Reduce the VAV box airflow — operate at lower maximum airflow by resizing the box or reducing the zone design load. Sound power decreases approximately 50 dB × log(V2/V1) for airflow reduction; (2) Specify a low-noise VAV box with a downstream attenuator section (some manufacturers offer integral sound attenuators); (3) Increase downstream duct length from VAV box to the diffuser (more lined duct = more attenuation); (4) Add a prefabricated silencer downstream of the VAV box in the primary branch duct; (5) Relocate the VAV box to serve the conference room from a less sensitive adjacent space or from above the corridor. Never locate the VAV box directly above the conference room ceiling if NC-25 or below is required.

What spring deflection is required for isolating a centrifugal chiller?

The required static deflection depends on the equipment operating speed and the desired transmissibility. The natural frequency of a spring-mass system is fn = 3.13/√δ (in Hz, with δ in inches of static deflection). For the isolator to achieve < 10% transmissibility, the operating frequency must be at least √2 × fn higher than the natural frequency. A 1,500 RPM (25 Hz) centrifugal chiller requires spring isolators with approximately 1.5 inches static deflection for 90% isolation efficiency. Large centrifugal chillers (1,000–3,000 tons) on grade slabs typically use 2.5–3.0 inch deflection spring isolators with inertia bases. On elevated floor structures (which are flexible and add to the vibration problem), longer deflection springs (3.0+ inches) or air springs with rigorous structural analysis are required.

How much attenuation does acoustic duct lining provide?

Acoustic duct lining (fiberglass or mineral wool, typically 1-inch or 2-inch thickness) provides attenuation in dB per foot based on the duct dimensions, lining thickness, and frequency. From ASHRAE Handbook HVAC Applications Table 49-9, for a 24"×24" duct with 1-inch lining: approximately 0.5 dB/ft at 125 Hz, 1.0 dB/ft at 250 Hz, 1.5 dB/ft at 500 Hz, 2.0 dB/ft at 1,000 Hz, 2.5 dB/ft at 2,000 Hz, declining at higher frequencies. Doubling lining thickness to 2 inches roughly doubles low-frequency attenuation. For a 10-foot lined duct section with 1-inch lining, you can expect approximately 10 dB of attenuation at 500 Hz — useful for VAV box noise reduction but insufficient for fan noise at 125 Hz without supplemental silencers.

Engineering·10 min read·October 15, 2025

🔧 HVAC Noise and Vibration Control Design

Q: How much attenuation does acoustic duct lining provide?

Acoustic duct lining (fiberglass or mineral wool, typically 1-inch or 2-inch thickness) provides attenuation in dB per foot based on the duct dimensions, lining thickness, and frequency. From ASHRAE Handbook HVAC Applications Table 49-9, for a 24"×24" duct with 1-inch lining: approximately 0.5 dB/ft at 125 Hz, 1.0 dB/ft at 250 Hz, 1.5 dB/ft at 500 Hz, 2.0 dB/ft at 1,000 Hz, 2.5 dB/ft at 2,000 Hz, declining at higher frequencies. Doubling lining thickness to 2 inches roughly doubles low-frequency attenuation. For a 10-foot lined duct section with 1-inch lining, you can expect approximately 10 dB of attenuation at 500 Hz — useful for VAV box noise reduction but insufficient for fan noise at 125 Hz without supplemental silencers.

Engineering guide to HVAC noise and vibration control — sound power levels, NC/RC criteria, duct-borne noise attenuation, equipment vibration isolation, acoustic lining, and ASHRAE 2019 Handbook guidance.

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Why HVAC Noise and Vibration Control Matters

Mechanical system noise is one of the most frequent sources of occupant complaints in commercial buildings, and fixing acoustic problems after construction is exponentially more expensive than designing correctly from the start. HVAC noise sources include fans, compressors, pumps, chillers, cooling towers, VAV boxes, diffusers, and flow-induced turbulence in ductwork and piping. Vibration transmitted through building structure from rotating equipment causes both noise (structure-borne sound) and discomfort. The ASHRAE Handbook — HVAC Applications, Chapter 49 (Sound and Vibration Control) is the primary reference for mechanical engineers designing acoustic systems.

Topics covered

HVAC noise controlvibration isolationnoise criteriaNC curveRC curvesound power levelduct-borne noisesound attenuatoracoustic liningspring isolatorneoprene padASHRAE acousticsroom acoustics HVACVAV box noise