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.

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.

Noise Criteria: NC, RC, and WELL Standards

Acoustic criteria for occupied spaces are defined using standardized curves that account for human hearing sensitivity at different frequencies:

NC (Noise Criteria) curves: Single-number ratings (NC-15 through NC-65) representing the maximum octave-band sound pressure level in each octave band. NC-25 to NC-35 is the typical range for offices and commercial spaces. NC-20 to NC-25 for executive offices and conference rooms. NC-40 to NC-50 for manufacturing and mechanical spaces.
RC (Room Criteria) curves: Updated version of NC curves developed by ASHRAE that also accounts for low-frequency noise (which NC underweights) and provides a rating quality (N = neutral, R = rumbly, H = hissy). ASHRAE recommends RC criteria over NC for critical spaces. RC-25 to RC-35 for office occupancies.
WELL Building Standard: The WELL Building Standard v2 (Feature A07) limits background HVAC noise in regularly occupied spaces to NC-35 maximum, with NC-30 preferred. WELL Platinum certification often requires NC-25 or lower.
LEED: LEED v4.1 EQ Credit Acoustic Performance requires HVAC background noise levels not to exceed 40 dBA in regularly occupied areas, with NC-35 documentation required.

HVAC Sound Sources and Sound Power Levels

Designing a quiet HVAC system begins with understanding sound power levels (SWL, in dB referenced to 10⁻¹² watts) of each major component:

Fans: The dominant noise source in most HVAC systems. Fan SWL is rated per AMCA 300 in octave bands from 63 Hz to 8,000 Hz. Fan noise increases sharply with fan speed (sound power ∝ speed⁵) and pressure rise. At reduced speed (part-load), fan noise decreases dramatically — a VFD-controlled fan at 70% speed produces approximately 12 dB less noise than at full speed.
VAV boxes: Generate noise from air turbulence across the control damper, especially at reduced airflow (high damper restriction). AHRI 880 provides standard test methods for VAV terminal unit sound power levels. High-performance VAV boxes (pressure-independent, with low-velocity distribution through the discharge collar) are quieter. Always calculate downstream duct-borne noise from VAV boxes for adjacent quiet spaces.
Air outlets (diffusers and grilles): Generate self-noise from high air velocity through the neck and face. NC levels from diffusers are available from manufacturer catalogs — select diffusers at neck velocities consistent with the target NC level for the space (typically 500 FPM for NC-35, 400 FPM for NC-25).
Chillers, boilers, and cooling towers: Primarily affect exterior equipment areas and lower-floor mechanical rooms. Coordinate with structural engineer for vibration isolation of equipment frames and with architect for mechanical room wall/slab construction.

Duct-Borne Noise Propagation and Attenuation

Sound generated by fans and VAV boxes propagates downstream through supply ductwork and ultimately into occupied spaces through diffusers. The ASHRAE Handbook provides octave-band transmission loss and insertion loss values for duct elements:

Straight duct: Unlined rectangular sheet metal duct provides very little attenuation (0.1–0.3 dB/ft at 125–500 Hz). Lined duct (1-inch or 2-inch fiberglass internal lining) provides 0.5–2.0 dB/ft attenuation in the 250–2,000 Hz range. Round duct provides slightly less attenuation than rectangular at the same dimensions.
Elbows: Sheet metal elbows provide 1–7 dB of attenuation at mid-high frequencies, but lined elbows (with acoustic turning vanes or liner) can provide 6–12 dB. Low-frequency attenuation from elbows is negligible.
Sound attenuators (duct silencers): Prefabricated sound attenuators with parallel splitter baffles of fiberglass or mineral wool provide significant noise attenuation — typically 10–25 dB at 250–2,000 Hz with 3–4 feet of length. Dynamic insertion loss (DIL) data is published by manufacturers per ASTM E477 test standard. Specify active-type (with internal media exposed to airflow) for broad-spectrum attenuation or tubular type for low-frequency attenuation. Include self-noise rating — at high velocities, silencers generate their own aerodynamic noise.
End reflection loss (ERL): When sound in a duct exits through a small opening into a large room, low-frequency sound is reflected back into the duct. ERL is significant at low frequencies for small openings (diffuser necks), providing 5–15 dB of natural attenuation at 63–250 Hz — an important but often overlooked noise reduction mechanism.

Vibration Isolation for Mechanical Equipment

Rotating mechanical equipment — fans, pumps, compressors, chillers — transmits vibration through the building structure as structure-borne noise if not properly isolated. Vibration isolation system selection depends on equipment RPM and the deflection required:

Neoprene pads: Effective for high-speed equipment (RPM > 1,200) and light loads. Static deflection typically 0.1–0.2 inches. Suitable for small pumps, fan coil units, and terminal equipment. Not appropriate for fans below 600 RPM.
Spring isolators: Steel coil springs provide static deflections of 1.0–3.0 inches. Isolate low-speed equipment (fans, chillers, large pumps) and provide transmissibility of < 5% when operating frequency is more than 4× the spring's natural frequency. Spring isolators must include neoprene snubbers to prevent lateral movement and high-frequency noise transmission through the springs.
Air springs (pneumatic isolators): Very high deflection (3–6 inches) for extremely sensitive applications — large centrifugal chillers above occupied concert halls or recording studios. Expensive and require an air supply for inflation.
Inertia base: A concrete-filled steel base (typically 6–12 inches thick) adds mass to the equipment-spring system, lowering the natural frequency and improving isolation at low speeds. Required for equipment below 400 RPM or where floor flexibility makes standard spring isolators ineffective.

Flexible Connections and Acoustically Treated Passages

Vibration transmitted through piping and ductwork connections bypasses vibration isolators if rigid connections are used at the equipment. Required flexible connections include:

Flexible duct connectors: Canvas or neoprene fabric connectors between fan discharge/inlet and rigid ductwork. Minimum 6 inches long to provide effective decoupling.
Flexible pipe connectors: Braided metal flexible hose or elastomeric couplings on pump suction and discharge piping. Minimum 18–24 inches for vibration decoupling.
Pipe isolation hangers: Spring-loaded or neoprene-insert pipe hangers within 50 feet of vibrating equipment to break structure-borne noise transmission path in piping.

Topics covered

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

🛠️ Related Free Tools

Put this knowledge to work on your iPhone

Browse our full catalog of professional iOS apps — from electrical code tools to AI builders.

Browse All 95+ Apps