HVAC noise and vibration control

Noise is one of the most common complaints about HVAC systems, and one of the hardest to fix after construction. A rumble from a rooftop unit, a hiss from a diffuser or a hum transmitted through the structure can make an otherwise well-performing system unacceptable to occupants. Most HVAC noise problems are predictable and can be avoided in design at relatively low cost.

This guide explains the sources and transmission paths of HVAC noise, the criteria used to set targets, and the practical measures for controlling noise and vibration in new and existing buildings.

Source, path and receiver

Every noise problem has three parts:

• Source: fans, compressors, pumps, cooling towers, VAV boxes, dampers, diffusers and turbulent airflow.
• Path: how sound travels from the source to the listener, through ductwork, through walls and floors, or through the structure as vibration.
• Receiver: the occupant, whose sensitivity depends on the space type and activity.

Control can happen at any point, but it is usually most effective and cheapest at the source and along the main path.

Noise criteria

Room noise from HVAC is usually evaluated with one of the following rating methods, described in the ASHRAE Handbook, HVAC Applications:

• NC (Noise Criteria): the most widely used, based on octave band sound pressure levels compared with a set of curves.
• RC (Room Criteria) and RC Mark II: also consider the spectral balance, identifying rumble (too much low frequency) or hiss (too much high frequency).
• dBA: a single A-weighted number, simple but less useful for diagnosing HVAC noise because it underweights low frequencies.

Typical HVAC background noise design targets:

These are typical ranges drawn from ASHRAE guidance; project targets should be agreed with the owner and, for schools and healthcare, checked against provincial or institutional requirements. CSA Z317.2 includes noise guidance for healthcare spaces.

Major noise sources

Fans

Fans generate broadband noise plus a tonal component at the blade passage frequency. Noise rises when a fan operates away from its peak efficiency point, especially in the unstable region to the left of the peak pressure. Poor inlet conditions also increase noise. Selecting fans near peak efficiency and providing good inlet and outlet conditions are the first steps. See fan selection, curves and system effect.

Airflow in ducts

Turbulence at fittings, dampers and high velocities generates regenerated noise. Velocity is the controlling factor, since regenerated noise rises steeply with velocity. Abrupt fittings close to outlets and partially closed dampers near diffusers are common culprits.

Terminal units

VAV boxes and fan-powered boxes produce radiated noise (through the casing into the ceiling) and discharge noise (down the duct). Manufacturers publish sound data per AHRI 880; AHRI 885 provides a method for estimating room sound levels from that data.

Compressors, pumps and towers

Rooftop units, chillers and pumps produce strong low-frequency noise and vibration. Rooftop units mounted directly over occupied spaces are a frequent source of complaints, because the roof deck offers little sound isolation and the supply and return openings are large.

Transmission paths and control measures

Duct-borne noise

Sound travels along the duct with the air. Controls include:

• Duct lining: acoustic lining absorbs mid and high frequencies. Many healthcare facilities avoid lining in the airstream for hygiene reasons, so double-wall duct with perforated inner liner or silencers is used.
• Silencers: dissipative (with fill) or packless designs. Selected for insertion loss and pressure drop at the design airflow.
• Elbows and plenums: lined elbows and lined plenums provide meaningful attenuation.
• Duct length and branching: natural attenuation occurs through branch power division and end reflection.

Duct breakout

Low-frequency sound inside a duct can pass through the duct wall into the ceiling space and room below. Rectangular ducts break out more than round ducts. Controls include using round or flat-oval duct near the fan, increasing duct wall gauge, wrapping with mass-loaded lagging, and routing large ducts away from sensitive spaces.

Structure-borne noise and vibration

Rotating and reciprocating equipment transmits vibration into the structure, which then radiates noise from walls, floors and ceilings, sometimes far from the source. This is the most difficult path to fix later.

Airborne noise through construction

Mechanical rooms adjacent to occupied spaces need walls, floors and doors with adequate sound transmission class (STC). Penetrations by ducts and pipes must be sealed, as gaps undermine the rating.

Vibration isolation

ASHRAE Handbook, HVAC Applications provides a table of recommended isolator types and static deflections, based on equipment type, speed, horsepower and the floor span supporting it. General principles:

1. Mount rotating equipment on isolators with enough static deflection for the operating speed. Equipment on long-span floors or roofs needs greater deflection than equipment on grade.
2. Use spring isolators for most fans, pumps and chillers on upper floors; neoprene pads are suitable only for some small or on-grade equipment.
3. Use inertia bases for pumps and larger fans to add mass and control motion.
4. Provide flexible connectors on ducts and pipes at isolated equipment, and resilient hangers for the first several hangers of piping and ductwork.
5. Make sure isolators are not short-circuited by rigid conduit, pipes, debris under bases or blocked springs.
6. Coordinate isolators with seismic restraint, required in many parts of Canada by the NBC. Seismic snubbers must allow the isolators to work under normal conditions. See seismic restraint of HVAC equipment in Canada.

Diagnosing an existing noise problem

1. Talk to occupants: when does it happen, where, and what does it sound like (rumble, hum, hiss, whistle, rattle)?
2. Correlate with equipment operation. Turn units on and off, change fan speeds and observe.
3. Measure sound levels in octave bands and compare with the NC or RC target.
4. Check for obvious defects: blocked or bottomed-out isolators, rigid connections, high VAV inlet pressure, closed dampers near diffusers, loose panels.
5. Identify the path: duct-borne, breakout, structure-borne or airborne.
6. Apply the targeted fix.

Common fixes include reducing duct static pressure, slowing fans, relocating balancing dampers upstream, repairing isolators, adding silencers or lagging, and replacing rigid connections. Many noise issues are found during air balancing, when pressures and damper positions are reviewed.

Frequently asked questions

Why is low-frequency noise so hard to control?

Low-frequency sound has long wavelengths, passes easily through lightweight construction and duct walls, and is poorly absorbed by typical lining. Controlling it usually needs mass, distance or source reduction.

Do VFDs make noise?

Motors driven by VFDs can produce a high-pitched tone at the carrier frequency. Adjusting carrier frequency or using motor filters usually resolves this. Some fans also have resonance at certain speeds, which can be skipped in VFD settings.

Key takeaways

• Treat noise by source, path and receiver; fixing the source is usually most effective.
• Set NC or RC targets for each space type early in design using ASHRAE guidance.
• Velocity, fittings and damper locations drive regenerated noise.
• Proper vibration isolation, coordinated with seismic restraint, prevents structure-borne noise.
• Diagnose systematically before applying fixes; many problems trace to high static pressure or failed isolators.