HVAC noise complaints are among the most common post-occupancy MEP failures in commercial and healthcare buildings — and nearly all of them were avoidable. The duct breakout noise from an oversized AHU, the low-frequency rumble from an unbalanced fan, the high-pitched whistle from a terminal box at excessive velocity — each of these is a predictable consequence of design decisions that were made without acoustic analysis.

The critical insight is that HVAC noise is a design parameter, not a commissioning problem. By the time a building is occupied and tenants are complaining about noise, the solutions available are expensive and disruptive. Silencers can be retrofitted — but only if there is space. Fan speeds can be reduced — but only at the cost of airflow and cooling capacity. Equipment can be relocated — at enormous cost. All of these interventions are preventable through acoustic design at the schematic and detailed design stages.

Noise Criteria and Design Targets

The primary metric for occupied space HVAC noise is the NC (Noise Criteria) curve — a family of curves in the octave band frequency spectrum that define the maximum permissible sound pressure level at each frequency band. ASHRAE Applications Handbook Chapter 48 and CIBSE Guide B provide NC targets for different space types.

The Three HVAC Noise Sources

Understanding where HVAC noise comes from is the first step in controlling it. The three primary sources require different design responses.

Duct Velocity Limits: The Primary Design Tool

The most cost-effective acoustic design tool is simply keeping duct velocities within limits that do not generate excessive regenerated noise. Oversized ductwork designed to achieve low velocities is almost always cheaper than undersized ductwork with downstream silencers.

Silencer Selection and Insertion Loss

Where equipment noise cannot be reduced sufficiently through duct sizing and equipment selection alone, silencers (sound attenuators) are inserted in the duct run to absorb airborne sound energy. Silencer selection requires knowing the sound power entering the silencer at each octave band frequency, the target sound power at the outlet, and the required insertion loss (IL) in each band.

• 01

  Establish Sound Power at Source

  AHU manufacturer provides sound power data (Lw) in octave bands from 63 Hz to 8000 Hz for the specified fan speed and airflow. This is the starting point — without manufacturer data at the actual operating point, silencer sizing is guesswork.

• 02

  Calculate Duct Attenuation

  Unlined ductwork, lined ductwork, duct bends, and plenum boxes all provide some natural attenuation. ASHRAE calculations for each duct element reduce the sound power level before it reaches the silencer.

• 03

  Determine Required Insertion Loss

  Lw(required) = target NC level + room correction factor − estimated room absorption. The difference between the calculated sound power reaching the space and the required target is the insertion loss the silencer must provide at each octave band.

• 04

  Select Silencer from Manufacturer Data

  Silencer manufacturers provide IL data by frequency band for each silencer length and velocity. Select the silencer that meets the required IL at all critical frequency bands, at the design air velocity, within the allowable pressure drop budget.

• 05

  Verify Regenerated Noise in Silencer

  Silencers themselves generate noise at high velocity — typically 5–8 m/s face velocity limit for sound-sensitive applications. If silencer face velocity is too high, the silencer generates more noise than it attenuates. Select silencer cross-section for acceptable face velocity.

Vibration Isolation: Equipment and Connections

All rotating plant — AHUs, chillers, cooling towers, pumps — must be isolated from the building structure to prevent structure-borne vibration transmission. The isolation method depends on the disturbing frequency and the required isolation efficiency.

The KVRM Acoustic Design Approach for HVAC

• 01

  NC Target Definition

  Room-by-room NC targets defined at schematic design stage based on space function and occupancy. Healthcare projects reference HTM 08-01 and NBC 2016 acoustic requirements.

• 02

  Duct Sizing for Velocity Control

  All ductwork sized to keep velocities within the NC-appropriate limits. No ductwork sized purely for space reasons without velocity check.

• 03

  Fan Selection with Sound Data

  AHU fans selected with manufacturer octave-band sound power data at the actual operating point. Backward-curved centrifugal fans are specified in preference to forward-curved for lower noise generation at equivalent airflow.

• 04

  Silencer Specification

  Where equipment noise exceeds room targets after duct attenuation, silencers are specified with insertion loss requirements at each octave band. Silencer face velocity confirmed within limits.

• 05

  Vibration Isolation Schedule

  All rotating plant specified with isolation type, natural frequency, and static deflection requirements. Flexible connections specified at all pipe and duct connections to vibrating equipment.

Conclusion: Acoustic Design Is Not an Afterthought

HVAC acoustic design is not an optional refinement for luxury projects. It is a functional requirement — occupant comfort, productivity, and in healthcare applications, patient wellbeing — that must be designed in from the schematic stage. The tools exist, the standards are clear, and the cost of getting it right at design stage is a fraction of the cost of remediation after construction.

Every low-velocity duct, every correctly sized silencer, every spring-isolated AHU represents a design decision that was made before the concrete was poured. By the time the occupants move in, those decisions are irreversible. Make them correctly.