How to Diagnose and Reduce HVAC Noise

Symptoms

A constant broadband rushing, rumbling, or roaring sound is present whenever the HVAC system is running. The noise may fluctuate as the system cycles on and off or changes fan speed. It masks quiet speech and affects recording quality. In measurement terms, the background noise level exceeds the NC (Noise Criteria) rating appropriate for the space, reducing the effective signal-to-noise ratio of the sound system and lowering speech intelligibility scores.

Common Causes

HVAC noise enters a room through three paths. Airborne noise travels through ductwork from the air handling unit, carrying fan noise, motor noise, and turbulence directly into the room via supply and return diffusers. Structure-borne vibration from compressors, fans, and pumps transmits through building structure to walls, floors, and ceilings which radiate the vibration as audible sound. Breakout noise penetrates through thin duct walls, particularly for large rectangular ducts near the air handler. Each path has a different frequency character: duct-borne noise is broadband with low-frequency emphasis, structure-borne vibration peaks at specific frequencies related to motor RPM and fan blade pass frequency, and turbulence at diffusers creates high-frequency hissing.

Measurement Procedure

1. Open SonaVyx SPL Meter with octave band analysis enabled.
2. Measure with the HVAC system running, capturing LAeq over 30 seconds.
3. Record the 1/1 octave band levels from 63Hz through 8kHz.
4. Turn off the HVAC system and repeat the measurement for comparison.
5. The difference between HVAC-on and HVAC-off reveals the noise contribution of the HVAC system per octave band.
6. Compare the HVAC-on octave band levels to the NC curves displayed in SonaVyx.

Interpretation

NC ratings indicate the maximum acceptable background noise for different space types. Recording studios require NC-15 to NC-25. Concert halls and theaters need NC-20 to NC-30. Churches and classrooms target NC-25 to NC-35. Office spaces accept NC-35 to NC-45. If your HVAC noise exceeds the target NC rating for your space type, treatment is needed. Identify which octave bands exceed the NC curve — this tells you which HVAC noise source to address first. Low-frequency exceedance (63-250Hz) points to fan and motor noise. Mid-frequency exceedance (500-2kHz) suggests duct turbulence. High-frequency exceedance (4-8kHz) indicates diffuser noise.

Solutions

For duct-borne noise, install lined duct silencers between the air handler and the room. Internal duct lining with fiberglass absorbs mid and high frequency noise. For structure-borne vibration, mount air handlers on vibration isolation springs and install flexible connectors between equipment and ductwork. For diffuser noise, increase diffuser size or reduce air velocity — noise drops dramatically when face velocity decreases below 2 m/s. Replace rectangular ducts with round ducts for lower breakout noise. In severe cases, relocate the air handler further from the critical space and extend the duct run with additional silencers.

Verification

After treatment, remeasure octave band levels with HVAC running. The NC rating should now meet the target for your space type. Run an STI measurement to verify that the reduced background noise has improved speech intelligibility, which should increase by 0.05-0.15 STI points for each 5dB reduction in background noise.

Open SPL Meter for HVAC noise measurement