Noise complaints from occupied spaces are one of the most common callbacks in HVAC projects. The fan was specified correctly, the airflow is on target, the system balances out-and then someone in the conference room hears a low hum through the ceiling and the calls start coming in. Most of these problems trace back to one step: the noise rating was either ignored during selection or misinterpreted on the schedule.
The Two Ratings You'll See
Fan sound data typically appears in two forms. The first is sone rating, which describes the perceived loudness of the fan itself, usually measured at a specified distance and duct condition. The second is sound power level, expressed in decibels across octaves bands. This is the raw acoustic energy the fan generates.
Sone ratings are useful for comparing fans in a catalog. A fan rated at 10 sones is roughly twice as loud as one rated at 5 sones. The scale follows human perception, which makes it practical for quick comparisons. The limitation is that sone ratings don't tell you what happens after the air leaves the fan-how the ductwork, silencers, and terminal devices modify the sound before it reaches the occupied space.
Sound power levels, by contrast, describe the source. They don't account for the room, the duct path, or the distance from the fan to the listener. But they provide the input data for acoustic modeling, which is how you actually predict whether a space will be too loud.
Why Catalog Ratings Mislead
Published fan noise data almost always reflects AMCA Standard 300 test conditions. The fan is mounted on a test rig with straight inlet and discharge ducts, no elbows, no transitions, no system connections. Real installations are nothing like this.
Inlet restrictions, discharge elbows close to the fan, and turbulent airflow entering the wheel all increase noise output. A fan that looks quiet on paper can be significantly louder once it's connected to actual ductwork. The difference isn't small-it can be 5 to 10 decibels, which is noticeable to anyone in the space.
What Actually Controls the Sound
Three factors dominate fan-generated noise. The first is tip speed. Faster tip speeds produce more noise across all frequencies, especially the higher octaves. This is why large, slow-turning fans are quieter than small, fast-turning ones moving the same airflow.
The second is the impeller type. Airfoil and backward curved wheels are generally quieter than forward curved or radial blade designs at comparable operating points. The blade shape affects how air leaves the wheel, and smoother exit flow means less turbulence and less noise.
The third is system effect. Poor inlet conditions-such as a tight plenum, a screen, or a close-coupled elbow-create turbulent air entering the fan. The wheel doesn't receive a clean, uniform flow, and the resulting instability generates noise that wouldn't appear under test conditions.
Standards and Compliance
ASHRAE Standard 90.1 and various green building programs include acoustic criteria, but the most referenced standard for fan noise is AMCA Standard 300, which governs the test methods for fan sound. Many specifications also reference NC (Noise Criterion) curves or dBA limits for occupied spaces.
The gap between these standards and actual field performance is where problems occur. A fan that meets the NC requirement at the factory test can exceed it in the field if the installation introduces system effects. The only way to close that gap is to account for installation conditions during selection, not after commissioning.
Practical Selection Approach
MIDI Industrial provides sound power and sone data for all centrifugal and EC fan models. Our selection process includes system effect considerations that affect both performance and noise. When a project has strict acoustic requirements, we recommend reviewing the complete air path-from fan discharge through ductwork to terminal devices-rather than relying on the fan rating alone. The fan is only one component in the chain, and the chain is only as quiet as its loudest link.