A duct silencer is useful when sound travels along the air path. It will not, by itself, cure vibration transmitted through supports or noise radiating from a poorly isolated fan casing. Establish the dominant path before buying additional attenuation.
Why one decibel figure is an incomplete specification
Fan noise contains different frequencies, and a silencer does not reduce them equally. An attractive overall reduction can conceal weak performance in the band causing the complaint. Compare insertion-loss data across the relevant frequency bands, together with the operating airflow and flow direction.
The ASHRAE Handbook chapter on noise and vibration control treats attenuation, pressure drop and self-generated noise as separate selection considerations. A useful quotation therefore gives more than casing dimensions and a headline acoustic figure.
The resistance trade-off
Internal splitters provide acoustic surfaces but also restrict the free air path. Increasing velocity through the remaining passages raises resistance and can create new aerodynamic noise. A shorter, smaller unit that fits the space may consequently demand more fan pressure while solving less of the acoustic problem.
Compare alternatives at the same airflow. If a larger cross-section reduces velocity, allow for its transitions and support requirements. Check whether the selected fan can deliver the required duty after adding the silencer; increased fan speed can erode the expected noise benefit.
Installation changes the result
- Leave suitable connection space so abrupt bends or obstructions do not create uneven entry flow.
- Support the casing independently where required; do not transfer its weight into a weak flexible connection.
- Specify lining protection and access for the actual moisture, cleanliness and cleaning conditions.
- Consider noise breaking out through the duct wall or bypassing the treated branch.
For an existing installation, record where the noise is heard, how it changes with fan speed, the available straight length and the duct dimensions. For a new system, provide the sound-power schedule, airflow and room acoustic target.
shengtongJH can coordinate round or rectangular duct-silencer options around these conditions. Send the duty and duct layout for a configuration that fits the acoustic requirement and available space.