A sound attenuator (duct silencer) is selected by matching its insertion loss to the octave bands where the fan is loudest, while keeping regenerated noise and pressure drop under control; typical units deliver 10–25 dB of attenuation at 250–2000 Hz for 50–150 Pa of added resistance. Pick the silencer on the band that violates your noise target, not on a single-number rating, because a fan that is 80 dB at 500 Hz but quiet elsewhere needs attenuation exactly there.
How a Silencer Works
Absorptive silencers line the duct with mineral or fibre infill behind perforated sheet. Sound energy entering the chamber is converted to heat in the liner. Performance peaks at mid and high frequencies (500–4000 Hz) where wavelengths are short enough to be absorbed, and falls at 63–125 Hz where long waves simply pass through. A 1.2 m long, 300 mm deep splitter bank typically yields 15–20 dB at 1 kHz but only 4–6 dB at 125 Hz, which is why low-frequency complaints need a different strategy entirely.
The lining itself must survive the airstream: fibre liners need an erosion veil in velocities above 10 m/s, and any exposed infill in a kitchen or lab exhaust will clog or wash out. Specifying the wrong facing is a frequent cause of silencers that work for six months then lose half their rating as the material compacts and bridges.
Reading the Insertion Loss Curve
Insertion loss (IL) is the reduction in sound pressure at a receiver when the silencer is inserted. Compare the fan’s octave-band sound power (usually supplied as 8 values from 63 to 8000 Hz) against your room criterion curve. If the fan puts out 78 dB at 500 Hz and your limit is 60 dB, you need at least 18 dB IL at 500 Hz. A straight 1.5 m silencer gives roughly 18–22 dB there, so it fits; a 0.6 m unit giving 8 dB does not, and buying it would waste money while still failing the limit.
Comparing Silencer Types
| Type | IL at 1 kHz (dB) | Added pressure drop (Pa) | Best frequency | Notes |
|---|---|---|---|---|
| Rectangular absorptive | 15–30 | 40–150 | 250–4000 Hz | Common supply/return |
| Circular pod | 10–22 | 30–120 | 500–4000 Hz | Round duct, lower cost |
| Linear / low-velocity | 8–18 | 15–50 | 500–2000 Hz | Clean rooms, low loss |
| Reactive (expansion) | 10–25 | 20–80 | 63–250 Hz | Low-frequency, gas streams |
Pressure Drop and Regenerated Noise
Every silencer restricts flow. A bank of 100 mm splitters in a 1000×1000 mm duct at 6 m/s adds about 45 Pa; tightening to 50 mm splitters doubles velocity in the gaps and can add 120–180 Pa plus self-generated noise of 70–85 dB that defeats the purpose. Keep silencer face velocity at 4–7 m/s. Also account for 10–20% extra pressure when filters and silencers stack on the same fan, because each element eats into the same available static pressure.
Worked Example: A Rooftop Supply Fan
A 4-72 centrifugal supply fan emits 82 dB at 500 Hz and 79 dB at 1 kHz; the roof boundary limit is 60 dB. We need 22 dB at 500 Hz and 19 dB at 1 kHz. A 1.8 m rectangular absorptive silencer with 100 mm splitters at 5 m/s face velocity delivers 22–24 dB across 250–2000 Hz and adds only 60 Pa. At 8 m/s the same unit would add 140 Pa and regenerate 80 dB, so we widen the frame to hold velocity at 5 m/s—a 15% larger opening that protects both the noise and the pressure budget.
Decision Checklist: Which Silencer
- Choose rectangular absorptive when you need 15–30 dB across 250–4000 Hz and can afford 40–150 Pa on a supply or return main.
- Choose a circular pod when duct is round and budget or space limits length to under 1 m; expect 10–22 dB at 1 kHz.
- Choose a linear low-velocity unit for clean rooms where added pressure drop must stay below 50 Pa and air must stay laminar.
- Choose a reactive silencer when low-frequency hum (<250 Hz) dominates and absorptive types fall short of the needed attenuation.
Placement and Length Rules
Install the silencer close to the noise source—right at the fan discharge or inlet—so it treats sound before the duct radiates it. A 0.9 m unit gives about two-thirds the attenuation of a 1.8 m unit at the main band, so double length only when the target is strict. Keep 1–2 duct diameters of straight run on both sides to avoid flow separation that raises regen noise by 3–6 dB and undermines the whole investment.
Standards and Testing
Ratings should follow ISO 7235 or AMCA 300 for laboratory insertion-loss and self-generated-noise tests, socomparisons between brands are meaningful. Ask for the full 8-band curve, not a single-number claim, and verify the test face velocity matches your duty. A silencer rated at 3 m/s that you run at 7 m/s will both lose attenuation and regenerate noise above its stated curve.
Sourcing and Testing
Keruite manufactures HVAC fans and air-handling equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment. Requesting the fan’s octave-band sound power from the same lab lets you size the silencer against measured data rather than a catalogue average, which is the difference between meeting a 60 dB limit and discovering a 68 dB failure on the first site test.
Combining Silencers With Filters and the Plenum
In real plants the silencer rarely sits alone; it shares the air path with filters and coils that each add 30–200 Pa. Stack them and the total can reach 400–600 Pa, so size the fan for the sum, not the silencer alone. Place the silencer upstream of the filter when the layout allows, so the attenuated air is already quiet before it passes the resistive element, and keep 1–2 duct diameters between elements to let the velocity profile recover. A filter loaded to 250 Pa on top of a 120 Pa silencer can erase 15% of fan margin if the duty was never summed at design time, and the room then quietly fails its limit.
Field verification closes the loop: after installation, measure the receiving-room level with a Class 1 sound level meter at the octave bands and compare the result to the prediction. If the 500 Hz band reads 4–6 dB high, the usual culprit is regenerated noise from a too-tight silencer or a poorly sealed casing radiating around the unit rather than through it. Tightening the casing and dropping face velocity by 1 m/s typically recovers 3–5 dB. Keep the as-tested curve on file; it is both the evidence that the specification was met and the basis for any warranty claim against the supplier if the unit under-performs in service.
FAQ
What insertion loss do I actually need?
Subtract your noise limit from the fan’s octave-band sound power at each frequency, then take the largest gap; that band sets the requirement. A fan at 80 dB against a 60 dB room needs 20 dB at the worst band, not an average. Size the silencer to that single worst band plus 3 dB margin, and confirm the 8-band curve rather than a single number.
Why does my silencer add so much pressure drop?
Splitters that are too thin force air through narrow gaps, raising velocity and loss from 45 Pa to over 150 Pa. Keep face velocity at 4–7 m/s and use 100 mm or wider splitters on mains above 1 m² cross-section. Also add 10–20% fan margin for stacked filters, because every element draws from the same available static pressure budget of the fan.
Do silencers work on low-frequency hum?
Absorptive silencers are weak below 250 Hz, giving only 4–6 dB at 125 Hz. For transformer or large-fan hum use a reactive expansion chamber or a longer, deeper splitter bank. Measure the 63–125 Hz bands first because they usually decide the result, and never specify an absorptive unit expecting help where it physically cannot perform.
How long should the silencer be?
A 0.9 m unit gives roughly two-thirds the attenuation of a 1.8 m unit at the main band. Use 1.2–1.5 m for offices and 1.8 m or more near residences or hospitals where limits are 45–55 dB. Keep 1–2 diameters of straight duct on each side, because flow separation there can add 3–6 dB of regenerated noise that erases the silencer’s benefit.