How the two differ in kind
Noise from a ventilation system reaches a room by more than one route, and the two products here address different ones. Getting the diagnosis wrong means fitting an expensive component that does nothing about the complaint.
DUCT-BORNE noise travels inside the duct as sound in the airstream, from the fan or from turbulence at fittings, and comes out of the grille into the room being served. A SILENCER — an attenuator, a lined section with splitters or a lined bend — sits in the duct and absorbs that energy as it passes through, so less of it arrives at the grille. Its performance is stated as insertion loss, the reduction achieved by inserting it, and it is frequency-dependent: most attenuators do far more at mid and high frequencies than at low ones.
BREAK-OUT noise takes a different path. Sound energy inside the duct excites the duct wall, which radiates into whatever space the duct passes through — a corridor, an office, a bedroom above a plant room. A silencer downstream is irrelevant to it, because the noise is leaving before it gets there. LAGGING addresses it, by adding mass and a decoupling layer around the duct so the wall radiates less.
Two things temper both. A silencer adds pressure drop, which the fan has to overcome; a fan pushed to work harder generates more noise and consumes more energy, so an oversized silencer can leave the system noisier than a smaller one. And attenuation stops paying once another path dominates: past a certain point the limit is break-out, or flanking through the structure, or the terminal device itself, and a second silencer buys very little.
The factors that actually differ
Show
| Duct silencer (attenuator) | Lagging the duct | |
|---|---|---|
| Path it addresses | Noise travelling INSIDE the duct toward a grille. | Noise radiating OUT through the duct wall into the space the duct passes through. |
| Where it goes | In the duct run, usually near the fan and before the branch it protects. | Around the duct, over the length passing through the sensitive space. |
| Pressure drop | Real, and it has to be in the fan's static pressure. Oversizing can make the system noisier. | None. It does not touch the airstream. |
| Frequency behaviour | Strong at mid and high frequencies, much weaker at low ones, which is why fan rumble is hard to attenuate. | Governed by mass, so also weaker at low frequencies — but a decoupled mass layer performs far better than insulation alone. |
| What it does NOT fix | Break-out along the duct upstream of it, and any structure-borne path. | Anything coming out of the grille. |
| Space required | A straight length in the duct run, and a bigger cross-section than the duct if velocity is to be controlled. | The thickness of the lagging around the duct — which in a tight ceiling void is frequently the constraint. |
| Thermal insulation | Incidental and local. | Often the same operation — thermal duct wrap and acoustic lagging overlap, though acoustic lagging needs a mass layer that thermal wrap does not have. |
| Cleanliness | Lined attenuators can shed fibres and collect dirt; where hygiene matters, use a type designed for it. | External, so no hygiene implication for the airstream. |
| Cost | A component, plus the fan energy its pressure drop costs for the system's life. | Material and labour along the run, with no ongoing cost. |
| What else to check first | Velocity. Noise generated at fittings and terminals is often the real source, and a silencer upstream does nothing about it. | Structure-borne transmission. A fan hard-mounted to the structure is not a duct problem at all. |
Which one, and when
Choose duct silencer (attenuator) when…
- The noise is coming out of the grille — the diagnostic that decides it.
- A fan serves a sensitive space and the duct-borne path is the direct route.
- There is a straight run available at a sensible place, near the source rather than near the terminal.
- The fan's static pressure can accommodate the added drop without being pushed up a curve.
Choose lagging the duct when…
- The duct passes through a sensitive space and the noise is heard along its route rather than at a grille.
- A high-pressure or high-velocity duct runs above a quiet room — the classic break-out case.
- A plant room duct passes through occupied space before it reaches its attenuator.
- There is no room in the run for an attenuator, but there is room around the duct.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- How do I tell break-out noise from duct-borne noise?
- By where it is loudest, and the test is easy. Duct-borne noise comes out of the terminal, so it is loudest at the grille and falls away as you move across the room. Break-out radiates from the duct itself, so it is loudest beneath the duct run — which may be nowhere near a grille — and traces the route of the duct across the ceiling. Two further clues: break-out is common where a high-velocity or high-pressure duct passes over a quiet space on its way somewhere else, and it is often broad and rumbling rather than the hiss associated with a terminal. A third possibility to rule out before spending anything is structure-borne transmission, where the fan or the duct is hard-mounted to the building and the noise is arriving through the structure rather than through the air.
- Can a silencer make things worse?
- Yes, in two ways, and both come from treating attenuation as a free addition. The first is pressure drop: an attenuator resists airflow, the fan has to overcome that resistance, and a fan working further up its curve generates more noise and uses more energy — so a silencer that is generously sized for attenuation and ungenerously sized for airflow can leave the system noisier than before. The second is self-generated noise: air moving through an attenuator at high velocity makes noise in the attenuator itself, which sets a floor below which its insertion loss is irrelevant. Both are avoided the same way, by sizing the cross-section for an acceptable face velocity rather than simply matching the duct size, which is why attenuators are frequently larger in section than the duct they sit in.
- Why does low-frequency noise resist everything?
- Because both mechanisms weaken as frequency falls. Absorptive attenuation works by sound passing through porous material and losing energy to friction, and that requires the lining to be thick relative to the wavelength — low-frequency wavelengths are long, so a practical lining thickness does comparatively little. Mass-based lagging follows the mass law, which also gives less at low frequencies, and it runs into the resonance of the mass-air-mass system the lagging creates. So fan rumble and low-frequency plant noise are genuinely hard, and the answers are structural rather than product-based: reduce it at the source with a slower, larger fan; use a reactive or combination attenuator designed for the specific low-frequency content; increase the distance; and isolate the plant properly so nothing is arriving through the structure.
- Where should a silencer go in the run?
- Near the source rather than near the terminal, and with straight duct either side of it. Close to the fan means the noise is attenuated before it can break out along the length of duct downstream — put the attenuator at the far end and the whole run upstream has been radiating into every ceiling void it crossed. Straight duct either side matters because an attenuator preceded or followed immediately by a bend, a transition or a damper sees disturbed flow, which increases both its pressure drop and its self-generated noise, and it will not achieve its catalogue performance. The manufacturer states the straight lengths required. There is a competing consideration: a silencer immediately at the fan discharge sits in the most turbulent air in the system, so the usual compromise is close but not adjacent.
- Is acoustic lagging the same as thermal duct wrap?
- Related but not the same, and substituting one for the other is a common disappointment. Thermal wrap is a layer of insulation to control heat gain or loss and to prevent condensation; it has low mass, and low mass is precisely what does not stop sound radiating from a duct wall. Acoustic lagging adds a MASS layer — a heavy limp barrier such as a loaded vinyl — over a resilient layer that decouples it from the duct, so the duct wall's vibration is not transmitted straight into the mass. The resilient layer matters as much as the mass: a heavy barrier bonded tightly to the duct is far less effective than the same barrier on a soft layer. Where both jobs are needed, a combined system does them together, and it is thicker than either alone.
- How much attenuation do I actually need?
- The difference between the noise the system delivers and the noise criterion for the room, calculated per frequency band rather than as a single number — which is why both source data and attenuator performance are published as spectra. The calculation starts with the fan's sound power, subtracts what the duct run itself attenuates, adds the noise generated at fittings and at the terminal, applies the room's own absorption, and compares the result with the target. Doing it by band matters because the deficit is usually concentrated at particular frequencies, and an attenuator chosen on its headline figure may deliver its performance where you did not need it. The other reason to calculate rather than estimate is the pressure drop: the smallest attenuator that meets the criterion is the right one.
- Do lined ducts and bends help?
- Yes, and they are frequently overlooked because they are not a component anybody orders. Duct itself attenuates, and lined duct attenuates considerably more per metre than bare duct; a lined bend does more still, because sound reflecting off the turn is absorbed rather than passed on. On a long run with several bends the accumulated effect can be a substantial part of the total attenuation required, which is why the calculation includes it rather than crediting everything to the silencer. Two caveats. Lining reduces the free area, so it has to be allowed for in the sizing. And lining raises the same hygiene question as a lined attenuator — in environments where fibre shedding or cleanability matters, the lining has to be a type rated for it, or the attenuation has to come from elsewhere.
- What should I check before buying either?
- Velocity, and the terminal. A great deal of what gets diagnosed as fan noise is actually generated locally, by air moving too fast through a duct, a bend, a damper or the grille itself — and no attenuator upstream of the source can do anything about noise produced downstream of it. A damper throttled nearly shut to balance a system is a particularly common culprit, and the fix is balancing the system properly rather than silencing the symptom. Check the design velocities against what is recommended for the room's noise criterion, check whether any damper is heavily throttled, and check the terminal's own published noise rating at the flow it is actually passing. If any of those is the source, the attenuator will be fitted, paid for, and the complaint will remain.