Understand what a VMC silencer does — and does not — treat
Airborne noise in the duct: the only case where a silencer is relevant
A silencer makes sense when the noise is airborne noise carried by the duct (supply or extract). Typically, a remote unit that is audible at a terminal. The silencer absorbs part of the acoustic energy thanks to a duct lined with absorbent material, without treating the source. Before specifying one, confirm with a sound level meter and an airflow meter that the level changes with speed and drops when the duct is pinched or extended.
What a silencer will never fix: structure-borne noise, radiating casing, vibrations
A VMC silencer will do nothing about structure-borne noise transmitted through hangers, rigid sleeves or an uncompoupled wall penetration. It also will not mask a casing radiating noise into a false ceiling. In those cases, you must deal with the fixings, decoupling, airtightness of branch connections and, if needed, acoustic insulation of the unit casing.
When a “VMC silencer” makes things worse: pressure loss, whistling and reduced airflow
Poor sizing creates extra pressure loss. The fan compensates, or not. Either way, you pay in acoustics and airflow. To go further on common mistakes in VMC duct network sizing, check in particular diameters, lengths and bend radii.
- Reduced diameter or crushed fittings. Whistling appears at the terminals.
- Silencer fitted too close to the unit or an elbow. Turbulence and tonal noise.
- Unprotected absorbent in a dusty network. Clogging, airflow drop, difficult maintenance.
Diagnosing a noisy VMC before fitting a silencer
Measure and classify the noise: whistling, whooshing, vibration, resonance (site references)
Before considering a sound level meter or a VMC silencer, start by objectifying the issue. Measure the sound level at the terminal and at the unit, then classify the signature.
| Signature | Useful measurement | Site clue |
|---|---|---|
| Whistling | Air velocity, m/s | Constriction, leak, local turbulence |
| Whooshing | Airflow, m³/h | Excess flow or terminals too open |
| Vibration | Vibration, mm/s | Transmission through hangers, frame, supports |
| Resonance | Pressure, Pa | Rigid duct or “drum-like” volume |
Find the source: excessive airflow, unsuitable terminals, tight elbows, undersized duct
Trace the airflow path. Check the unit curve, available pressure and whether the network is coherent. To go further on tools and method, see how to measure ventilation airflow.
- Excessive airflow, unit setting, unbalanced terminals.
- Unsuitable terminals, diameter, control, local pressure losses.
- Tight elbows and harsh tees that generate turbulence.
- Undersized duct, high air velocity, whistling.
Address the cause first: settings, balancing, suspension, decoupling, airtightness
Correct the network and installation first. Adjust and balance airflow, fix leaks, then redo the hangers with decoupling. A silencer will not make up for pressure loss created by a cramped assembly; it may even bring the whistling to light.
Choosing the right VMC silencer: dimensions, performance and durability
Effective length and expected octave-band attenuation: what to aim for on a standard installation
On a residential network, an effective VMC silencer is chosen first by its effective length (excluding branch connections). In practice, aim for 600 to 1,000 mm in inline installation. Attenuation should be read as insertion loss by octave band, not as overall dB. Order-of-magnitude values to verify on the manufacturer datasheet.
| Octave band (Hz) | Target attenuation (dB) |
|---|---|
| 125 | 2 to 6 |
| 250 | 4 to 10 |
| 500 | 6 to 15 |
| 1000 | 8 to 20 |
Keep the same diameter to avoid pressure loss: do not “neck down” the duct
Keep the same nominal diameter as the duct. A local reduction increases velocity, pressure loss and causes whistling. If adaptation is necessary, use gradual transitions and recalculate the available airflow at the unit.
Absorbent material and resistance to clogging: what lasts in ventilation (and what mats down)
Choose a protected absorbent layer (liner, microperforated facing) to limit fibre release and clogging. “Bare” mineral wool and overly soft foams mat down faster and lose effectiveness. Check fire class, and access for inspection and cleaning.
Installing a VMC silencer without hurting airflow: location and rules to follow
Where to place it: on the discharge and on branches, not randomly in the network
A silencer treats airborne noise travelling through the duct. Place it where acoustic energy is most strongly “carried”: at the unit discharge and, if needed, on branches feeding the most sensitive terminals. Avoid dropping it into the middle of a manifold “just to try”. If the noise is structure-borne (hangers, fixings, penetrations), the VMC silencer will not change anything until the mechanical cause is dealt with.
Minimum distances: keep away from the unit and elbows to avoid turbulence
| Point of attention | Minimum straight length | Purpose |
|---|---|---|
| Unit outlet → silencer inlet | 3 to 5 D | Stabilise the flow |
| Elbow → silencer (upstream) | 2 to 3 D | Limit turbulence |
| Silencer → elbow (downstream) | 1 to 2 D | Avoid whistling |
Keep the diameter of the network. Any reduction or crushed duct increases pressure loss and can shift the airflow settings. To limit drift, also pay attention to VMC duct airtightness, often behind “invisible” losses and unstable airflow.
Clean installation: airtightness, clamps, support, access for checking and maintenance
Install the silencer with suitable connectors, continuous seals and tape compatible with the network. Suspend it without stressing the duct, using clamps and decoupled supports. Leave an accessible area for inspection, retightening and checking clogging. A misaligned or poorly sealed silencer often creates whistling, then the unit is “compensated” for. That is where airflow falls apart.
Validate the result and avoid common mistakes (2026)
Check after installation: terminal airflow, residual noise and whistling
After installation, repeat your measurements. Check terminal airflow and overall balancing, then listen for residual noise under steady operation. If whistling appears, suspect first excessive air velocity or pressure loss created by the assembly. A VMC silencer will not make up for an unbalanced network.
Common site mistakes: silencer too short, wrong orientation, compression, crushed duct
- Silencer too short or installed hard against the unit or an elbow. Attenuation drops.
- Wrong installation direction. Some models require a specific flow direction.
- Compressing the absorbent, or joints tightened too much. You lose the acoustic benefit.
- Crushed duct or reduced diameter. You create pressure loss and noise.
When to move to a whole-system ventilation solution: noisy unit, network to be redone, ventilation product page
Move to a full overhaul if the unit radiates noise, if the hangers transmit structure-borne noise, or if the network is undersized (diameters, lengths, elbows). In those cases, deal with the cause first, then add acoustic treatment only afterwards. If needed, switch to the whole-system ventilation solution to frame a coherent solution.
