Reducing noise from single- and dual-flow CMVs

Summary of the article written in collaboration with Gabriel Gormezano, Econology, a French online retailer of building ventilation equipment.

Full article: https://www.econology.fr/le-blog/ameliorer-bruit-reseau-vmc

Ventilation systems, whether single or double flow, can be a major source of noise pollution. This noise can be caused by a number of factors: motor noise and vibration, turbulence in the airflow inside the ducts or near the outlets. The best way to limit these nuisances is to follow the installation and maintenance recommendations provided by the manufacturers.

However, it is also possible to intervene on an existing network to reduce noise problems. This article looks at ways of identifying the causes and mitigating noise pollution from a ventilation system.

I. Equipment

A. Sound traps

Sound traps, also known as silencers, combine acoustic dissipation and reflection to reduce noise pollution. Their installation significantly reduces noise levels downstream (in the direction of sound propagation) of the trap, but may slightly increase noise upstream.

Sound traps should be chosen according to duct diameter to avoid excessive pressure losses. Their effectiveness varies according to noise frequency and duct diameter.

B. Acoustic rings

Acoustic rings, placed directly behind the air vents, also work by dissipation and reflection. They have the advantage of being installed without dismantling the ducts, but their efficiency is generally lower than that of sound traps. In addition, they slightly reduce airflow by reducing duct cross-section. This function may be desirable when network balancing is imperfect: too much flow at one port and not enough at another.

C. Ventilation ducts

Two important acoustic phenomena are involved in ducts: wall radiation and attenuation.

Duct radiation can be seen as acoustic leakage from the duct: noise propagating in the duct escapes through its walls. The greater the radiation, the greater the noise outside and near the duct. The greater the radiation or the greater the distance from the noise source, the lower the sound power propagating in the duct. This is because the noise has escaped through the walls of the duct.

Irrespective of radiation, ducts attenuate noise propagation, either by dissipating noise (absorption) or by reflecting noise back to its source. The greater the attenuation, or the greater the distance from the noise source, the lower the sound power propagating in the duct. This is because noise propagation in the duct has been attenuated.

In addition to these two acoustic phenomena, there is a solid-borne vibration phenomenon: the transmission of solid-borne vibrations, generally generated by the central ventilation unit, along the ducts. The more axially rigid the duct, the more vibrations it transmits. These vibrations are problematic because they can generate noise (transfer of solid-borne vibration into airborne vibration = noise).

Ventilation ducts play a key role in the propagation or attenuation of noise. Here's a brief description of the main types:

1. Uninsulated flexible ducts

These ducts, in aluminum or PVC, are the most economical.

• Advantages : Low cost, high flexibility, low vibration transmission.

• Disadvantages : Very thin walls, resulting in high acoustic radiation and virtually no sound attenuation.

• Recommended use: For installations where acoustic radiation outside the duct is not a problem. Particular care must be taken to ensure that the duct is not pinched or bent too tightly once installed.

  
  

2. Insulated flexible ducts

This type of duct, often surrounded by glass wool thermal insulation, is designed to prevent condensation. The addition of glass wool around the central duct also reduces its acoustic radiation (the wool insulates the duct) and increases acoustic attenuation (the wool absorbs noise propagating through the duct).

• Advantages : Insulation reduces acoustic radiation and attenuates internal noise. The thickness of the insulation (25 or 50 mm) has a direct impact on acoustic efficiency.

• Disadvantages : More expensive than uninsulated ducts.

• Recommended use : For installations requiring thermal insulation (e.g. uninsulated attics) and noise reduction. In fact, its relatively low radiation capacity and acoustic attenuation reduce noise transmission outside duct walls and along ducts. Particular care must be taken to ensure that, once installed, the duct is not pinched or bent too tightly.

  
  

3. Insulated rigid ducts

These ducts, often made of expanded foam or metal with an insulating coating, offer good thermal insulation and easy installation thanks to their rigidity.

• Advantages : Intermediate reduction of acoustic radiation, low pressure drop thanks to smooth surface and straight shape.

• Disadvantages : Not very flexible, they transmit more solid-borne vibrations.

• Recommended use : For simplified straight-line installations or when thermal insulation is important.

4. Semi-rigid ducts

Made from high-density polyethylene, these ducts are becoming increasingly popular for CMV installations.

• Advantages : Very low acoustic radiation thanks to thick walls, moderate transmission of structure-borne vibrations thanks to axial flexibility, easy installation in false ceilings, reduced pressure drop thanks to smooth inner lining and relatively straight shape with large-radius elbows.

• Disadvantages : Marginal internal sound attenuation, often requiring the addition of sound traps to limit noise propagation.

• Recommended use : For modern networks equipped with sound traps, requiring a compromise between acoustic performance and ease of installation.

5. Galvanized conduits

These rigid metal ducts, often used for their “industrial” appearance, have little thermal insulation.

• Advantages : Good mechanical strength, lower sound radiation with greater wall thickness.

• Disadvantages : High solid-borne vibration transmission, low internal acoustic attenuation.

• Recommended use : In environments where aesthetics or durability take precedence over the propagation of airborne and structure-borne noise along ducts.

II. Noise causes and solutions

Noise emitted by a controlled mechanical ventilation (CMV) system can come from a variety of sources. Poor management of this noise nuisance can adversely affect occupant comfort. Here's an overview of the main causes of noise in a CMV system, along with practical solutions to remedy them.

A. Noise generated by vibrations from the VMC central unit

Solid vibrations from the central ventilation unit are transmitted to its support and then propagate through the solid elements of the building, radiating as acoustic waves. These vibrations can propagate throughout the home, and even to neighbors.

Solutions :

• Cord suspension : If possible, hang the unit from cords (in the attic or cellar) to separate the control unit from its support.

• Anti-vibration mounts or rubber insulation : Install anti-vibration mounts suited to the weight of the unit.

• Avoid contact with false ceilings : If the unit is installed between a ceiling and a false ceiling, ensure that it does not touch the latter.

• Unit maintenance : Check unit for abnormal vibrations.

B. Noise generated by vibrations in the ventilation ducts

Vibrations from the control unit can be transmitted to the ductwork, which then diffuses them in the form of radiated noise. This phenomenon is amplified if the ducts are axially rigid.

Solutions :

• Limit points of contact with radiating surfaces : Reduce contact areas between ducts and false ceilings or walls.

• Use flexible sheaths or gasketed connection accessories : Install sections of flexible sheath between the control unit and the rigid sheaths to break the vibration path.

• Apply resilient tape : Wrap ducts with vibration-absorbing tape.

• Choose suitable sheaths : Opt for sheaths designed to minimize vibration transmission (see above).

C. Noise generated around the VMC central unit

The VMC central unit may emit perceptible noise in the vicinity, particularly if installed near a bedroom or in a resonant space (attic, cellar).

Solutions :

• Choose a low-noise CMV : Compare models on the basis of ErP data and give preference to quiet units.

• Strategic positioning : Install the control unit in an area where noise will be less of a nuisance (e.g. above the bathroom rather than the bedroom).

• Reduce acoustic resonance : Add sound-absorbing materials (foam, wool) to the walls of the space housing the device.

• Sound insulation box : Make or buy an insulating box to reduce radiation from the unit.

D. Operating noise propagated through the ventilation network

The central ventilation unit's motor and fan generate mechanical noise that propagates through the ducts and exits through the air vents. This noise includes tonal (humming and whistling) and broadband (wind noise) components.

Solutions :

• Sound traps : Install these devices at the central unit outlet or behind the VMC vents.

• Sound-absorbing ducts : Use sound-attenuating ducts such as those thermally insulated with glass wool.

• Silent CMV : Choose models that emit low levels of noise from the air inlets.

• Acoustic rings : Install these devices just behind the diffusers. Beware of induced pressure losses.

E. Turbulence noise in the ductwork

Airflow in the network can generate turbulence and noise, especially in the presence of obstacles such as elbows or ducts with non-smooth inner surfaces.

Solutions :

• Unobstructed air inlets and outlets : Check that VMC vents and window air inlets are unobstructed.

• Reducing head losses : Eliminate pinches, limit elbows, and use smooth, large-diameter ducts (as recommended).

• Sound traps or acoustic rings : Install these devices just behind the diffusers. Beware of induced pressure losses.

• Network cleaning : Remove any debris likely to generate whistling noises.

F. Noise from neighboring dwellings (intercom)

In multi-family dwellings, CMV ducts can transmit noise from neighboring rooms (voices, music, household appliances, etc.).

Solutions :

• Sound-absorbing ducts : Use sound-attenuating ducts such as those thermally insulated with glass wool.

• Sound traps: Install sound traps.