How to reduce the noise of an air handling unit (AHU)?

This is the question that arises in many contexts, with respect to one of the main equipment of a Heating, Ventilation, Air Conditioning (HVAC) system:

  • when it comes to preventing noise pollution, by ensuring that its sound power level is low enough for this assembly generally consisting of a fan with filter, and a heating and/or cooling sub-set (in addition to ducts for air inlet and outlet)
  • when noise disturbance is proven
    • for an inhabitant or user of the building in question: owner or tenant of accommodation, tourist, commercial traveler or other in a hotel room, pupil or student - or even professor - of an educational establishment (school, university), child - or supervisory staff - in a place where young children are welcomed (nursery), patient in a care establishment (hospital, clinic, retirement home or assisted living facility for elderly people - ALFEP -), musician in a music rehearsal room or - then also valid for a spectator - in another musical place, guest or member of staff in a catering room, worker in an office (the list is endless)
    • for a neighbour, regardless of his social or professional position (e.g. among those mentioned above or other)
  How to reduce the noise of an air handling unit (AHU)
How to reduce the noise of an air handling unit (AHU)

Compressors and fans are the organs of an air handling unit (AHU) regularly incriminated for their undesirable noise emissions - including the case of Establishments Classified for Environmental Protection - ECEP -:

  • exceeding of the admissible limit for emergence: this is the difference between the A-weighted equivalent continuous pressure levels of the ambient noise - air handling unit (AHU) considered in operation - and of the residual noise - in the absence of noise generated by the air handling unit (AHU) considered, but measured over its operating period) in zones with regulated emergence (ZRE); depending on the context: 5 or 6 dB(A) during the day and 3 or 4 dB(A) during the night
  • presence of a tonal component: when, considering an unweighted third-octave spectrum due to the operation of the air handling unit (AHU), the level in a frequency band exceeds by 5 dB or more the average level of the two adjacent frequency bands from 400 Hz to 6300 Hz or 10 dB that of the two adjacent frequency bands from 63 to 315 Hz
  • exceeding the permissible level at the property boundary, set by prefectural decree - typically 70 dB(A) during the day, 60 dB(A) during the night -

When it comes to reducing the excessive noise generated, inside a building, by an air handling unit (AHU) which constitutes a collective equipment, good practices, in terms of soundproofing, are the following:

  • with regard to the propagation of structure-borne noise: the installation of anti-vibration supports (depending on the case: springs, elastomer pads and hangers) not only for the machine, but also for the ducts constituting the aeraulic network
  • with regard to the propagation of airborne noise: what makes it possible to limit sound transmission from noisy space being inside the ducts to the locations that one wishes to be protected from noise:
    • implementation of double-skin ducts, e.g. when the aeraulic circuit is installed in a suspended ceiling (or in a technical floor) made with tiles which only offer low noise insulation
    • increase in the sound reduction index of partitions separating the elements of the ducting network from living, living, working, sleeping, listening, speaking rooms, etc.
  • with regard to the noise of the air openings (i.e. for points where air is delivered or collected): the implementation of silencers:
    • circular, square or rectangular
    • whose effectiveness is based on the presence of a material that sufficiently absorbs sound in the frequency range of interest (e.g. mineral or polyester wool) - rock wool with a density of 35 kg/m3 to 50 kg/ m3 with a thin anti-defibration glass cloth constitutes a usual solution -, and on a passage rate and an active length appropriate to the case being considered

When it comes to reducing the excessive noise of an air handling unit (AHU) in the neighborhood, the means of action, in terms of soundproofing, are (possibly) the following (which can be combined):

  • the implementation of dissipative silencers, with baffles, in the form of boxes: one at the intake and the other at the discharge of the air handling unit (AHU), ensuring the necessary separation of the air flow, to avoid the recycling phenomenon, which is highly undesirable; these are generally frames made of folded, galvanized sheet metal filled with rock wool with a glass cloth surfacing (the air passage speed is generally sufficiently low – otherwise the total pressure loss would be too great for the proper functioning of the facility and/or the flow noise would compromise the efficiency of the device - so as not to make it necessary to use perforated metal protections; the air flow in the (cool) air inlet silencers is often horizontal, while that in the (warmer) air outlet silencers is vertical
  • the installation of casing elements (soundproofing canopy), allowing (when associated with the silencers described above) to ensure continuity of the acoustic insulation, only allowing the complete treatment of the noise "at the source", and of which the result is then a reduction in the overall external sound power level of the air handling unit (CTA) - which is an intrinsic characteristic -; these are metal soundproofing panels, with, from the inside (i.e. on the side of the noise source) to the outside (i.e. on the opposite side):
    • a perforated sheet (generally: at least 30% for perforation ratio)
    • mineral or polyester wool (often: rock wool with a density of around 70 kg/m3)
    • a metal facing (most of the time: with a thickness of a fraction of a millimeter - 0.5 mm, 0.75 mm - if they are panels manufactured using an industrialized process except for constructions aiming for acoustic performance and/or superior robustness and/or more or less handcrafted - 1 to 1.5 mm then -)

Such panels should have two features, in terms of soundproofing:

    • sound reduction: the corresponding normalized index, often denoted R - variable according to the quality of the assembly - is, when measured in a laboratory - commonly around 30 dB(A) for a sound spectrum based on a unique level in all 1/1 octave frequency bands from 125 Hz to 4 kHz (pink noise); this component of acoustic performance has to do with limiting the noise transmission from inside of the enclosure towards outside
    • sound absorption: the corresponding normalized coefficient, often denoted alpha, can - when measured in a laboratory - reach 100% at medium and high frequencies; this component of acoustic performance has to do with limiting the amplification of sound levels inside the enclosure, which would be counterproductive
  • the fitting of an acoustic screen (noise barrier), which can be achieved by means (in addition to a sometimes very substantial metal frame) of soundproofing panels such as those described above, and with, in the event of necessary - i.e. when the noise barrier wall is installed very close to the air handling unit (AHU) and when it is - then - necessary to limit the propagation of noise from the openings which are necessary for its proper aeraulic operation: with mufflers such as those also described above; unlike the combination mentioned above, an acoustic screen (noise barrier wall) only allows sound limitation:
    • in well-defined locations
    • which is less

ITS masters all aspects of the question: how to reduce the noise of an air handling unit (AHU) ? With possibly:

  • on-site acoustic diagnosis with sound levels measurements, comparison with regulatory values or with limits coming from technical specifications (including: identification and prioritization of noise sources, search for tonal components, emergence calculations) by a human resource qualified in physical measurements (specialized in instrumental techniques)
  • sound impact calculations and study of soundproofing solutions (including sizing of silencers in terms of acoustics and aeraulics, calculations of sound screens (noise barrier walls) efficiency and of and sound pressure levels at specified locations in the environment) ; an acoustic engineer is then involved, with a degree in building physics, experienced in the use of predictive acoustics software such as SILDIS® [1] , with an extensive knowledge of construction technologies and with a solid experience in solving sound nuisance problems in industry, in the construction sector and in the environment
  • marketing of all soundproofing components and systems: metal acoustic insulation panels, silencers, canopies or turnkey noise barriers [2]

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[1] Computer Aided Design (CAD): SILDIS® calculation software for acoustics and aeraulics in the construction sector

[2] Products and solutions