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Silencers in Ventilation Systems

 Ventilation Attenuators


Acoustic Silencers in Ventilation Systems 

The silencers reduce noise in ventilation ductwork which is generated by ventilation and air-conditioning equipment such as fans and air handling units as well as noises coming from individual rooms. Thanks to the aesthetic design and compatibility with the remaining parts of the ventilation system, the silencers can also be installed in visible places.

The air flow through the ventilation ducts can generate noise in the rooms. To prevent this it is necessary to reduce the noise from the system; therefore it is recommended to use silencers (attenuators). The silencers enable maintaining proper acoustic parameters in the rooms – even reducing noise in the rooms which require a high degree of acoustic comfort, such as e.g. sleeping rooms or reading rooms.

The noise can be generated not only by the flow of air through the ducts. In addition, it can be caused by incorrectly suspended ducts or badly insulated mountings of fans.

To reduce this undesired effects, it is possible to use:

  • Anti-vibration pads
  • Flexible couplings of hoses,
  • Acoustic insulation of ducts and fans,
  • Silencers,
  • Grills and diffusers featuring adequate acoustic characteristics.

The acoustic silencers are the fundamental component of each ventilation system. The silencers are designed mainly to silence noises in ventilation systems. In most cases, they are mounted between the fan and the diffusing or exhaust fan and before the air diffusers.

Depending on the type of ducts used in the system, you can choose from rectangular and round cross-section silencers; flexible silencers are also available.

The SLC rectangular silencer, available among the Alnor products, consists of an enclosure and internal baffles. The enclosure is made of zinc-coated sheet steel, its ends are provided with frames made of sheet metal profiles and its is reinforced by transverse corrugation of the sheet metal.

The baffles consist of a frame made of zinc-coated sheet steel and a sound-absorbing cartridge which absorbs the acoustic energy. The sound-absorbing cartridge is a combination of non-flammable mineral wool panels; its external surface is covered with a special wear resistant fabric which protects the mineral wool.

Two kinds of baffles can be distinguished with regard to their structure:

  • A – absorptive baffles, the entire surface of which is covered with a fabric. This type of the baffles is used for low and medium frequency applications - the  SLC silencer.
  • B – absorptive-resonator baffles which are made as the absorptive ones but the sound-absorbing cartridge on both sides of the baffle is shielded with sheet steel zinc-coated on half of the baffle surface - the SRC silencer.

The baffles are mainly used with big diameters and high frequencies.

The rectangular silencers are mounted in ventilation lines with vertically positioned baffles.

The round silencers are used to silence the noise in ventilation systems based on the round ducts system.  The silencers are built of a spirally coiled round cross-section tube which works as an external jacket and a layer of mineral wool, protected with an insulating fabric and a perforated steel hose from the inside. The polyester fabric between the perforated cartridge and the mineral wool prevents the wool from getting inside the hose.

The SIBRL and SIBL round silencers are additionally provided with an internal vertical baffle which improves the noise silencing efficiency.

The SIBOL and SIBROL silencers are additionally provided with the round baffle.


In ventilation systems where the spatial restrictions or other conditions make it impossible to use straight silencers, the BSIL silencing elbows will prove to be an excellent solution.

The silencer is made of two interconnected elbows. The internal arch is made of perforated sheet steel.

The space between the external jacket and the perforated cartridge is filled with a silencing material.

In order to prevent the silencing material from being sucked into the hose, the perforated cartridge is isolated with a fabric.

Flexible acoustic silencers are easy to install which makes it possible to use them in ceiling voids and hardly accessible parts of the system.

The flexible SLEAL acoustic silencer is made of the perforated ALUDUCT hose. The isolation used is 25 mm thick and it is wrapped with the ALUDUCT AD-L jacket which is 45 microns thick and reinforced with a steel wire.

The flexible SLEASD acoustic silencer is made of the perforated ALUDUCT AD-L hose. It comes with 25 mm thick insulation, wrapped with 45 microns thick aluminium-polyester jacket.

The flexible SLEAF acoustic silencer is made of two AF-AL type hoses. It comes with 50 mm thick insulation

On both sides of the flexible silencers, there are standard nipple reducers. The flexible silencers offered by Alnor are available in two length options - 500/600 mm and 1000/1200 mm. 

The plenum boxes can also work as silencers at the ends of the lines. The boxes filled with acoustic insulation make the sound spread uniformly over the entire surface of the box. Remember not to install the air inlet and outlet opposite each other since such arrangement may prevent silencing of high sounds. 

Silencers should be installed behind fans, flow regulators.  The silencers can be used in main lines or additionally in line branches as required; they are designed to silence the noise generated by the flow of air in elbows, T-connectors or dampers. 

To properly select the type of the silencer, you need to consider the permissible flow velocity depending on the flow rate and the specific noise of the silencer. 

The round silencers are mounted by means of self-drilling screws or rivets in the part of the duct in which the coupling flange is inserted. All elements with a diameter bigger than 315 mm are made with a coupler and, to connect them with ducts, they additionally require ordering two nipples. 

The rectangular silencers are installed with the use of bolts at steel corners and a KLQ clamp fastening together two flange frames.

Thanks to the good appearance and compatibility with the remaining parts of the ventilation system, the silencers can be installed in visible places.

Furthermore, the acoustic comfort experienced by the users of the rooms in which the ventilation system has been installed is increased by vibration-dampening assembly elements and flexible couplings of the ducts.