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Ventilation Silencers for Air Duct Systems | Product Selection Guide

Ventilation Attenuators  for air duct

What is the Function of Sound Attenuator?

A duct silencer is a component of ventilation system used to reduce noise transmitted inside ventilation ductwork. Duct silencers are also referred to as sound attenuators, sound traps or mufflers.

Sound attenuators in ventilation ductwork are installed near sources that generating noise, which includes ventilation and air-conditioning equipment such as fans and air handling units, air flow regulators, air dampers and fire dampers. Thanks to the aesthetic design and full compatibility with other components of the ventilation system, duct silencers can also be installed in visible places as a part of the exposed ductwork.

What are the causes of noise in the ductwork?

Mostly the noise in the ducting is generated by air handling equipment as well as noise travelling from one room to the next. But the noise generation can be caused also by the passage of air through straight ducts and impact of air flowing through components such as bends, branches, mixing boxes etc. To prevent this it is necessary to reduce the noise from the ventilation system; therefore it is recommended to incorporated duct silencers into the system project (attenuators).

The duct silencers enable maintaining proper acoustic parameters in the rooms – even reducing noise in the rooms which require a high degree of acoustic comforts, 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.

Where are sound attenuators installed?

The acoustic silencers are the fundamental component of every ventilation system. The duct silencers are designed mainly to attenuate noises in ventilation ducting. In most cases, they are mounted between the fan and the diffusing or exhaust fan and before the air diffusers. Duct silencers should be installed behind fans, flow regulators. The sound attenuators can be used in main ductwork runs or additionally inline branch-offs as required; they are designed to silence the noise generated by the flow of air in elbows, T-pieces or air dampers. 

Types of Sound Attenuators

Depending on the type of ducts used, You can choose from:

Rectangular Sound Attenuators with Baffles

Rectangular duct silencer

Rectangular splitter silencer (sound attenuators with silencing baffles ) consists of a casing and sound-insulating baffles (splitters). The casing is made of Z275 galvanised sheet steel as a standard, a rectangular duct connection is a frame made of flange profiles (see PQ flange profiles) and corner pieces (see NQ corners).

The sound attenuators' baffles consist of a frame made of galvanised sheet steel and a sound-absorbing infill. The sound-insulating infill is a combination of non-flammable mineral wool panels; its outer surface is covered with a special abrasion-resistant fabric which protects the mineral wool.

Attenuating baffles are used in sound attenuators, but can also be supplied individually. In order to minimize transport cost, instead of ordering ready-to-install rectangular sound attenuators, we offer baffles for on-site installation in rectangular ducting. The attenuating baffles are mainly used with larger diameters and high frequencies.

The rectangular silencers are installed in rectangular ventilation ducting with vertically positioned attenuating baffles. In order to install rectangular attenuators, You need to fasten bolt at steel corners and G clamp (see KLQ clamps) and cleats (see PWQ & PWQ-R cleats) to join together two flange frames.

3 Types of Attenuating Baffles (for rectangular attenuators)

Attenuating baffles for silencer Sliding nut for channtel strut 41X21MMSliding nut for channtel strut 41X21MM

K-SLC baffle used for SLC attenuators
Attenuating baffles
=steel frame +mineral wool liner

This baffle type is generally used for
sound insulation at low and medium frequencies.

K-SRC baffle
used for SRC attenuators

Perforated attenuating, resonator
=steel frame + mineral wool liner + ½ of a baffle length covered with steel sheet:
This baffle type is generally
used for sound insulation at medium and high frequencies.

K-SPC baffle used for SPC attenuators
Perforated attenuating, resonator
= steel frame + mineral wool liner + baffle
covered over its entire length with
perforated steel sheet at an open surface area ratio of 36%.

Testing of Rectangular Sound Attenuators

The insertion loss, the pressure loss and the sound power level of splitter silencers had been measured at the Fraunhofer IBP according to DIN EN ISO 7235. The sound power levels had been measured for two different flow velocities of 10m/s and 20m/s in the air gap between splitters.

Check technical data for SLC Rectangular Sound Attenuator or Test Report by Fraunhofer Institute 

Circular Sound Attenuators - Rigid

Sound attenuators in a circular casing

Circular silencers are used in circular ductwork as a noise-dampening solution. The silencers are made of a spiral duct (sound attenuators SIL, SIBL, SIBOL, SIRL, SIBROL) or plain duct (sound attenuators SI-GL and SIL-GL) as an outer duct and perforated steel sheet inner tube. Between these two walls, there is a layer of sound-attenuating, non-combustible mineral wool. The mineral wool is covered with polyester fabric and perforated steel sheet that prevent the mineral wool from getting inside the ducting. 

The circular silencers are mounted using of self-drilling screws (see WGO) or rivets in the part of the duct in which the spigot connection is inserted into the duct. All duct silencer with a diameter larger than 315 mm are made with female ends and to connect them with ducts, You need to separately order two male couplings (see NS/NSL couplers). 

Sound attenuator with baffle//podFor higher noise attenuation efficiency in larger diameters, circular sound attenuators can be equipped with a central baffle or pod. The SIBRL and SIBL circular silencers are additionally equipped with an internal vertical baffle (photo on the left), while SIBOL and SIBROL (photo on the right) duct silencers are additionally provided with the internal pod.

Curved Sound attenuators

In ventilation systems where it is limited, confined space or other conditions make it impossible to use straight duct silencers, the silencing elbows (curved silencer) will prove to be an excellent solution. The curved silencer is made of two interconnected bends. The inner bend is made of perforated sheet steel. The space between the outer and inner jacket is filled with a sound-absorbing material. In order to prevent the sound-attenuating material from being sucked into the ductwork, the perforated jacket is faced with a fabric.

Sound attenuators in a rectangular casing

Compact sound attenuators
Another compact sound attenuator type recommended for confined spaces are sound attenuators with rectangular casing for circular duct connection. These duct silencers have very low installation height, therefore, can be easily used for limited suspended ceiling installation. Compact sound attenuators are available with 25mm (SQLL-25), 50mm (SQLL-50) and 30mm (SQQL-30) mineral wool insulation. There's also rectangular silencer model with circular spigots, where one spigot is located on the sidewall. 

Flexible Sound Attenuators
Flexible sound attenuator

Flexible duct 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. 

How to select the sound attenuator

Sound attenuator softwareTo properly select the type of the duct silencer, you need to consider the permissible flow velocity depending on the flow rate and the specific noise of the silencer. Many sound attenuator manufacturers offer special silencer selection tool, including Alnor, our silencer selection tool is called AlnorSILENT. It's available for free, You can download it HERE

AlnorSILENT software calculates the sound attenuator performance characteristics based on the user's input data. It helps HVAC designers to analyse the key product features of the whole range of sound attenuators in Alnor's offer. The software product database contains sound insulation performance data for circular and rectangular splitter silencers. AlnorSILENT features a very user-friendly, intuitive, and highly functional GUI with multiple panels. 

The panels make it possible to review Product Data Sheets, 3D models and performance data tables at a glance. AlnorSILENT can provide several alternative sound attenuators to choose from for the expected sound insulation performance. The 3D models of silencers can be exported to IFC and DXF files. Once a duct silencer is chosen, AlnorSILENT will export detailed lists of technical specifications for easy cost estimation of the ventilation system.

Sound attenuator software

Advantages of AlnorSILENT software

• Select the sound attenuator to match the specified ventilation fan and the expected sound insulation performance
• Several alternative duct silencers to choose from for the expected sound insulation performance
• Full lists of technical specifications
• A comprehensive database of sound performance data for circular and rectangular sound attenuators in one place
• Export of 3D models to IFC and DXF files

PVC sound attenuator with splittersSound attenuator for corrosive/ chemically-laden air

For standard commercial and residential building, a sound attenuator made of galvanized steel is usually sufficient. However, there are facilities and industry sectors where ventilation systems require a higher resistance to specific chemical substances like acids, a higher level of humidity, higher temperature etc.  

Depending on the type of aggressive environment we can offer sound attenuators made of stainless steel, aluminium and plastics, like PVC,  PP, PPs and more. The construction of sound attenuator made of polymers is similar to a traditional metal one, where a casing and inner tube (in circular silencers) are manufactured from the chosen plastic material and attenuating material is made of mineral wool.

For more information about the physical and chemical properties of plastic materials go HERE.

Sound attenuator for the MVHR systems

Sound attenuators aren't only used in industrial ventilation systems. In domestic mechanical ventilation systems with heat recovery (MVHR) noisy unit/ductwork can be a huge issue for inhabitants. Most of the main causes of noise in the MVHR system can be eliminated with a good design, installation and unit selection. More about this matter we discuss in the article Is the MVHR system noisy? One of the “good practices” is mounting a ventilation silencer which will effectively reduce any potential noise transfer from the unit and into the room via the network of ducting.
Sound attenuators for MVHR system

Other attenuating noise solutions

Sound attenuators aren't the only ductwork components used to reduce noise in the ventilation systems. Since air handling units are the main source generating vibrations and noises in the ductwork, we have quite a range of sound-insulating solutions designed especially for this purpose. 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.
Flexible duct connectors

1. Flexible duct connectors (compensators)

Flexible duct connectors that are dedicated to silencing, insulating noise generated by HVAC equipment, like fans and AHUs. Flexible duct connectors are available with male or female ends, with flanges or made of flange profile to be compatible with connections of the AHUs. Flexible duct connectors (2 pcs of metal sheet + fabric) are used to production of expansion joints and compensators that absorb movement, reduce vibrations and sounds acting as a dampener.
Check technical sheet of flexible duct connectors (AMT) or ready compensators for ductwork

2. Supports and Suspension brackets with anti-vibration lining

Another type of noise dampening solutions are supports and suspension accessories with anti-vibration mounts or linings that aim to suppress the structure-borne noises. 

3. Acoustic insulation

Duct insulation doesn't play only the role of the thermal insulator (condensation growth protection) but also an acoustic insulator. Insulation of ventilation ductwork reduces breakout noise and the structure-borne noise transmitted into the building.

4. Plenum boxes with insulation lining

The plenum boxes can also work as "sound attenuators" at the ends of the ductwork runs. The plenum boxes are lined with sound insulation make the sound spread uniformly over the entire surface of the box. Keep in mind, not to install the air supply and air extract opposite each other since such arrangement may prevent attenuation of high sounds.