Air Duct Noise Silencers – All You Need to Know
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Air ducts are loved for bringing conditioned air into dwelling spaces throughout the building. But, these can often behave much like old-fashioned speaking tubes, which conduct invasive and bothersome noise into occupied areas.
Check out my guide on Soundproofing Air Vents
So, how exactly does one alleviate this problem? How would air duct noise silencers help this issue?
This speaking tube effect is often coupled with startling pops and booms echoing into previously peaceful spaces, which can become incredibly frustrating and disturbing for those residing within the area.
Stick around to find out everything you need to know about air duct noise silencers, as well as how to stifle these related noises.
Is it normal for ductwork to make noise?
Common sounds resonating from air ducts include booms, nags, squeals, buzzing, clicks, and pops, most of which occur once it’s been switched on and the conditioned air begins to rush through the duct.
When the thin sheet of metal within the ductwork accommodates these sudden changes in pressure caused by the airflow, it expands and makes these noises.
Bangs and pops can also become audible after the duct has been cleaned, since the airflow may cause the duct to expand in a way that it didn’t in the presence of dust and debris. That being said, air ducts can make noises if they are not clean as well.
Other reasons for popping include delayed ignition, undersized ductwork, or the duct joint between the trunk and plenum, and much more. This would need to be addressed and identified before finding an appropriate and effective solution.
The size, thickness, and shape of the ducts also make a difference. Round or circular ducts handle air pressure better than angular ducts which pop and bang more easily.
Unlike commercial ductwork, most residential ductwork is made with a thin metal, which is more vulnerable to sudden expansion and thus relative noises.
When hearing startling sounds coming from the air duct, the first reaction may be alarm or concern. Fortunately, most reasons for air ducts making noise are no reason to worry but should be addressed in case there is a malfunction.
These noises can disturb a peaceful living space, so it’s best to follow appropriate steps in order to alleviate these effects.
What is an air duct noise silencer?
Air duct noise silencers, also known as industrial sound attenuators, sound traps, or mufflers, are specifically designed to control airborne sound waves resonating within ducts, openings in buildings, enclosures, or from noisy equipment.
Duct silencers provide effective means of reducing noise emanating from the duct itself as well as sound traveling through the duct from elsewhere.
These are installed near sources that generate noise within the ventilation ductwork, which includes ventilation equipment like fans and air handling units, air flow regulators, air dampers, and fire dampers.
They can also be installed as part of the exposed ductwork thanks to aesthetic design and compatibility with other components of the ventilation system.
In most cases, air duct noise silencers are mounted between the fan and the exhaust fan as well as before the air diffusers and should be installed behind fans.
They can be used in main ductwork or additionally in formation branch-offs as required, and are designed to reduce noise emanating due to the flow of air in elbows, T-pieces, or air dampers.
Examples of Duct Silencers and Their Applications
The length, open area, and configuration of the duct silencer will be determined by the noise control requirements as well as the design of the duct system in question.
Spatial limitations, requirements, and pneumatic pressure drops are all considered when incorporating duct silencers into the existing system construction. General types of duct silencers include rectangular duct silencers, circular duct silencers, and flexible duct silencers.
In order to determine which type of duct silencer would be most compatible with the ventilation system in question, the allowable flow velocity subject to the flow rate and precise noise of the silencer will need to be considered.
Rectangular Duct Silencers
The sound silencer contained within the rectangular casing involves both housing and internal attenuating baffles. The housing is framed with sheet metal flange profiles, and it consists of galvanized steel.
The baffle is rounded on one side, entailing a sound-absorbing insert that absorbs acoustic energy, and galvanized steel is used for the frame.
The sound insert contained in the rectangular duct silencer is made with a combination of non-flammable mineral wool boards, and its exterior is covered with distinct abrasion-resistant fabric which in turn shields the wool interior.
These mineral wool boards are coated with a black glass fiber veil, its surface is suited to the airflow speed of 20 m/s, and the maximum temperature of the thermal energy carrier is 250°C.
In terms of design, there are three distinguishable types of attenuating baffles.
- L: absorption baffles
- R: absorption-resonator baffles
- P: absorption-resonator baffles, perforated
SLC, SRC, and SPC rectangular baffles are fixed in ventilation runs with the baffles being positioned vertically, and the bolts at steel corners are fastened using a G clamp and cleats in order to join the two flange frames together.
SLC attenuators: K-SLC attenuating baffles are used, including the steel frame and mineral wool liner. These are generally used for sound insulation at low to medium sound frequencies.
SRC attenuators: K-SRC perforated attenuating, resonating baffles are used, including the steel frame, mineral wool liner, and ½ of a baffle length covered with steel sheet. These are generally used for sound insulation at medium to high sound frequencies.
SPC attenuators: K-SPC perforated attenuating, resonator baffles are used, including the steel frame, mineral wool liner, and baffle covered over its full length with a perforated steel sheet at an open surface area ratio of 36%.
Circular Duct Silencers (Rigid)
Circular silencers are contained in a circular casing, are made of a spiral duct (sound attenuators SIL, SIBL, SIBOL, SIRL, and SIBROL) or a plain duct (sound attenuators SI-GL and SIL-GL), and are used in circular ductwork as a noise-dampening solution.
There is a layer of non-flammable, sound attenuating mineral wool between these two walls, which is coated with polyester fabric and steel sheeting which prevents the mineral wool layer from escaping into the ductwork.
These are fixed with self-drilling screws or rivets in the area of the duct where the spigot connection is inserted into the duct, and silencers with a diameter of 315mm or more are designed with female ends to connect with ducts, requiring separate male couplings.
For larger diameters, circular duct silencers can be equipped with a central baffle or pod. The SIBRL and SIBL circular duct silencers contain an internal vertical baffle, while SIBOL and SIBROL contain an internal pod.
- Silencing Elbows/ Curved Duct Silencer:
These are generally an excellent alternative to straight or rigidly shaped silencers and are used in ventilation that is spatially limited or has other conditions that make standard duct silencer types impossible to use.
This curved silencer comprises of two interconnected bends. The inner bend is comprised of perforated sheet steel, and the space between the outer and inner jacket is filled with a sound-absorbing material. The perforated jacket is covered in fabric, which prevents the sound-attenuating material from being drawn into the ductwork.
- Circular Connection in a Rectangular Casing
These are another fantastic alternative for working within a spatially confined ventilation system and suspended ceiling installation since its compact design has a very low installation height.
These are available with 25mm (SQLL-25), 50mm (SQLL-50), and 30mm (SQQL-30) mineral wool insulation. Rectangular silencer models with circular spigots are also available, where one spigot is positioned on the sidewall.
Many circular duct silencers include features such as cylindrical pressure-tight casing, streamlined acoustic core baffle, acoustic media which is sheltered by galvanized perforated sheet metal, spun head for reduced entrance pressure loss, and a tapered tail designed for optimal velocity pressure recovery.
Flexible Duct Silencers
These are quite easy to install and can be used in tricky spaces like ceiling voids and areas of the system which are not easy to access, due to its flexibility. On both sides of these flexible duct silencers, there are standard reducers, and they are available in varying lengths.
SLEAL Duct Silencer: It’s comprised of perforated ALUDUCT hose, and the insulation used is 25mm in thickness, wrapped with the ALUDUCT AD-L jacket which is 45 microns dense and strengthened with a steel wire.
SLEASD Duct Silencer: It’s comprised of perforated ALUDUCT AD-L hose, and contains insulation 25mm thick, which is wrapped in an aluminum-polyester jacket 45 microns thick.
SLEAF Duct Silencer: These are comprised of two AF-AL type hoses, containing insulation which is 50mm in thickness.
The typical duct silencer applications include supply and return ductwork, fan plenums, and air handling units, terminal boxes and mixing boxes, fume, and smoke exhaust, and an alternative to acoustically lined ducts.
- Acoustical Silencers reduce sound reverberation
- Gen-Set Silencers control sound produced by generators or engines
- Sound Traps reduce the volume of sound emanating from noisy machinery.
- HVAC Silencers are duct silencers for building ventilation systems
Both standard and custom duct silencers are available from many companies, and many industrial silencers include feature options such as a galvanized, carbon, or stainless steel casing, a duct extension, a fully welded construction, a weather hood, a bird screen, a painted exterior, access doors or panels, and filters.
Sometimes, ventilation systems require a higher level of resistance to specific chemical substances like acids, increased humidity, or increased temperature.
For ventilation systems that are situated within aggressive environments, duct silencers made of other materials such as stainless steel, aluminum, and plastics such as PVC, PP, and PPs.
Duct silencer comprised of polymers are similar to metal ones in terms of design and functionality, with the inner tube, which is used in circular silencers, and casing being created with the compatible materials. The non-flammable attenuating mineral wool is still used.
How do you soundproof HVAC ducts?
The fans that move the air within modern HVAC systems often produce various noises, which will resonate through the ductwork until these intolerable frequencies eventually find your unprepared ears.
There are various ways in which to either lessen or resolve the issue entirely, and duct silencers are certainly the most effective option since it’s specifically designed for this purpose.
An effective solution regarding the use of duct silencers would be to position one or more duct silencers between the fan and the occupied spaces, and the type of air duct silencers used for this purpose would be greatly dependent on the space available and the specifications of the HVAC duct in particular.
While duct silencers are quite compatible, these factors would still play a massive role in choosing the best option for your specific air duct system and needs.
- Silent-Mod Duct Silencers
These are available in an array of specifications, models, and lengths, and accommodates shape and pressure drop versus airflow and noise reduction.
Silent-Mod HVACs are designed for maximum strength and reduction of sound transmission through sidewalls and is created using 22 – 18 gauge galvanized steel, without any flimsy vent noise silencer materials which could vibrate in the airstream such as 26 or 24 gauge.
Popular types of Rectangular and Circular Duct Silencers for HVAC ducts include Elbow Dissipative, Straight Dissipative, Reactive (No-Fill), Elbow, and Straight, Cross-Talk, Axial Cone and Transitional.
There are various other methods of soundproofing an HVAC system, which are somewhat effective. But, these methods do not have such a massive impact on the behavior and control of sound waves and vibrations within various components of the ductwork specifically.
Such alternative methods of soundproofing an HVAC duct would include insulating the ducts with liners, covering unused vents, choosing flexible ductwork where possible, making repairs if needed, creating a Sound Maze within the air duct, sealing gaps, and using soffits.
There are products available that could assist in soundproofing the HVAC vents, including Quiet Wrap Pipe Soundproofing Wrap, RoadBlockR, VibraBlock, and Utterly Quiet Fabric-Covered Foam.
How can you make your ductwork quieter?
When it comes to standard commercial and residential buildings, the use of a compatible type of duct silencer which is made of quality galvanized steel is generally sufficient in achieving the desired outcome. Most issues can be eliminated with good design, installation, and unit selection.
While this would certainly be effective in resolving the problem on a holistic scale, there are a few other things that could make ductwork less noisy.
Since the majority of air duct noises are a result of air handling units and generated vibrations, there are a few other alternative solutions that are designed to deal with these specific problems – of course, if they have been accurately addressed and diagnosed.
- Flexible duct connectors (compensators)
These are specially designed for the purpose of stifling and insulating sound which is created by air duct equipment such as fans and AHUs.
They act as a sound dampener, reducing vibrations and sounds through the production of expansion joints and compensators that absorb sound.
Flexible duct connectors can be made of flange profiles in order to be compatible with the connections of the AHUs, which can come with female or male ends, and they are comprised of two pieces of metal sheet and fabric.
- Supports and Suspension brackets with anti-vibration lining
These accessories which have anti-vibration lining act as a dampening solution by aiming to suppress the noises which originate from within the structure of the ductwork.
- Acoustic insulation
As well as being responsible for acting as thermal insulation, and condensation growth protection, duct insulation also acts as an acoustic insulator.
The inclusion of this insulation within the ductwork results in the reduction of noise that originates from within the structure of the ductwork, preventing it from breaking out and being transmitted through the duct and into the building or room.
- Plenum boxes with insulation lining
These plenum boxes, although different from air duct silencers, have the ability to behave similarly at the ends of the ductwork runs, to a certain degree.
These are lined with sound insulation, which results in sound waves being spread evenly across the entire surface area of the box.
However, the installation of these plenum boxes would need to be quite accurate. The air supply and air extract cannot be positioned and installed directly opposite each other since arranging these components in such a way would prevent the efficient reduction of high-frequency sound waves.
While there are multiple methods of reducing air duct noise, the use of air duct silencers will not only reduce unpleasant fan noise significantly but will greatly reduce the amount of cross-talk that is transmitted and conducted through these vents.
The level of acoustical comfort experienced by those who have efficiently installed air duct noise silencers is thus increased through the addition of vibration-dampening assembly elements as well as flexible couplings within the ductwork.
This will alleviate the primary reasons for addressing the noise emanating from your ductwork, rewarding much-deserved peace and quiet as well as securing privacy.
