Silencer Types And Applications (HVAC)

Selection of quieter equipment can eliminate many noise problems before they even begin. Treatment options along the path are the next best option and can include silencers, barriers, absorption, lagging, or other options. The last resort is typically treatment at the receiver, with hearing protection for loud occupational exposure.

Octave band frequency (Hz)

Approaches to Noise Control

Sound Sources

Wherever possible, it is important to obtain source sound power levels from the supplier. The sound power levels should be derived from tested performance according to a recognized standard applicable to that equipment. Fans are commonly tested per AMCA 300 or 320, Air Handling units according to AHRI 260, terminal units per ASHRAE 130. Testing according to a recognized standard ensures that the results can be compared and evaluated between manufacturers. Estimates of power levels should only be used as a last resort where manufacturer supplied data is not available. Common sound sources in an HVAC system are fans, element generated flow noise, VAV components, and mechanical equipment.


Once the sources are known, the position in relation to the receiver can be determined. This will allow the path by which the noise is transmitted to be identified. It is important to note that noise typically travels through multiple paths, both airborne and structural, so possible paths must be acknowledged and evaluated appropriately. Additionally, once one path is treated, another path may become dominant and any further treatment to the first path will not be effective.

Source, Path, Receiver

The Sound Paths diagram (below) illustrates possible transmission paths for the sound and vibration from the source to the receiver. In this example, the sound source is an air handler that contains both a fan and a compressor; the receiver is the human occupant in the adjacent room. Silencers are an effective and economical means of incorporating noise control into a system to reduce noise from duct-borne paths. The main types of silencers, including absorptive or dissipative, film lined, and reactive or packless, are discussed in the following sections.

Sound paths

Path A – Structure-borne path, where the vibration of the air handler passes through the floor.

Path B – Airborne path, where noise from the equipment radiates directly to the receiver.

Path C – Duct-borne path, where noise from the equipment radiates through the walls of the ductwork or passes through the supply/return ductwork into the occupied space.


After the source and path have been identified, it is a matter of assessing the receiver and determining what sound levels are considered acceptable so that the most effective and economical solution to the noise problem can be selected. Calculation of the sound pressure level at the receiver is the final component to the sourcepath-receiver concept, and combines contributions from all path types. The main consideration for specifying a target or design sound criteria is the intended use of the space. The most commonly referenced source for typical design criteria for an indoor occupied space is the ASHRAE Applications Handbook. Outdoor sound criteria are typically specified by local ordinance in terms of overall A-weighted value specified at a property line

Silencer Types and Applications


Acoustic silencers, which are also known as dissipative silencers, utilize sound-absorbing media to attenuate sound levels. As the noise inside the duct passes through the silencer, the acoustic energy enters the baffles through the holes in the internal perforated metal liner. This perforated metal liner protects the acoustic media from being eroded by the air at high velocities, but has a large enough free area to be acoustically transparent. Once inside the baffle, the acoustic energy interacts with the absorptive media, which typically consists of strands of glass fiber.


The friction between the acoustic energy and the glass fibers converts the acoustic energy into heat, thereby reducing the amount of acoustic energy and decreasing sound levels at the discharge of the silencer. For silencers that will experience high gap velocities when installed, a sheet of fiberglass cloth can be placed between the perforated metal and the glass fiber media for protection against erosion with no effect on the acoustic performance of the silencer. Typical absorptive applications include rectangular, elbow, and circular ductwork, air handling units, generator silencers, ventilation, fan plenums, and general HVAC supply, return and exhaust.

Film Lined Absorptive

Film lined absorptive silencers work on the same principle as the standard absorptive silencers. However, these silencers have a thin polymer film liner wrapped around the acoustic media to protect the media from airborne contaminants and moisture that may be present in the air stream of some systems. It is important to note that the addition of the film liner will negatively affect the sound absorbing capabilities of the acoustic media.

Film Lined Absorptive

To minimize these effects, a thin acoustic standoff liner is placed between the film and the perforated metal. Typical film lined silencer applications include laboratories, cleanrooms, and hospitals where contaminants and moisture in the air stream may be of concern.


Packless silencers, sometimes referred to as reactive silencers, contain no absorptive material and are constructed solely of solid and perforated sheet metal. Attenuation of sound is achieved by using multiple resonant chambers of varying size located behind the perforated metal liner. As sound passes through the silencer, the acoustic energy dissipates in a manner similar to the Helmholtz resonator concept, which results in decreased sound levels at the discharge of the silencer. Since these silencers are tuned to a narrow band of frequencies, it is more difficult to achieve significant attenuation over broadband frequencies.


Packless silencers are typically specified when glass fiber media is not acceptable or when it is necessary to sterilize the entire duct system. This may include laboratories, cleanrooms, hospitals, or electronics manufacturing.

Effect of Acoustic Media

The graph below is a comparison of the different silencer types. The graph shows that the standard absorptive silencer has the best overall performance across the full range of frequencies. The addition of a film liner such as Mylar significantly decreases the effectiveness of the absorptive silencer, but can be greatly improved at the higher frequencies by placing an acoustic standoff between the perforated metal and the film liner.

Comparison of 5 ft Long Silencer Performance

The graph also shows the unique performance characteristics of the packless silencer, which utilizes tuned resonating chambers instead of acoustic media. The standard absorptive silencer provides a fairly predictable broad insertion loss curve across the frequency ranges, while the packless silencer peaks in the mid frequency ranges. This peak is due to the size and shape of the internal chambers that are tuned to remove that frequency.


Rectangular silencers are the standard for silencing noise transmitted through ductwork. Their straightforward design and relative low cost make them the first choice for the highest sound attenuation and lowest pressure drop on the air distribution system. They are available with a number of options that allow them to be integrated into any HVAC system with ease.

Rectangular silencer

A wide range of designs with low to high pressure drop and a large selection of media types can be applied to systems ranging from 0-2500 fpm.


Elbow silencers have many of the same performance characteristics of rectangular silencers, but are extremely versatile and an excellent choice for systems where straight lengths of ductwork are not available. Elbow silencers perform at or above the levels of rectangular silencers with a small increase in pressure drop on the system, and can be configured to suit most duct sizes without the use of transitions.

Elbow silencer


Circular silencers are an excellent solution when round ductwork is utilized in the system. They eliminate the need for squareto-round transitions that cause undesirable pressure drops and system effects. Circular silencers are available in a wide range of sizes which allow for varying ranges of attenuation and pressure drop.

Circular silencer

Axial Fan

Axial fan silencers are designed to be close coupled to an axial fan. These silencers are engineered to provide noise attenuation at the source and improve aerodynamic performance at the inlet and discharge of the fan. The axial fan silencer’s center pod is properly sized to help reduce the pressure loss over the fan hub.

Axial Fan

A discharge axial fan silencer decelerates the air to maximize the static pressure regain.


A standard silencer configuration cannot always be adapted to work in every situation. In these cases it is necessary to develop a unique silencer that meets the requirements of the application. When noise control products must be applied to a system with limited space or when the silencer is directly coupled to a fan, a custom silencer can be designed to provide the required performance.

Custom silencer

Typical custom designs include transitional silencers, T or Z shaped silencers, and silencers that require a size that is not available in the standard model line.

Air Transfer Products

Air transfer silencers allow air to move from one area to another without compromising the acoustic integrity of the wall. These silencers are typically not ducted and are part of the wall construction.


Cross-talk silencers are used to transfer air to adjacent areas while providing the necessary attenuation to prevent transferring unwanted noise. They are commonly used to prevent speech intrusion into adjacent rooms.

Cross-Talk silencer

Cross-talk silencers are manufactured in a variety of configurations that can be used to solve many different acoustic air transfer problems.

Thin Line Return Dissipater

Thin line return dissipaters utilize multiple baffles filled with acoustic media that are staggered in a frame that is only 4 in. thick. They are great for applications in place of standard transfer grilles to reduce sound transmission between adjacent spaces or to attenuate return air noise.

Thin Line Return Dissipater

Acoustic Louvers

Acoustic louvers can be used for allowing air to flow through an opening while providing the necessary sound attenuation. Acoustic louvers use acoustic media to absorb the sound energy and a perforated metal to protect the media from erosion by the air flow.

Acoustic Louvers

Acoustic louvers are designed to be aerodynamic to help minimize the pressure drop.

Acoustic Panels and Enclosures

Used for controlling excessive noise from equipment, Price acoustic panels and enclosures are available in many shapes and sizes to meet specific noise reduction requirements.

Acoustic Panels and Enclosures

Typical applications include barrier systems, custom enclosures, and inlet and exhaust plenums; with applications ranging from large units for gas turbines to smaller units for pumps, motors, compressors or any other unwanted HVAC or industrial noise source.

Engineering Guide Noise Control - Price Engineer’s HVAC Handbook


What are the primary approaches to noise control in HVAC systems?
The primary approaches to noise control in HVAC systems are: selection of quieter equipment, treatment along the path (including silencers, barriers, absorption, lagging, or other options), and treatment at the receiver (with hearing protection for loud occupational exposure). These approaches should be considered in sequence, with selection of quieter equipment being the most preferred option and treatment at the receiver being the last resort.
How do I obtain accurate sound power levels for HVAC equipment?

It is essential to obtain source sound power levels from the equipment supplier, derived from tested performance according to a recognized standard. This ensures that the sound power levels are accurate and reliable, allowing for effective noise control measures to be implemented.

What are the different types of silencers used in HVAC systems?

There are several types of silencers used in HVAC systems, including reactive silencers, dissipative silencers, and hybrid silencers. Reactive silencers work by reflecting sound waves back towards the source, while dissipative silencers absorb sound energy. Hybrid silencers combine both reactive and dissipative principles to achieve optimal noise reduction. The choice of silencer type depends on the specific application, noise frequency, and desired level of noise reduction.

How do I select the appropriate silencer for my HVAC system?

To select the appropriate silencer for your HVAC system, consider factors such as the noise frequency, sound power level, airflow rate, and pressure drop. It is also essential to consult with the equipment supplier and consider the specific application, including the type of fan, compressor, or pump being used. Additionally, consider the space constraints, maintenance requirements, and cost of the silencer.

What are the benefits of using silencers in HVAC systems?

The benefits of using silencers in HVAC systems include reduced noise levels, improved occupant comfort, and increased productivity. Silencers can also help reduce noise-induced stress and fatigue, and improve overall indoor air quality. Furthermore, silencers can help reduce the risk of noise-related health problems, such as hearing loss and cardiovascular disease.

How do I ensure the effective installation and maintenance of silencers in my HVAC system?

To ensure the effective installation and maintenance of silencers in your HVAC system, follow the manufacturer’s instructions and recommendations. Ensure that the silencer is properly sized and installed in the correct location, and that all connections are secure and airtight. Regularly inspect and clean the silencer to prevent dust and debris buildup, and replace worn or damaged components as needed.