Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

This is an AMP page - Open full page! for all features.

Ventilation Systems - Classification of Noise Reduction Silencers

Sponsored Links

The purpose of a duct silencer is to reduce the noise from the air-handling system caused by the

  • fan
  • passage of air through straight ducts
  • impact of air flowing through components such as elbows, branches, mixing boxes etc.

DIL - Dynamic Insertion Loss

The Dynamic Insertion Loss - DIL - is the difference between the sound power or intensity levels measured in the same point of the duct work before and after the insertion of the silencer. The insertion loss depends on the flow - if its forward or reverse. The flow is forward if air flows in the same direction as the propagation of sound.

SN - Self Noise

The Self Noise - SN - is the noise power level in decibels generated by the silencer when inserted in the air flow. The Self Noise depends on the direction of the flow - if its forward or reverse.

Absorptive or Dissipative Silencers

Absorptive or dissipative silencers use sound absorbing materials to attenuate sound waves.

Dissipative silencers are widely used in HVAC duct systems. Typical dissipative silencers are configured in a parallel baffle arrangement.

The thickness of acoustical linings or baffles should be selected with reference to the predominant frequency of the noise. The incident sound energy is partially transformed to heat by causing motion in the fibers during its passage through the material. Absorptive silencers include lined duct attenuators, packaged cylindrical and rectangular attenuators, acoustic louvers and lined plenum chambers.


Typical DIL - Dynamic Insertion Losses (decibel) - with absorptive silencers are indicated in the table and diagram below

Ventilation Systems - Classification of Noise Reduction Silencers
DIL - Dynamic Insertion Loss - (dB)


Frequency (Hz)
4 24 8 14 26 34 41 45 25
5 24 6 12 22 28 37 38 22
6 24 5 10 18 23 33 30 19
8 24 4 9 17 22 29 25 18
10 36 6 11 21 27 39 25 19
12 36 5 9 18 23 32 20 18
16 36 5 8 11 23 19 17 15
  • (1 in) = (25.4 mm)


Reflective or Reactive silencers

The primary function of a reactive silencer is to reflect sound waves back to the source. Energy is dissipated in the extended flow path resulting from internal reflections and by absorption at the source. The operation principle of the reactive silencers is a combination of lambda/4- and Helmholtz-resonators acting as acoustic filters. Reactive silencers have tuned cavities or membranes and are designed to attenuate low frequency noise from machines.

The reactive silencer may have excellent low frequency performance, is non-fibrous and cleanable and has small or negligible pressure loss. The simplest kind of a reactive muffler is the expansion chamber. In general reactive silencers are used for fixed speed machinery producing pure tones. The reactive silencer is suitable for engines requiring very low exhaust system back pressures for a maximum engine performance.

Reactive silencers are rarely used in HVAC systems.

Diffuser or Depressive Silencers

Diffuser type silencers have perforated pepper pots to slow down flow velocity and prevent the generation of low frequency noise and are mainly used for applications involving nozzles, control valves, jet engines etc.

The total pressure drop is divided in several stages across the nozzle, the valve and the diffuser. This allows a better pressure ratio between upstream and downstream and reduces the noise level.

Active Silencers

Active noise control is sound field modification, particularly sound field cancellation, by electro-acoustical means. Active silencers use microphones and electronics to determine and attenuate noise.

In its simplest form, a control system drives a speaker to produce a sound field that is an exact mirror-image the offending sound (the "disturbance"). The speaker thus "cancels" the disturbance, and the net result is no sound at all. Such silencers can be effective at low frequencies under 300 Hz.

Active noise control is best suited for applications with relatively steady noise fields - like fans, engines or similar. Active silencers are not suitable for broadband noise reduction.

Sponsored Links

Related Topics


Room acoustics and acoustic properties. decibel A, B and C calculations. Noise Rating (NR) curves. Sound transmission through walls. Calculate sound pressure, sound intensity and sound attenuation.

Noise and Attenuation

Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.

Related Documents

Sound Attenuation in Ducts due to End Reflection

Low frequency noise transferred from main duct to end terminals is reflected back to main duct.

Sound Transmission through Duct Walls

Sound transmission from ducts to surrounding rooms.

Sound Transmission through Flanking Paths

Sound transmitted through duct walls, floors and ceilings.

Unlined Sheet-Metal Ducts - Sound Attenuation

Noise attenuation in unlined sheet-metal ducts.

Ventilation Systems - Acoustic Calculation Procedure

Acoustic noise calculation procedure HVAC systems.

Sponsored Links

Search Engineering ToolBox

  • the most efficient way to navigate the Engineering ToolBox!

SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!


We don't collect information from our users. Only emails and answers are saved in our archive. Cookies are only used in the browser to improve user experience.

Some of our calculators and applications let you save application data to your local computer. These applications will - due to browser restrictions - send data between your browser and our server. We don't save this data.

Google use cookies for serving our ads and handling visitor statistics. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected.

AddThis use cookies for handling links to social media. Please read AddThis Privacy for more information.