Duct Attenuators
 IAC rectangular and cylindrical duct attenuators provide effective, predictable noise reduction at substantial savings over other noise reduction methods because IAC products are laboratory-developed and tested under controlled conditions. Moreover, we manufacture duct silencers to specific metal tolerances and with controlled acoustic infill flow resistances to achieve consistent results.
 IAC was founded in 1949, and our first air conditioning silencers were developed in 1950. Since then, IAC has pioneered the development of performance rated silencers to ensure quiet Air Handling Systems.
To maintain this position of leadership, IAC operates fully equipped state-of-the-art Aero-Acoustic Laboratories. We use these facilities to design and develop new silencers and other noise control products, as well as quality control.
Should none of our standard range of silencers meet your requirements we will develop one for you. If you have a special requirement, please contact our Building Services Team.
IAC Duct Attenuator Range of Products
IAC designs and manufactures duct attenuators in response to specific requirements from acoustical consultants, consulting engineers, owners and contractors. Our duct silencers provide the most economical choice for solving the wide diversity of noise control problems encountered in HVAC engineering.
IAC standard rectangular duct attenuator cross sections range from 150mm x 150mm (6in. x 6in.) to 1200mm x 1200mm (48in. x 48in.). For small mains, branches and duct run outs there are module sizes to fit every need. When large silencer banks are required, multiple-module assemblies can be arranged to provide almost limitless dimensional flexibility.
 Quiet-Vent® Silencers solve a common problem in air handling – the passage of noise from one space to another through vents provided for air passage. These modern low-cost units are easy to install, permit circulation of air in either direction, yet effectively stop noise in both directions.
- Quiet-Duct® Rectangular Silencers are suitable for conventional applications including low-frequency silencers having acoustic performance specifically engineered for the 63 Hz, 125 Hz and 250 Hz octave bands.
- Clean-Flow™ Rectangular Silencers are ideal for systems requiring a higher degree of cleanliness and hygiene such as hospitals or clean room applications. Linings on the fill material guard against erosion of particulate matter into the airstream. Specific internal construction features protect the lining against chaffing or premature failure and are necessary to maintain the rated aero-acoustic performance.
- Conic-Flow® Tubular Silencers are available for silencer applications including low-frequency silencers having acoustic performance specifically engineered for the 63 Hz, 125 Hz and 250 Hz octave bands.
- D-Duct™ Acoustic Diffuser Silencers are appropriate for use on axial fan systems. The combined interior diffuser cone and exterior square jacket casing make these units aerodynamic regain devices as well as silencers.
- Rectangular Packless Silencers are the ultimate solution for ultra-clean environments and corrosive / flammable environments. The complete absence of fill, combined with ease of cleaning and draining makes Packless silencers well suited for engine test cells, chemical plants, refineries and facilities that handle or process hazardous materials.
- Tubular Packless Silencers are excellent for small diameter circular duct systems such as fume hoods. The Packless design of these units makes them equally suitable to the types of systems mentioned for the Rectangular Packless silencers.
 IAC’s Green Duct Attenuator range is the first British made ‘green’ silencer range for air handling applications. With acoustic infill material made from specially developed post-industrial organic fibre, IAC's range of green Duct Attenuators offers a solution to the ever-increasing requirements for eco-friendly HVAC systems.
Duct Attenuator Design
All of the duct attenuators manufactured by IAC are of the passive design because they do not require mechanical or electrical means to function. They do their job very simply by providing a trouble free static means for the dissipation of sound energy by converting it into very minute quantities of heat. Many of the original air conditioning attenuators developed by IAC in 1950 are still in use today.
Active duct attenuators operate electronically by means of microprocessors, loudspeakers and microphones. They cancel sound by feeding back an additional noise source that is 180 degrees out of phase with the original noise. In theory, noise reduction is effective for low frequencies, usually below 300, Hz. Initial research to develop a commercial product was particularly strong in Great Britain but, today, research goes on throughout the world.
HVAC noise control requirements are rarely confined to a narrow low frequency range. The broad band low and high frequency attenuation capabilities of typical passive attenuators are practically always required.
Cross section through typical IAC Quiet-Duct Silencer
1. Die-formed single piece splitter constructed throughout.
2. Acoustic baffles designed for maximum attenuation at low frequencies, the toughest job of all.
3. Straight-through air passages designed for maximum air handling at minimum pressure drop.
4. Solid, rounded noses increase noise reduction.
5. Bell-mouth entrance and exit minimize turbulence.
6. No protruding fastener heads to cause turbulence or self-noise.
7. Solid air-impingement surfaces and self-cleaning air passages minimise dirt entrapment.
8. Acoustic fill protected against erosion by perforated-metal containments.
Forward and Reverse Flow
In 1972, IAC developed silencer Dynamic Insertion Loss and Self-Noise ratings both under FORWARD FLOW (+) and REVERSE FLOW (-) conditions for Rectangular and Cylindrical Duct Attenuators.
Forward Flow (+) occurs when air and sound waves travel in the same direction, as in an air conditioning supply system or a fan discharge.
Reverse Flow (-) occurs when sound waves and air travel in opposing directions, as in a typical return air system.
Since attenuation values are generally higher in the first five octave bands in the Reverse Flow (-) mode compared to the Forward Flow (+) mode, more economical silencer selections can often be made on return air systems. These phenomena are illustrated below.
Under Forward Flow conditions
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Under Reverse Flow conditions
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Duct Attenuator Proven Certification
IAC’s Aero-Acoustic Research Centre permits forward and reverse flow dynamic insertion loss, self-noise, and pressure drop rating of duct attenuators and other elements in closed loop wind tunnel and other facilities. Dual reverberation rooms permit testing of system components or assembled air handling units. The National Institute of Standards and Technology, National Voluntary Lab Accreditation Program (NVLAP) accredit the IAC Aero-Acoustic Laboratory for acoustical testing services.
IAC duct silencers are rated under operating conditions in the most advanced aero-acoustic research and development laboratory available. Regardless of their size or configuration, they are developed, tested and rated in accordance with the most current industry standards. There is no guesswork with hit-or-miss empirically-developed calculations or otherwise inaccurately-rated silencers.
Manufacturing
IAC operates modern equipped manufacturing plants in the United Kingdom, New York, Hong Kong, and South Carolina. With numerically controlled automated machinery, trained personnel with more combined experience in the noise control industry than any other organization engaged in a related activity operate these facilities.
Download our Duct Silencers catalogue to learn more about the many sizes and standard types of silencers that are available from IAC or contact our Building Services Team.
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