A sound attenuator, or duct silencer, sound trap, or muffler, is a noise control acoustical treatment of Heating Ventilating and Air-Conditioning (HVAC) ductwork designed to reduce transmission of noise through the ductwork, either from equipment into occupied spaces in a building, or between occupied spaces. In its simplest form, a sound attenuator consists of a baffle within the ductwork. These baffles often contain sound-absorbing materials. The physical dimensions and baffle configuration of sound attenuators are selected to attenuate a specific range of frequencies. Unlike conventional internally-lined ductwork, which is only effective at attenuating mid- and high-frequency noise, sound attenuators can achieve broader band attenuation in relatively short lengths. Certain types of sound attenuators are essentially a Helmholtz resonator used as a passive noise-control device.
Configuration
Generally, sound attenuators consist of the following elements:
An inner perforated layer of light gauge sheet metal (baffle) The baffle is then filled with sound-absorptive insulation In high velocity systems, or when there is a concern for particulate matter in the air stream, a bagged or mylar-faced insulation is used. Packless sound attenuators do not include sound-absorptive insulation. As a result, the high-frequency insertion loss of a packless sound trap is greatly reduced. Bagged insulation or packless sound attenuators are typically referred to as "hospital grade" attenuators. An outer non-perforated layer of sheet metal. The outer layer is typically heavy gauge sheet metal (18ga or stiffer) to minimize duct break-out and break-in noise. The gauge of circular sound attenuators is typically less of a consideration, as circular ductwork is considerably stiffer than rectangular ductwork and less prone to duct breakout noise. Sound attenuators are available in circular and rectangular form factors. Prefabricated rectangular sound attenuators typically come in 3, 5, 7, or 9-ft lengths. The width and height of the sound attenuators are often determined by the surrounding ductwork, though extended media options are available for improved attenuation. The baffles of rectangular sound attenuators are commonly referred to as splitters, whereas circular sound attenuators contain a bullet-shaped baffle. Sound attenuators are typically classified as "Low," "Medium," or "High" based on performance characteristics and/or duct velocity. An example classification scheme is listed below.
Properties The acoustical properties of commercially available sound attenuators are tested in accordance with ASTM E477: Standard Test Method for Laboratory Measurements of Acoustical and Airflow Performance of Duct Liner Materials and Prefabricated Silencers. These tests are conducted at NVLAP-accredited facilities and then reported by the manufacturer in marketing or engineering bulletins. Outside of the US, sound attenuators are tested in accordance with British Standard 4718 (legacy) or ISO 7235.
Dynamic insertion loss The dynamic insertion loss of a sound attenuator is the amount of attenuation, in decibels, provided by the silencer under flow conditions. While flow conditions in typical low velocity duct systems rarely exceed 2000–3000 ft/min, sound attenuators for steam vents must withstand airflow velocities in the 15,000-20,000 ft/min. range. The acoustic performance of a sound attenuator is tested over a range of airflow velocities, and for forward and reverse flow conditions. Forward flow is when the air and sound waves propagate in the same direction. The insertion loss of a silencer is defined as
I L ( d B ) = 10 log ( W 0 W m ) {\displaystyle IL\ (dB)=10\log({\frac {W_{0}}{W_{m}}})}
where:
W 0 {\displaystyle W_{0}} = Radiated sound power from the duct with the attenuator
W m {\displaystyle W_{m}} = Radiated sound power from the duct without the attenuator Some manufacturers report the static insertion loss of the silencer, which is typically measured with a loudspeaker in lieu of a fan to represent a zero flow condition. These values can be useful in the design of smoke evacuation systems, where sound attenuators are used to attenuate exterior noise that breaks into the exhaust ductwork. The insertion loss of a sound attenuator is sometimes referred to as transmission loss.
Regenerated noise The internal baffles of a sound attenuator constrict airflow, which in turn generates turbulent noise. Noise generated by a sound attenuator is directly related to the airflow velocity at the constriction, and changes proportionally with the face area of the sound attenuator. The change in generated noise can be expressed as
G e n e r a t e d N o i s e ( d B ) = 10 log ( A 1 A 0 ) {\displaystyle Generated\ Noise\ (dB)=10\log({\frac {A_{1}}{A_{0}}})}
where:
A 1 {\displaystyle A_{1}} = The new face area of the sound attenuator
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