Sound attenuator
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.
[2] Certain types of sound attenuators are essentially a Helmholtz resonator used as a passive noise-control device.
[7] Sound attenuators are typically classified as "Low," "Medium," or "High" based on performance characteristics and/or duct velocity.
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.
[9] 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.
[10] The acoustic performance of a sound attenuator is tested over a range of airflow velocities, and for forward and reverse flow conditions.
= 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.
The internal baffles of a sound attenuator constrict airflow, which in turn generates turbulent noise.
Conversely, if the attenuator shrinks by a factor of 10, while keeping the airflow velocity constant, the generated noise will decrease by 10 dB.
Since turbulence generated noise caused by duct fittings changes at a rate of
[4] There is a prediction formula that can be used to estimate duct silencer regenerated noise if no data exists[13][14]
= reference dimension (0.0394 in) Similar to other duct fittings, sound attenuators cause pressure drop.
Catalog pressure drop values obtained through ASTM E477 assume ideal, laminar airflow, which is not allow always found in field installations.
The ASHRAE Handbook provides pressure drop correction factors for different inlet and outlet conditions.
[15] These correction factors are used whenever there's a turbulent wake within 3 to 5 duct diameters upstream or downstream of the attenuator.
Abrupt transitions cause the pressure drop and regenerated noise to significantly increase.
However, significantly longer lengths of lined duct are required to achieve equal attenuation, at which point the pressure drop of large extents of lined duct is significantly greater than incurred through a single sound attenuator.
= passage velocity The perimeter, area, and length of the sound attenuator are also parameters which affect its pressure drop.
[10] Attenuation is primarily achieved through sound reflection, area change, and tuned chambers.
[10] Dissipative silencers are used when broadband attenuation with low pressure drop is desired.
[11] In typical ductwork, high frequencies propagate down the duct as a beam, and minimally interact with the outer, lined edges.
[19] Generally, longer attenuators with thicker baffles will have a greater insertion loss over a wider frequency range.
Purpose-built sound attenuators to prevent crosstalk between two closed, private spaces.
Crosstalk attenuators are passive devices and should be sized for extremely low pressure drops — typically less than 0.05 inches w.g.
In the early 1970s, American SF Products, Inc. created the KGE Exhaust Register, which was an air distribution device with an integral sound attenuator.
The required sound attenuator insertion loss is the difference between the calculated path and the target background noise level.
[16] Ideally, sound attenuators should straddle the wall of the mechanical equipment room provided there are no fire dampers.
[23] If there is significant distance between the attenuator and the mechanical room penetration, additional duct cladding (such as external fiberglass blanket or gypsum lagging) may be required to prevent noise from breaking into the duct and bypassing the attenuator.
[23] Sound attenuators can also be used outdoors to quiet cooling towers, air intake of emergency generators, and exhaust fans.
