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.
- 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.
{| class="wikitable"
|+Sound Attenuator Classification at 1000 ft/min
!Shape
!Pressure Drop
!Attenuator Classification
|-
|Rectangular
|<0.10 in. w.g.
|"Low"
|-
|Rectangular
|0.10-0.30 in. w.g.
|"Medium"
|-
|Rectangular
|> 0.30 in. w.g.
|"High"
|-
|Cylindrical
|< 0.03 in w.g.
|"Low"
|-
|Cylindrical
|> 0.03 in w.g.
|"High"
|}
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
<math>IL\ (dB)=10\log( \frac{W_0}{W_m})</math>
where:
<math>W_0</math>= Radiated sound power from the duct with the attenuator
<math>W_m</math>= 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. airflow velocities are a critical component of attenuator sizing.
Regenerated noise should always be reviewed, but it is usually only a concern in very quiet rooms (e.g. concert halls, recording studios, music rehearsal rooms) or when the ductwork velocity is greater than 1500 ft/m.
<math>Lw=55log(V/V_0)+10log(N)+10log(H/H_0)-45</math>
where:
<math>Lw</math> = sound power level generated by the sound attenuator (dB)
<math>V</math> = velocity at the constricted cross-area (ft/min)
<math>V_0</math> = reference velocity (196.8 ft/min)
<math>N</math> = number of air passages (number of splitters)
<math>H</math> = height or circumference of the sound attenuator (in)
<math>H_0</math> = reference dimension (0.0394 in)
Pressure drop
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. These correction factors are used whenever there's a turbulent wake within 3 to 5 duct diameters upstream or downstream of the attenuator.
Where sound attenuator dimensions differ from surrounding duct dimensions, transitions to and from the sound attenuator should be smooth and gradual. Abrupt transitions cause the pressure drop and regenerated noise to significantly increase.
The pressure drop through a sound attenuator is typically higher than the pressure drop for an equivalent length of lined duct. 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.
Friction losses due to dissipative sound attenuators can be expressed as Industrial Acoustics Company, Industrial Sound Control, and Elof Hansson. This is a common feature in fan array design.
Crosstalk silencers
Purpose-built sound attenuators to prevent crosstalk between two closed, private spaces. Their design typically incorporates one or more bends to form a "Z" or "U" shape. This bend increases the efficacy of the sound attenuator without significantly increasing its overall length. Crosstalk attenuators are passive devices and should be sized for extremely low pressure drops — typically less than 0.05 inches w.g.
Exhaust registers
In the early 1970s, American SF Products, Inc. created the KGE Exhaust Register, which was an air distribution device with an integral sound attenuator.
Noise control implementation
First, the project noise control engineer (or acoustician), mechanical engineer, and equipment representative select the quietest possible equipment which meets the mechanical requirements and budget constraints of the project. Then, the noise control engineers will typically calculate out the path, without the attenuator first. The required sound attenuator insertion loss is the difference between the calculated path and the target background noise level. many higher education projects have adopted a limit on internal fiberglass liner. In these situations, the project acoustician must rely on duct silencers as the primary means of fan noise and duct-borne noise attenuation.
Sound attenuators are typically located near ducted mechanical equipment, to attenuate noise which propagates down the duct. This creates a trade-off: the sound attenuator should be located near the fan and yet the air is typically more turbulent closer to fans and dampers. If a sound attenuator is located over occupied space, the noise control engineer should confirm that duct breakout noise is not an issue prior to the attenuator.