1. Configuration

 

Circular sound trap (left of the grille)

Generally, sound traps consist of the follow 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 traps do not include sound-absorptive insulation. As a result, the high-frequency insertion loss of a packless sound trap is greatly reduced.
• 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 traps is typically less of a consideration, as circular ductwork is considerable stiffer than rectangular ductwork and less prone to duct breakout noise.^[1][2]

Sound traps are available in circular and rectangular form factors. Prefabricated rectangular sound traps typically come in 3, 5, 7, or 9-ft lengths. The width and height of the sound trap are often determined by the surrounding ductwork, though extended media options are available for improved attenuation. The baffles of rectangular sound traps are commonly referred to as splitters, whereas circular sound traps contain a bullet-shaped baffle.^[3]

2. Properties of Sound Traps

The acoustical properties of commercially available sound traps 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.^[4] These tests are conducted at NVLAP-accredited facilities and then reported by the manufacturer in marketing or engineering bulletins.

2.1. Dynamic Insertion Loss

The Dynamic Insertion loss of a sound trap is the amount of attenuation, in decibels, provided by the sound trap under flow conditions. The acoustic performance of a sound trap 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 sound trap is defined as^[5]

[math]\displaystyle{ IL\ (dB)=10\log( \frac{W_0}{W_m}) }[/math]

where:

[math]\displaystyle{ W_0 }[/math]= Radiated sound power from the duct with the attenuator

[math]\displaystyle{ W_m }[/math]= Radiated sound power from the duct without the attenuator

Some manufacturers report the static insertion loss of the sound trap, which is typically measured with a loudspeaker in lieu of a fan to represent a zero flow condition.^[3] These values can be useful in the design of smoke evacuation systems, where sound traps are used to attenuate exterior noise that breaks into the exhaust ductwork.

The insertion loss of a sound trap is sometimes referred to as transmission loss.

2.2. Regenerated Noise

The internal baffles of a sound trap constrict airflow, which in turn generates turbulent noise. Noise generated by a sound trap is directly related to the airflow velocity at the constriction, and changes proportionally with the face area of the sound trap.

The change in generated noise can be expressed as

[math]\displaystyle{ Generated\ Noise\ (dB)=10\log( \frac{A_1}{A_0}) }[/math]

where:

[math]\displaystyle{ A_1 }[/math]= The new face area of the sound trap

[math]\displaystyle{ A_0 }[/math]= Reference face area of the sound trap

For example, if the attenuator doubles in width, while maintaining a constant airflow velocity, the generated noise will increase by 3 dB. 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 [math]\displaystyle{ 50log }[/math],^[6] airflow velocities are a critical component of attenuator sizing.

2.3. Pressure Drop

Similar to other duct fittings, sound traps 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.^[7]

Where sound trap dimensions differ from surrounding duct dimensions, transitions to and from the sound trap should be smooth and gradual. Abrupt transitions cause the pressure drop and regenerated noise to significantly increase.^[8]

The pressure drop through a sound trap 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 trap.^[9]

Friction losses due to dissipative sound traps can be expressed as^[5]

[math]\displaystyle{ Friction\ Loss=\frac{P}{A}l(K_f\frac{1}{2}\rho v_p^2), \ N/m^2 }[/math]

where:

[math]\displaystyle{ \frac{P}{A} }[/math] = ratio of the sound trap perimeter and area

[math]\displaystyle{ l }[/math] = length of the duct

[math]\displaystyle{ K_f }[/math] = The friction loss coefficient

[math]\displaystyle{ \rho }[/math] = density of air

[math]\displaystyle{ v_p^2 }[/math] = passage velocity

The perimeter, area, and length of the sound trap are also parameters which affect its insertion loss. Friction loss at the sound trap is directly proportional to its noise attenuation performance, whereby greater attenuation usually equates to greater pressure drop.

3. Design Variations

Prefabricated sound traps rose to prominence in the late 1950s-early 1960s.^[10] Several manufacturers were among the first to produce and test prefabricated sound traps: Koppers,^[11][12] Industrial Acoustics Company,^[13] Industrial Sound Control^[14], and Elof Hansson.^[11]

Though rectangular dissipative attenuators are the most common variant of sound traps used today in architectural acoustics noise control, other design options exist.

3.1. Reactive Silencers^[5]

3.2. Dissipative Silencers

Dissipative silencers are used when broadband attenuation with low pressure drop is desired.^[5] In typical ductwork, high frequencies propagate down the duct as a beam, and minimally interact with the outer, lined edges. Sound traps with baffles that break the line of sight or elbow attenuators with a bend provide better high frequency attenuation than conventional lined ductwork.^[11]

These type of attenuators are commonly used on air handling units, ducted fans coil units, cooling towers, and ventilated equipment enclosures.

3.3. Crosstalk Silencers

Purpose-built sound traps to prevent crosstalk between two closed, private spaces. Their design typically incorporates one or more bends to form a "Z" or "U" shape. This increases the efficacy of the sound trap without significantly increasing it overall length. Since these are passive devices, cross talk silencers are sized for extremely low pressure drops —less than 0.05 inches w.g.

3.4. 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 trap.^[15]

4. Noise Control Implementation

Duct silencers are featured prominently in systems where fiberglass internal duct liner is prohibited. While fiberglass's contribution to air quality is insignificant,^[16] 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 traps are typically located near ducted mechanical equipment, to attenuate noise which propagates down the duct. Ideally, sound traps should straddle the wall of the mechanical equipment room provided there are no fire dampers.^[17]

Sound traps can be used outdoors to quiet cooling towers, emergency generator, and exhaust fans. Larger equipment will require an array of sound traps, otherwise known as an attenuator bank.