Sound Attenuators

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.[10] 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[11]

${\displaystyle IL\ (dB)=10\log({\frac {W_{0}}{W_{m}}})}$

where:

${\displaystyle W_{0}}$= Radiated sound power from the duct with the attenuator

${\displaystyle W_{m}}$= 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.[7] 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.

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

${\displaystyle Generated\ Noise\ (dB)=10\log({\frac {A_{1}}{A_{0}}})}$

where:

${\displaystyle A_{1}}$= The new face area of the sound attenuator

${\displaystyle A_{0}}$= Reference face area of the sound attenuator

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 ${\displaystyle 50log}$,[12] 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.[4]

There is a prediction formula that can be used to estimate duct silencer regenerated noise if no data exists[13][14]

${\displaystyle Lw=55log(V/V_{0})+10log(N)+10log(H/H_{0})-45}$

where:

${\displaystyle Lw}$ = sound power level generated by the sound attenuator (dB)

${\displaystyle V}$ = velocity at the constricted cross-area (ft/min)

${\displaystyle V_{0}}$ = reference velocity (196.8 ft/min)

${\displaystyle N}$ = number of air passages (number of splitters)

${\displaystyle H}$ = height or circumference of the sound attenuator (in)

${\displaystyle H_{0}}$ = 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.[16]

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.[17]

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.[18]

Friction losses due to dissipative sound attenuators can be expressed as[11]

${\displaystyle Friction\ Loss={\frac {P}{A}}l(K_{f}{\frac {1}{2}}\rho v_{p}^{2}),\ N/m^{2}}$

where:

${\displaystyle {\frac {P}{A}}}$ = ratio of the sound attenuator perimeter and area

${\displaystyle l}$ = length of the duct

${\displaystyle K_{f}}$ = The friction loss coefficient

${\displaystyle \rho }$ = density of air

${\displaystyle v_{p}^{2}}$ = passage velocity

The perimeter, area, and length of the sound attenuator are also parameters which affect its pressure drop.[16] Friction loss at the sound attenuator is directly proportional to its noise attenuation performance, whereby greater attenuation usually equates to greater pressure drop.

Reactive silencers are very common in muffler design of automobiles and trucks.[10] Attenuation is primarily achieved through sound reflection, area change, and tuned chambers.[11] The design of reactive silencers from scratch is mathematically intensive, so manufacturers often have a number of prefabricated designs.

Dissipative silencers attenuate sound by transferring sound energy to heat.[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. Sound attenuators with baffles that break the line of sight or elbow attenuators with a bend provide better high frequency attenuation than conventional lined ductwork.[19] Generally, longer attenuators with thicker baffles will have a greater insertion loss over a wider frequency range.[4]

These types of attenuators are commonly used on air handling units, ducted fan coil units, and at the air intake of compressors, gas turbines, and other ventilated equipment enclosures.[4][10] On certain air handling unit or fan applications, it is common to use a co-planar silencer—a dissipative silencer that is sized for the fan and mounted directly to the fan outlet.[23] This is a common feature in fan array design.

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.

In the early 1970s, American SF Products, Inc. created the KGE Exhaust Register, which was an air distribution device with an integral sound attenuator.[24]