duct silencer regenerated noise if no data exists where: = sound power level generated by the sound attenuator (dB) = velocity at the constricted cross-area (ft/min) = reference velocity (196.8 ft/min) = number of air passages (number of splitters) = height or circumference of the sound attenuator (in) = 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 where: = ratio of the sound attenuator perimeter and area = length of the duct = The friction loss coefficient = density of air = passage velocity The perimeter, area, and length of the sound attenuator are also parameters which affect its pressure drop. Friction loss at the sound attenuator is directly proportional to its noise attenuation performance, whereby greater attenuation usually equates to greater pressure drop. Design variations Prefabricated sound attenuators rose to prominence in the late 1950s-early 1960s. Several manufacturers were among the first to produce and test prefabricated sound attenuators: Koppers, Industrial Acoustics Company, Industrial Sound Control, and Elof Hansson. Though rectangular dissipative attenuators are the most common variant of attenuators used today in architectural acoustics noise control, other design options exist. Reactive silencers Reactive silencers are very common in muffler design of automobiles and trucks. Attenuation is primarily achieved through sound reflection, area change, and tuned chambers. The design of reactive silencers from scratch is mathematically intensive, so manufacturers often have a number of prefabricated designs. Dissipative silencers Dissipative silencers attenuate sound by transferring