Simulation of Attenuation due to Elbows and Side Branches and Breakout Transmission Loss (RP-1529) (OR-16-002)

The primary path of noise propagation in buildings is airborne transmission from building equipment through ducts. Attenuation is increased if elbows or side branches are introduced into a duct. In a companion paper, finite element analysis was used to predict the insertion loss of straight lined and unlined ducts and results were validated with measurement. In this work, finite element analysis is used to predict the attenuation of elbows and side branches. Results for elbows and branches are compared to the ASHRAE Handbook with good agreement. In addition, an important secondary noise transmission path is through duct walls into rooms. This path, which is often termed breakout noise, is also investigated using the finite element approach and results are correlated with an analytical solution and the ASHRAE Handbook with good agreement. Of note, it is demonstrated that the breakout transmission loss is much less than the insertion loss through lined rectangular ducts at some frequencies. This suggests that breakout noise may be the dominant noise path at some frequencies.