Breakout from HVAC ductwork

Noise radiated by HVAC ductwork is becoming more important with the recent trend to eliminate ceilings in many office buildings. The ASHRAE Applications Handbook has provided a method for calculating breakout from ductwork since 1987. This method requires determining the duct wall transmission loss, which is not the same at the conventional transmission loss obtained using ASTM E90 because of the shape of the duct wall and because the sound field inside the duct is not random incidence. This methodology assumes that the sound level inside the duct is the same throughout the entire duct length, which is only approximately true for short, unlined ducts, but not always the case. In 2003 the ASHRAE Handbook presented a revised duct breakout calculation method that takes into consideration the sound attenuation with distance inside the duct, assuming that the sound level decreases at a constant rate. The results of a 2017 ASHRAE research project studying the insertion loss versus length of acoustically lined ducts revealed that the attenuation rate is not constant with length. This paper demonstrates the difference between the various breakout calculation methods and highlights the importance of including the attenuation rate in the analysis.