Development of Acoustic Attenuators for Air‐Conditioning Ducts

The air‐borne noise path along ducts is important for frequencies above approximately 150 cps. While long, tortuous ducts often provide inherent attenuation, special treatment can be necessary for short ducts. Treatment effectiveness is judged in terms of air flow resistance in relation to minimum acoustic attenuation which will be provided for a range of installation conditions. Design of acoustic duct attenuators involves selection of suitable means for providing sound absorptive surfaces, effective over the wide frequency range involved, and their arrangement in the air stream. Methods are discussed for measuring acoustic attenuation in relation to air‐flow resistance. Attenuation measurements without air flow permit detailed examination of acoustic effectiveness of attenuator components and their arrangement. Subsequent attenuation measurements under rated air‐flow conditions are used to determine any potential source of self‐noise from turbulence.