This Aerospace Information Report (AIR) is limited in scope to the general consideration of environmental control system noise and its effect on occupant comfort. Additional information on the control of environmental control system noise may be found in 2.3 and in the documents referenced throughout the text. This document does not contain sufficient direction and detail to accomplish effective and complete acoustic designs. Purpose The purpose of this AIR is to provide aid for the reduction of environmental control system noise levels and to minimize their effect on passengers and crew members through engineering design. The reader should be aware that the material included in this document is for general guidance purposes only.