The Applied Physics Laboratory of Johns Hopkins University, operating under contract with the Bureau of Ordnance of the U.S. Navy, set up during the war a testing laboratory for ram jet motors at Forest Grove, Maryland, short distance outside of Washington, D. C., in a location surrounded by open country. Postwar building in the neighborhood made it necessary to quiet this installation if operation were to continue at the same location, and the Kellex Corporation undertook design of the revisions. Since very large volumes of air and hot gases had to enter and leave the test cells, it was necessary to design a duct system which offered extremely low resistance to gas flow, but high attenuation to sound. This paper gives a brief description of the resulting construction which has allowed the laboratory continue operation in the midst of a residential community.