It is anticipated that some of the auxiliary equipment to be used in conjunction with the new water tunnel under construction at The Pennsylvania State College will be particularly noisy in the ultrasonic frequency region where certain low level acoustic measurements are to be made. This potentially noisy equipment includes such items as a deaerator, an energy dissipator and a pressure control system which are located in a water loop, external to the main tunnel section. To effectively isolate this loop acoustically from the remainder of the tunnel where the acoustic measurements will be made, it has been considered desirable to design an acoustic filter which can be inserted at the junctions of the loop with the tunnel. This filter should serve the multiple function of reducing the sound transmission through the pipe walls and through the water in the pipes while permitting the passage of relatively large volumes of water at small loss of pressure.The problem of isolation in the pipe path is a fairly common one and essentially requires the use of sound attenuating gasket material. However, the isolation of the water path is more complex in that water must be allowed to flow while sound is attenuated. In this case a honeycombed structure of a pressure release material is used.This paper discusses the hydraulic consideration of the problem and presents the results of acoustic transmission measurements on the filter components. The measurements include: 1. The frequency characteristics of the acoustic filter under free field conditions, 2. The transmission properties of the isolating gaskets mounted in the pipe flanges, and 3. The effective response of the filter and isolation gaskets combined.