Broadband passive noise control in air/water using structural acoustic silencers

The effectiveness of introducing flexible structural layers into fluid and air conveying ducts for controlling noise is investigated through theoretical and experimental means. Previous work has shown that using flexible rather than rigid walls can theoretically achieve high transmission losses. However, for hydraulic applications these relatively thin layers cannot sustain the high static pressures seen in real situations and for pneumatic applications, radiated noise from these walls (called breakout noise) is a significant issue. Designs that introduce a second duct below the flexible wall region of the main duct allows for equilibration of the static pressure and elimination of breakout noise. However, plane waves can now propagate through the duct without exciting the plate, thus reducing the effectiveness of the device. Designs suitable for hydraulic/pneumatic applications, that overcome these issues and achieve appreciable transmission loss are investigated. Results based on 2.5 D finite element simulations will be compared with experimental results. [Research funded by ONR and NSF.]