ASA 124th Meeting New Orleans 1992 October

4aSA4. Wave-number analysis of active structural acoustic control of sound transmission through a panel.

Gopal P. Mathur

Boi N. Tran

Douglas Aircraft Co., McDonnell Douglas Corp., MC 36-60, 3855 Lakewood Blvd., Long Beach, CA 90846

Active control of sound transmission through elastic structures, such as panels using structural inputs, has received considerable attention in recent years. The active control technique of improving structural transmission loss (TL) by applying control forces directly to the transmitting structure using single or multiple piezoelectric actuators mounted on the structure rather than using acoustic canceling sources is known as active structural acoustic control (ASAC) method. This paper presents results of experimental investigations of ASAC method in improving the TL of a panel. Wave-number analysis was employed to understand the mechanism of improving the TL of a panel using multiple piezoceramic actuators. Experiments were conducted to measure near-field sound radiation from a baffled panel (dimensions: 1.75 mx1.14 mx1.27 mm) subjected to acoustic excitation for both with and without active control conditions. The active control of the panel TL was achieved by four piezoceramic actuators mounted on the panel. An array of four microphones placed about 1 m away from the panel were used as error transducers for a multiple-input multiple-output active control system. The transmitted acoustic field was measured by a two-dimensional array of microphones (array size: 22x14). The wave-number frequency spectra of the sound field were obtained by implementing Fourier transforms on the spatial domain data. Sound intensity measurements were also made to determine improvements in panel transmission loss. Comparison of measured wave-number spectrum for the active control condition with that of baseline (without control) showed that sound radiation from the acoustically fast lower-order modes at low wave numbers is significantly reduced. Results from the near-field acoustical holographic measurements will also be discussed. The relative reduction in sound power levels due to ASAC estimated from the near-field holographic measurements compared very well with those obtained from sound intensity measurements.