Active structural acoustic control of broadband sound transmission through a panel

The ability of active control systems to reduce propulsion-related noise tones in aircraft cabins is well documented. However, for a variety of reasons, analogous solutions for broadband random noise have yet to be demonstrated. In contrast to tonal noise problems, broadband noise sources (such as turbulence) tend to be distributed both spatially and spectrally, and cannot be isolated to simple structural-acoustic transmission paths. Consequently, the design of a broadband noise control system poses additional and more demanding constraints than are encountered in tonal controller design. In this paper, we describe initial laboratory tests of a multichannel active control approach being developed for broadband random noise control in aircraft cabins. The tests were conducted in a transmission loss (TL) facility with a flat aluminum panel (1.75m x 1.12m) mounted in the test window and exposed to broadband acoustic excitation. The noise transmitted through the panel was measured with a spatially distributed array of microphones. A multichannel controller, coupled with an acoustic sensor and eight piezoceramic actuators (bonded to the panel surface), was used to reduce the transmitted noise in the 200-2000 Hz band throughout a large quiet zone. Overall noise reductions of up to 18 dB were demonstrated. Introduction In the last 10 years, active control of low-frequency tonal noise in aircraft cabins has progressed from the laboratory to commercial market acceptance." Active noise control techniques can successfully reduce aircraft cabin noise caused by engine or propeller tones in the low-frequency region (approx. 50-300 Hz). However, broadband random noise (e.g., due to boundary layer turbulence acting on the exterior of the aircraft) is a major contributor to cabin noise in many commercial aircraft, and is dominant in the mid-frequency range (approx. 300-1500 Hz). Conventional passive noise control treatments, though effective in the highfrequency range, impose unacceptable weight penalties when extended to treatment of low-frequency noise. It is therefore desirable to consider active control alternatives. Active acoustic noise control approaches employ cancelling sound sources (i.e., speakers) to control sound in enclosed spaces such as an aircraft cabin. In the mid-frequency range, acoustic control requires many cancelling sources to match the temporal and spatial sound patterns radiated from complex fuselage structures. Active structural-acoustic control (ASAC), on the other hand, employs control forces—applied directly to the vibrating structure—to minimize the sound transmitted through the structure. There is some indication that (for tonal noise control problems, at least) structural actuators are preferable to acoustic actuators, due to their closer match with the primary source patterns in the cabin. Also, structural transducers are reasonably light weight, compact, and amenable to integrated, smart structure designs. In this paper, we describe results of broadband ASAC applied to a large aluminum panel— representative of a fuselage section—using various test configurations, including details of the transmission-loss test facility and measurements, the controller architecture and optimization criteria, and real-time broadband ASAC results for both random and repetitive noise sources. The issue of time-delay, introduced by the actuator/panel dynamics, is treated in an appendix. Test facility and measurements Figure 1 shows a plan-view schematic of the McDonnell Douglas transmission-loss facility in Long Beach, CA, where the tests were performed. The facility has two anechoic rooms separated by a "window" in which a fuselage test panel is mounted. Acoustic noise is generated by a loudspeaker in the source room and transmitted through the panel into the receiver room, where the transmission loss of the panel is measured. For the broadband ASAC tests, the following electromechanical transducers complemented the facility: • An acoustic loudspeaker (the primary noise source) • A microphone sensor, mounted directly in front of the loudspeaker • An array of 14 microphone receivers (R1-R14), mounted in two coplanar arcs • An array of 8 electronically controlled piezoceramic (PZT) actuators (A1-A8) bonded to the panel. The microphone array was used to sample the noise pattern in a wide arc through the receiver room, roughly coplanar with the opposing loudspeaker source. Coarse sampling in the out-of-plane dimension was done by lowering the entire array. The placement of actuator patches on the panel is shown in Figure 2. Figure 3 and Figure 4 show the source and receiver rooms, respectively. Real-time implementation of the ASAC controller was performed by SRI's Advanced Signal Processor (ASP). For these tests, the ASP was configured with 1 control input, 8 control outputs, and up to 16 error inputs. Data were sampled at 13.3 kHz, and control filters were 2048 taps (-150 ms) long. Copyright ©1996 by the American Institute of Aeronautics and Astronautics, Inc. Noise Source (loudspeaker)