Analytical and experimental evaluation of active structural acoustic control (ASAC) of helicopter cabin noise

Abstract

Application of active structural acoustic control (ASAC) for helicopter cabin noise reduction is investigated using both analytical modeling and experimental tests. With ASAC, forced structural inputs from piezoelectric actuators bonded directly to the structure are used to cancel noise. These actuators are small, lightweight, inexpensive, and easily installed on aircraft surfaces, and were found to provide the necessary control authority to cancel cabin tonal noise when installed on thin sound-radiating surfaces. An analytical model of helicopter fuselage was developed and validated using experimental transfer function data. Estimates of noise reductions with different actuator configurations were determined numerically. An optimum actuator configuration was used for flight tests. . An ASAC system was developed and tested on an MD900 Explorer helicopter in flight. Cabin noise reductions were demonstrated with this system. INTRODUCTION With rapidly growing applications for rotorcraft worldwide, both passengers and crewmembers need quiet, comfortable transportation similar to that of commercial fixed wing aircraft. Helicopter interiors are significantly noisier (20 to 30 dB) than those of commercial fixed wing aircraft. Traditionally, aircraft interior noise reduction has been achieved with passive materials and devices, which are usually effective in the high frequency region, and in cases where the cabin noise is generated by aero / thermodynamic sources (e.g., rotors, engines, turbulence, etc.). However, significant weight penalties generally prevent them from being useful in the lowto mid-frequency region, and where the noise sources are mechanically generated (e.g., drive system, gearbox, etc.). In helicopters, for example, noise radiated into the cabin is typically transmitted initially as gearbox vibrations through the airframe structure before it radiates into the acoustic cavity. * Boeing Technical Fellow, Boeing-Long Beach, Member AIAA a Senior Engineer, Boeing-Mesa t Boeing Technical Fellow, Boeing-Mesa, t Roanoke Electric Steel Professor, AIAA Assoc. Fellow Presented at the 40th AIAA Aerospace Sciences Meeting at Reno, NV, January 14-17, 2002. Copyright ©The American Institute of Aeronautics and Astronautics, Inc. All rights reserved Controlling these vibrations before they become airborne sound could prove to be a more practical solution with potentially lower weight penalties than those normally incurred by purely passive techniques. Alternative means of aircraft cabin noise reduction using active control within the airframe structure were investigated for potential application to the uniquely complex helicopter cabin noise problem. Because there has been a lack of an integrated approach in applying advanced noise control approaches to new and existing rotorcraft, an analytical methodology was required for evaluation and design optimization of active (and passive) noise control concepts for helicopter interiors. Advanced noise control technologies, and the analytical means to optimize their integration into new and existing rotorcraft, were necessary to achieve acceptable levels of comfort for passengers and crew. The need for low cost, low weight rotorcraft interior noise control methods exist for both civilian and military helicopters. Military helicopters demand low noise crew stations to meet speech intelligibility requirements for reliable communication which can directly impact mission effectiveness. Specific crew station noise control requirements are specified in the U. S. Army MIL-STD-1294A. Lower interior noise levels on commercial rotorcraft will improve speech intelligibility for crewmembers, 1 American Institute of Aeronautics & Aeronautics (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. thereby improving flight safety. Reduced passenger cabin noise levels will improve passenger acceptance and provide a competitive advantage in the worldwide commuter aircraft infrastructure. HELICOPTER CABIN NOISE A large part of helicopter cabin noise is transmitted as vibratory responses through structural components before it is radiated into the enclosed space. The transmission gearbox is generally considered the major source of interior noise in helicopters, The main objective of this 3-year effort, which started in 1997, was to develop and demonstrate in flight an Active Structural Acoustic Control (ASAC) system, which reduces midfrequency transmission noise on helicopters [1]. The MD900 Explorer helicopter, shown in Figure 1, was selected as the candidate flight test vehicle for evaluating Figure 1. MD900 Explorer helicopter. and demonstrating the potential for active structural acoustic control. It is a 6250 lb., twin engine helicopter with a NOTAR®-equipped anti-torque system. Its main 0