AIAA 2002-1355 1 American Institute of Aeronautics and Astronautics EVACUATED ENCLOSURE MOUNTED ACOUSTIC ACTUATOR AND PASSIVE ATTENUATOR Benjamin K. Henderson 1 , Steven A. Lane 2 Air Force Research Laboratory/VSSV, Kirtland AFB, NM 87117-5776 Steven F. Griffin 3 Boeing-SVS, 4411 The 25 Way, Suite 350, Albuquerque, NM 87109 Joel Gussy 4 University of Michigan, 3055 Whisperwood Dr, Ann Arbor, MI 48105 Kevin M. Farinholt 5 Virginia Tech Mech. Eng. Dept., 307 New Engineering Bldg., Blacksburg, VA 24061-0261 1 Research Aerospace Engineer, AFRL/VSSV, Senior Member AIAA 2 Research Aerospace Engineer, AFRL/VSSV, Non-member AIAA 3 Senior Engineer, Boeing-SVS, Senior Member AIAA 4 Student, Michigan University, Member AIAA 5 Student, Virginia Tech, Non-member AIAA ABSTRACT It is often desired that acoustic actuators or passive attenuators be mounted in relatively small, sealed enclosures to meet space constraints or in order to make the device unobtrusive. However, the large stiffness contribution from the air volume of such an enclosure can significantly increase the natural frequency of a conventional system, degrading performance at low frequencies. In this work, a novel concept is presented in which a diaphragm is mounted in a small, sealed enclosure, yet the overall system exhibits an extremely low natural frequency. This is accomplished by partially or completely evacuating the air from the enclosure to minimize the air-spring effect. A buckling suspension, exhibiting a nonlinear spring-rate, is utilized to counteract the large loads associated with the pressure difference across the diaphragm while retaining a low stiffness in the range of operation. This work presents theory, modeling, simulations, and experimental results from a prototype that demonstrates the utility of the proposed concept. INTRODUCTION The structural dynamics of enclosed volumes can generate an acoustic response that, through structural- acoustic interaction, can damage a delicate payload in a commercial space launch. Proposed approaches to mitigate this acoustic environment often involve the use of acoustic actuators, such as acoustic actuators (loudspeakers), or passive attenuator elements. Such elements are commonly mounted in sealed enclosures, or cabinets, to enhance acoustic radiation by preventing dipole behavior. However, the mechanical resonance frequency (i.e. the natural frequency) of the speaker may be significantly increased due to the air-spring effect of the air volume within the enclosure. This degrades the low-frequency performance of the speaker, since speakers radiate sound less efficiently below their natural frequency. Therefore, speakers designed for low-frequency applications are often mounted in large enclosures. Large-volume enclosures have less of an air-spring effect, which allows for a lower resonance. Although this is acceptable for most applications, many applications have constraints on the amount of space available for the speaker enclosures. Examples include compact sub-woofer designs, and active noise control actuators for automobile, aircraft, and launch vehicles. In such cases, it is desired to have speakers mounted in small enclosures that maintain good low-frequency performance. 43rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Con 22-25 April 2002, Denver, Colorado AIAA 2002-1355 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.