Piezoceramic Actuated Transducers for Interior Acoustic Noise Control
Green, Kimball W.
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Weight is a critical parameter in the design of any system launched into space. Current launch costs are on the order of 10,000 dollars per pound of payload capacity. Reducing weight and thus increasing payload capacity is always in the forefront of the design process. One method of increasing the payload capacity of launch vehicles is to reduce the acoustic environment in the interior of the fairing. A major problem is that passive methods currently used for noise suppression do not exhibit significant energy dissipation at low frequencies. This motivates the use of active noise control. Using active noise control for frequencies below 200 to 300 Hz in addition to the passive control means has potential to provide broadband noise suppression and thus a smoother, cheaper ride for any payload. The problem with this technique is that active noise control commonly uses electromagnetic speakers as the control element. The weight of the speaker adds more cost to the application due to the approximate cost per pound to send a launch vehicle and payload to space. At 10,000 dollars per pound of payload capacity, the added cost spent on protecting the payload can potentially reduce the amount of payload capacity a customer receives due to monies spent on non-payload mass. Therefore, necessity dictates a light weight noise control solution. This work investigates the feasibility of a transducer with less mass than that of a conventional loudspeaker which dissipates energy at the acoustic resonances of an enclosed cavity. The test setup involves using the transducer to lower the sound pressure levels of acoustic resonances which are excited by an external source, thus simulating the launch phase of a launch vehicle. The transducer is used as an actuator to add damping through feedback control. The transducer is comprised of three thin flexures that are actuated by piezoceramic material attached to both sides. The flexures actuate a speaker cone that is attached to the end of the flexures. The transducer can act as a sensor or an actuator due to the nature of the piezoceramics. The sound absorbing transducer is modeled to couple to the first acoustic resonance of a six foot cylindrical cavity. The cavity acts as a simplified model of a launch vehicle payload fairing. Equations of motion are derived to model actuator motion and the acoustic impedance of the cavity. A state-space model of the system was derived for two cases: a collocated sensor/actuator pair exciting the tube and an external source exciting the tube with the transducer acting as an absorber. The transducer is designed to affect the first mode, however damping is noticed in the next acoustic resonance. Analysis of the theoretical model indicated up to 70 percent reduction of the open-loop RMS values or a reduction of 10 dB. Experimental results with the optimized transducer produced a 35 percent reduction of the open-loop RMS value or 3.73 dB. The first acoustic resonance coupled well with the first structural mode of the transducer providing optimal noise suppression for the first mode. Damping was also noted in the second acoustic mode. Neglecting the inertia of the tip mass introduced errors in the predictions of the transducer resonances at higher frequencies. This problem limited the ability to control the higher modes of the cavity.
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