Sharon L. Padula and Daniel L. Palumbo*

Multidisciplinary Optimization Branch and *Structural Acoustics Branch, NASA Langley Research Center

May, 1997

RTA 538-03-14-01

Research Objective. Active Structural Acoustic Control (ASAC) is a promising technique for reducing noise and vibration in aircraft interiors by active control of actuators bonded to the fuselage skin. The goal of the present study is to select the best set of actuator locations for controlling noise at the blade passage frequency and at multiple harmonics produced by a typical propeller aircraft.

Approach. Data collected at McDonnell Douglas' Fuselage Acoustic Research Facility (see photo) provides noise measurements at eighteen interior microphones due to the primary noise source and due to each of the 64 piezoelectric actuators activated one at a time. This measured transfer function data allows accurate prediction of the noise reduction potential for various subsets of N out of the 64 actuators. A tabu search procedure selects the best subset of actuators by evaluating a tiny fraction of the 10¹³ possible combinations. An existing tabu search procedure which reduces noise at a single frequency was extended to consider noise due to multiple frequencies and to impose constraints on the total force applied to each actuator.

Accomplishment Description. Tabu search method was used to select the best N actuator locations (N=2, 4,..., 16) for noise at five different frequencies, with and without force constraints (see figure). The constrained case predicts modest noise reduction with diminishing returns for more than 12 actuators. The unconstrained case predicts unbelievably large noise reduction which increases with added actuator locations. The inconsistent noise predictions were explained when the noise reduction potential for two subsets of 14 actuators were evaluated over a range of force constraints. The actuator subset chosen with constraints performs very well even if the constraint level was significantly modified. The actuator subset chosen without constraints performs well only at unrealistically high forces (see figure).

Significance. Noise and vibration cause annoyance and fatigue for passengers on short haul civil transports. Previous laboratory experiments demonstrate that ASAC techniques can target a single annoying frequency and reduce noise more effectively and with less weight penalty than passive techniques. The present study suggests that optimization of the actuator locations and the use of realistic force constraints significantly improves ASAC performance, especially if multiple frequencies are targeted.

Future Plans. The tabu search method will be used to select actuators locations for ground-based and flight tests later this year. These tests will compare predicted noise reduction to actual noise reduction. If noise reduction is successful for several frequencies typical of propeller noise, then the tabu search method may be extended to target a large number of broadband frequency ranges typical of jet noise.

Figure: Optimized Actuator Array for Control of Multi-frequency Noise