Since advanced composite materials generally experience coincidence at lower frequencies than metals when used in aircraft fuselage sidewalls, they may allow more transmission of airborne noise thereby requiring heavier acoustical treatments. A sequential design approach of addressing first structural and then acoustical design does not take advantage of structural/acoustical coupling. A simultaneous approach is expected to help minimize the total sidewall mass. This thesis uses numerical optimization to examine structural/acoustical interactions and compare the sequential and simultaneous design approaches.

Acoustical performance is defined in terms of the infinite panel transmission loss at frequencies surrounding the coincidence region (1600 Hz - 12800 Hz for the panels studied.) Impedance transfer theory is used to predict the acoustical properties of a flat unstiffened anisotropic panel treated with a fibrous acoustic blanket, airgap, and limp-mass septum. Structural analysis is based on a fatigue damage resistance criterion.

Sequentially designed treated composite panels exibit transmission losses 15 dB - 45 dB higher (transmitted pressure is 6 - 180 times smaller) than a structurally equivalent, equal-mass aluminum panel. Depending on the type of acoustic excitation (specific incidence direction or diffuse source) and the acoustic frequency considered, the simultaneous approach alters the sequential minimum-mass panel in order to 1) improve low frequency performance by raising coincidence frequencies, 2) improve high frequency performance by lowering coincidence frequencies, or 3) make the coincidence region as narrow as possible. Since these structural alterations require that more mass be allotted to the panel and less to the treatment, they only occur for strong structural/acoustical interactions (i.e. near coincidence.) The simultaneous design approach can achieve a moderate improvement (TL increased up to 10 dB, transmitted pressure decreased by a factor or 3) over a sequential design for a particular acoustic performance index, although computation time is increased and acoustic performance may be sacrificed in other regions.