Approach to interior noise control. II - Self-supporting damped interior shell

Abstract

A companion paper presents theoretical and experimental data identifying the significance of panel critical frequency and structural damping in controlling trim panel dynamic response from excitation at attachment points. This paper explores a logical extension to the trim panel system. The shell presents several desirable nonacoustic properties that may offer design or construction economies. Of concern here is the design considerations that can turn potential acoustic problems into significant advantages. The high stiffness, necessary to make the shell self-supporting, may provide improved low-frequency performance in the vicinity of the double-wall resonance. The low critical frequency, usually implied by this stiffness, may be controlled through design of the panel dynamic properties, judicious location of the attachment points, and effective vibration isolation. Quantitative approaches to each of these issues are explored. A successful installation in one aircraft is described. Nomenclature Bs = flexural rigidity per unit width of the shell, N-m CL = longitudinal wave speed in frame material, m/s / = frequency, Hz fc = critical frequency, Hz h = thickness of the frame flange at the attachment point, m Kf = static stiffness of the fuselage system at the attachment point, N/m KI = isolator spring constant, N/m Ms =mass of the shell panel kg ms = mass per unit area of the shell panel, kg/m2 Yf = drive mobility point of fuselage side attachment point for the isolator, m/N-s Ys = drive point mobility of the shell attachment rx>int, m/N-s Zy = transfer impedance across an isolator, N-s/m p = density of the frame material, kg/m3 co =27r/rad/s