Heterogeneous (HG) Blankets for Improved Aircraft Interior Noise Reduction

Abstract

This study involves the modeling and optimization of heterogeneous (HG) blankets for improved reduction of the sound transmission through double-panel systems at low frequencies. HG blankets consist of poro-elastic media with small, embedded masses, operating similar to a distributed mass-spring-damper system. Although most traditional poro-elastic materials have failed to effectively reduce low-frequency, radiated sound from structures, HG blankets show significant potential. A design tool predicting the response of a single-bay double panel system (DPS) with, acoustic cavity, HG blanket and radiated field, later a multi-bay DPS with frames, stringers, mounts, and four HG blankets, was developed and experimentally validated using impedance and mobility methods (IMM). A novel impedance matrix formulation for the HG blanket is derived and coupled to the DPS using an assembled matrix approach derived from the IMM. Genetic algorithms coupled with the previously described design tool of the DPS with the HG blanket treatment can optimize HG blanket design. This study presents a comparison of the performance obtained using the genetic algorithm optimization routine and a novel interactive optimization routine based on sequential addition of masses in the blanket. This research offers a detailed analysis of the behavior of the mass inclusions, highlighting controlled stiffness variation of the mass-spring-damper systems inside the HG blanket. A novel, empirical approach to predict the natural frequency of different mass shapes embedded in porous media was derived and experimentally verified for many different types of porous media. In addition, simplifying a model for poro-elastic materials for low frequencies that Biot and Allard originally proposed and implementing basic elastomechanical solutions produce a novel analytical approach to describe the interaction of the mass inclusions with a poro-elastic layer. A full-scale fuselage experiment performed on a Gulfstream section involves using the design tool for the positions of the mass inclusions, and the results of the previously described empirical approach facilitate tuning of the natural frequencies of the mass inclusions to the desired natural frequencies. The presented results indicate that proper tuning of the HG blankets can result in broadband noise reduction below 500Hz with less than 10% added mass.