Modeling and Validation of Damped Plexiglas Windows for Noise Control

Windows are a significant path for structure-borne and air-borne noise transmission in general aviation aircraft. In this paper, numerical and experimental results are used to evaluate damped plexiglas windows for the reduction of structureborne and air-borne noise transmitted into the interior of an aircraft. In contrast to conventional homogeneous windows, the damped plexiglas windows were fabricated using two or three layers of plexiglas with transparent viscoelastic damping material sandwiched between the layers. Transmission loss and radiated sound power measurements were used to compare different layups of the damped plexiglas windows with uniform windows of the same nominal thickness. This vibro-acoustic test data was also used for the verification and validation of finite element and boundary element models of the damped plexiglas windows. Numerical models are presented for the prediction of radiated sound power for a point force excitation and transmission loss for diffuse acoustic excitation. Radiated sound power and transmission loss predictions are in good agreement with experimental data. Once validated, the numerical models were used to perform a parametric study to determine the optimum configuration of the damped plexiglas windows for reducing the radiated sound power for a point force excitation. Introduction Laminated glass has been shown to provide benefits for noise reduction in automotive and architectural applications. Recent experimental work by Gibbs et. al. shows the potential benefits of damped plexiglas windows based on comparisons of transmission loss and radiated sound power. The damped plexiglas windows were evaluated as replacements for conventional solid aircraft windows to reduce the structureborne and air-borne noise transmitted into the ∗Aerospace Engineer, Structural Acoustics Branch †Cooperative Education Student, Purdue University This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. interior of general aviation aircraft. The damped plexiglas windows were fabricated using two or three layers of plexiglas with transparent viscoelastic damping material sandwiched between the layers as shown in Figure 1. For windows that were nominally 1⁄4” thick, reductions in the radiated sound power as large as 3.7 dB over a 1000 Hz bandwidth were shown experimentally when comparing damped and uniform windows. An increase in transmission loss of up to 4.5 dB over a 50 to 4000Hz frequency range was also shown. This work extends the plexiglas window study to include finite element and boundary element modeling of the windows. Vibro-acoustic predictions were compared with vibration and acoustic response measurements for the windows installed in the NASA Langley Structural Acoustic Loads and Transmission (SALT) Facility. Good agreement between measurement and predictions were obtained. Comparisons between the numerical and experimental data validate the numerical modeling approach. Using the validated numerical models, a parametric study was used to examine the optimum layup of the damped plexiglas windows. The test facility, hardware, vibro-acoustic tests, and numerical models are described.