Damped Windows for Aircraft Interior Noise Control

This paper describes how interior noise studies on general aviation aircraft have shown that windows are a significant noise path. This noise comes predominantly from external pressure excitations that are distributed over the fuselage, such as turbulent boundary layer pressure fluctuations, external acoustic sources, or dynamic flow structures produced by propellers. For a single engine propeller aircraft, considerable variability in the cabin noise levels exist with the sound pressure levels near the windshield usually being the highest at most frequencies. From a recent flight test, it was concluded that treating the windshield and forward cabin windows with increased mass loading or damping treatments offered good potential for cabin noise reduction. This flight test provided the inspiration for this paper on damped windows for aircraft applications. Laminated glass has been shown to provide benefits for noise reduction in automotive and architectural applications. Recent work at NASA Langley Research Center has examined the acoustic benefits of laminated windows that consist of two or three layers of plexiglass with transparent viscoelastic damping material sandwiched between the plexiglass layers. These damped plexiglass windows were evaluated as replacements for conventional solid aircraft windows to reduce the structure-borne and air-borne noise transmitted into the interior of general aviation aircraft. For windows that were nominally 6.35 mm 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 Plexiglas windows. An increase in transmission loss (TL) of up to 4.5 dB over a 50 to 4000Hz frequency range was also shown. While this work focused on single-pane laminated windows, it has also been shown for architectural applications10 that laminated glass provides increased transmission loss for window assemblies consisting of two glass panels separated by an air gap. This has the potential for broader application of the laminated windows to multi-pane window configurations applicable to passenger windows for advanced turboprop and jet aircraft. The work presented in this paper extends the plexiglass window study to include transmission loss predictions based on statistical energy analysis (SEA) methods and TL equations in the literature. Transmission loss predictions are shown for finite element/boundary element predictions below 1 kHz, and SEA and TL equations in the range from 100 to 10k Hz. Finite element/boundary element predictions of the radiated sound power are also presented. The predictions are compared with acoustic response measurements for the windows installed in the NASA Langley Structural Acoustic Loads and Transmission (SALT) Facility. Good agreement is obtained between measurements and predictions. Using the validated numerical models, a parametric study examined the optimum layup of the damped plexiglass windows. The test facility, hardware, vibro-acoustic tests, and numerical models are described.

Language

  • English

Media Info

  • Media Type: CD-ROM
  • Features: Figures; Photos; References; Tables;
  • Pagination: pp 899-910
  • Monograph Title: Noise-Con 04. The 2004 National Conference on Noise Control Engineering

Subject/Index Terms

Filing Info

  • Accession Number: 01054365
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: Jul 26 2007 2:05PM