Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Having acoustic wave modifying structure
Reexamination Certificate
1998-06-19
2001-07-24
Kuntz, Curtis (Department: 2643)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Having acoustic wave modifying structure
C381S423000, C381S426000, C181S167000, C181S172000, C181S284000
Reexamination Certificate
active
06266427
ABSTRACT:
TECHNICAL FIELD
The present invention relates to damped structural panels and methods of making damped structural panels.
BACKGROUND ART
Structural panels such as aircraft fuselage panels, panels of automobiles, panels found on machinery, and panels found in household appliances, typically radiate noise due to vibratory motion induced in the panels. The resonant vibrations of the structural panels are often induced by unavoidable external sources. For example, engines, motors, compressors, etc., may induce vibrations in panels. Noise problems with structural panels are more apparent when panel thickness is reduced to minimize panel weight, such as in aircraft fuselage panels and other aerospace applications.
One known technique frequently employed to reduce resonant vibrations in structural panels is the use of viscoelastic damping treatments. In free-layer type damping treatments, a viscoelastic damping material, such as rubber, is added as a free layer to the surface of the structural panel. The damping treatment is usually applied to the entire surface of the panel. The viscoelastic material absorbs a portion of the total vibration energy by shear deformation. A more effective damping technique is to cover the free layer of viscoelastic material with a constraining layer of metal to form a constrained-layer type damping treatment. The addition of the constraining layer on top of the free layer improves the energy absorption characteristics of the damping layer.
Although the conventional damping treatments provide increased damping for resonant modes of the structural panel, the large amounts of viscoelastic material which are used to cover the entire surface of the structural panel are expensive and heavy. These conventional damping treatments are particularly disadvantageous for aerospace applications or any other applications in which thin, light panels are desired.
DISCLOSURE OF INVENTION
It is, therefore, an object of the present invention to provide a damped structural panel having reduced weight, while sufficiently damping sound radiation caused by bending waves during use of the structural panel.
It is another object of the present invention to provide an improved damped structural panel having reduced amounts of viscoelastic material required for effective damping.
In carrying out the above objects and other objects and features of the present invention, a damped structural panel is designed. The damped structural panel comprises a panel having bending modes including demanding bending modes which radiate sound more efficiently. These demanding bending modes have subsonic bending waves along at least one axis, and require damping treatment based on sound radiation properties of the panel. A viscoelastic material is applied within a limited area adjacent to the panel edges based on the demanding bending modes. The viscoelastic material damps sound radiation caused by bending waves in the demanding bending modes. The viscoelastic material may be applied at corners of the panel, along a plurality of the panel edges, or along all of the panel edges, depending on the panel configuration, design constraints, expected excitation frequencies, and desired damping characteristics.
Preferably, the panel has a thickness sized such that a first bending mode of the panel has a natural frequency of less than about 50 Hertz. Further, the panel is configured such that a coincidence frequency of the panel is at least about 6,000 Hertz. The limited area adjacent to the panel edges, within which the viscoelastic material is applied, is preferably defined by maximum wavelengths for bending waves normal to the panel edges in the demanding bending modes. The limited area extends inwardly from each panel edge for at least about one-fourth of the maximum wavelength for the bending waves normal to that panel edge.
Further, in carrying out the present invention, an aircraft comprising a body composed of structural panels, wings, and a thrust device is provided. At least one of the body structural panels includes a panel having viscoelastic material applied within a limited area adjacent to the panel edges to damp sound radiation caused by bending waves in demanding bending modes of the panel during operation of the aircraft.
Still further, in carrying out the present invention, a method of making a damped structural panel is provided. The method comprises forming a panel having bending modes including demanding bending modes which have subsonic bending waves along at least one axis, and which require damping treatment. The method further comprises determining a limited area adjacent to the panel edges based on the demanding bending modes, and applying a viscoelastic material within the limited area to damp sound radiation caused by bending waves in the demanding bending modes.
The advantages accruing to the present invention are numerous. For example, embodiments of the present invention provide damped structural panels having reduced weight and thickness, and requiring reduced amounts of viscoelastic material while sufficiently damping sound radiation caused by bending waves during use of the structural panel.
While embodiments of this invention are illustrated and disclosed, these embodiments should not be construed to limit the claims. It is anticipated that various modifications and alternative designs may be made without departing from the scope of this invention.
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Brooks & Kushman P.C.
Dabney T.
Kuntz Curtis
McDonnell Douglas Corporation
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