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Research Papers

An Experimental Investigation of Acoustic Black Hole Dynamics at Low, Mid, and High Frequencies

[+] Author and Article Information
Philip A. Feurtado

Applied Research Lab,
The Pennsylvania State University,
P.O. Box 30,
University Park, PA 16804
e-mail: paf932@arl.psu.edu

Stephen C. Conlon

Applied Research Lab,
The Pennsylvania State University,
P.O. Box 30,
University Park, PA 16804
e-mail: scc135@arl.psu.edu

Contributed by the Noise Control and Acoustics Division of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received January 4, 2016; final manuscript received May 31, 2016; published online July 19, 2016. Assoc. Editor: Ronald N. Miles.

J. Vib. Acoust 138(6), 061002 (Jul 19, 2016) (6 pages) Paper No: VIB-16-1004; doi: 10.1115/1.4033894 History: Received January 04, 2016; Revised May 31, 2016

The acoustic black hole (ABH) has been developed in recent years as an effective, passive, and lightweight method for attenuating bending wave vibrations in beams and plates and reducing the sound radiation and structural-acoustic response of structures. The ABH effect utilizes a local change in the plate or beam thickness to reduce the bending wave speed and increase the transverse vibration amplitude. Attaching a viscoelastic damping layer to the ABH results in effective energy dissipation and vibration reduction. Surface-averaged mobility and radiated sound power measurements were performed on an aluminum plate containing an array of 20 two-dimensional ABHs with damping layers and compared to a similar uniform plate. Detailed laser vibrometer scans of an ABH cell (including the ABH and surrounding homogeneous plate) were also performed to analyze the vibratory characteristics of individual ABH cells and compared with mode shapes calculated using finite elements. The results showed that the surface-averaged mobility was reduced by up to 14 dB for the fully damped ABH plate compared to a uniform reference plate while also reducing the mass of the plate. The results demonstrated that the dynamics of plates with embedded ABHs can be characterized by low, mid, and high frequency ranges, with low-order local ABH modes contributing significantly to low frequency ABH performance. The effects of damping layer thickness and diameter were also investigated to assess ABH performance optimization. It was shown that the damping layer can have the added benefit of mass loading the ABH and enhancing low frequency performance. The results will be useful for designing the low frequency performance of future ABH systems and describing ABH performance in terms of design parameters.

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References

Figures

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Fig. 1

Finite element analysis (FEA) model (a) and cross section and (b) of two-dimensional aluminum ABH with damping layer

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Fig. 2

FEA predicted modal loss factor for the first four ABH modes as a function of the diameter of a 3 mm thick damping layer. The modal loss factor is greatly reduced for damping layers less than about 3.5 cm in diameter. Ten centimeter represents full coverage of the ABH. See Fig. 3 for ABH mode shapes.

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Fig. 3

Experimental (top row) and computational (bottom row) mode shapes for the first four ABH modes. The mode shapes are characteristic of the radial symmetry of the ABH, containing nodal lines and nodal circles. The circles in the experimental results denote the edge of the ABH. See Table 2 for mode shape descriptors and frequencies.

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Fig. 4

Average modal loss factors of first four ABH modes for various damping layer thicknesses. A 3 mm thick damping layer provides the highest loss factor and was also used for measurements.

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Fig. 5

Aluminum plate with 4 × 5 array of embedded ABHs with full diameter damping layers in a frame with a mechanical point drive. The uniform surface opposite the drive was scanned with the vibrometer.

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Fig. 6

Surface velocity of points at the center, half radius, and outer edge of an ABH. The first ABH mode has a center frequency of 2.8 kHz. At low frequencies, the entire ABH vibrates as one unit. As the first ABH mode begins to cut on the center of the ABH, it begins to vibrate with higher velocity than the ABH edge.

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Fig. 7

One-third octave band surface-averaged mobilities for uniform plate and ABH plate with varying diameters of damping material. The damped ABH plate begins to reduce vibration only above approximately 3 kHz, corresponding to the frequency where the first ABH mode cuts on.

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Fig. 8

One-third octave band surface-averaged mobilities for uniform plate and ABH plate with varying diameters of damping material on a log frequency scale. At low frequencies below the first ABH mode, there is little difference between the uniform plate and the ABH plates. Varying the amount of damping material has negligible effect at low frequencies.

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Fig. 9

One-third octave band radiated sound power for a uniform plate and an ABH plate with varying diameters of damping material. The embedded ABH plate shows reduced radiated sound power at frequencies above the first ABH mode.

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