An Experimental Exploration Study of Acoustic Black Holes in Thick Plates
Open Access
- Author:
- Stimson, Emily
- Graduate Program:
- Acoustics (MS)
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 01, 2023
- Committee Members:
- Andrew Barnard, Program Head/Chair
Amanda D Hanford, Thesis Advisor/Co-Advisor
Tyler Patrick Dare, Committee Member
Micah Shepherd, Thesis Advisor/Co-Advisor - Keywords:
- Acoustic Black Holes
Thick plates
Multi-grid panels
Inherent damping
Structural acoustics
Embedded Structures
Local modes
Global modes
Mixed modes
Mode classification - Abstract:
- An Acoustic Black Hole (ABH) is an effect that occurs when a structure contains a power-law tapered thickness. This profile will create a means for vibrations to be focused into a thinner region of the structure, where the addition of a lossy material can help efficiently dissipate the vibrational energy, essentially "trapping" the vibrations. Embedding a set of damped ABHs into a structure then creates a metastructure with good vibration and noise reduction characteristics, but with a lower weight than traditional add-on treatments. For this project, four 1" thick aluminum panels with various ABH orientations, lengths, minimum thicknesses, and types are explored using experimental modal analysis and laser vibrometry. Each of the four panels contained one or multiple ABHs, labeled as an ABH or multi-grid panel, were compared to the mode shapes and loss factors of a Baseline panel. The frequencies for each of the first modes shapes, or "cut-on" frequencies, for the ABH and multi-grid panels all occurred at lower frequencies compared to the Baseline panel; even after adding a layer of polyurethane for damping to the Baseline and ABH panels, there was an observable downward shift in each panel's "cut-on" frequency compared to the undamped Baseline panel. All of the panels exhibited several different modal behaviors including: modes that were dominated by the geometry of the panel and the boundary conditions, modes that were dictated by the embedded ABHs in the panel, and a mixture between the two. This led to the investigation for the types of mode classifications: global, local, and mixed modes. Then comparing the general shapes, movements, and radiation efficiencies of the mode shapes to the likes of a piston, dipole, or quadrupole source. The panels' loss factors aided in this exploration, due to similar global and mixed modes appearing in the lower loss factor range, while particular locally focused modes were seen with loss factors clearly above the rest.