Degree Candidacy: Master of Science in Mechanical Engineering
Date: Thursday, May 8, 2014
Time: 12:00 PM
Location: 108 EIB
Advisor: Dr. Lonnie Thompson
Committee Members: Dr. Gang Li and Dr. Huijuan Zhao
Title: Finite Element Analysis of Structural Acoustic Interaction with Air or Water for Sound Transmission through Honeycomb and Double Walled Panels with Air Cavity
A measure of Sound Transmission Loss (STL) through panel structures is the ratio of the average power over the panel surface from an incident acoustic pressure wave interacting with the surface of one side of the panel with the transmitted average power on the other side of the panel. For panels filled with an air cavity defined by a depth between the two panels, the panel interacting with the incident acoustic wave vibrates producing structure-born sound to radiate though the cavity and interacts with the transmitted side panel, causing sound to radiate into the acoustic region on the transmitted side. For steady-state frequency response analysis, power is measured from the integration across the panel surface of the product of acoustic pressure and velocity component normal to the surface. In contrast to water coupling, the effect of air on the structural vibration response is relatively small. For air, since the acoustic impedance defined as the ratio of pressure to velocity is constant and given by the product of mass density for air multiplied by the speed of sound, the expression for STL is simplified as the ratio of incident to transmitted pressure amplitude.
In the present work, a finite element model for prediction of sound power transmission through single panel, air cavity filled double panel structures, lattice panel structures, and honeycomb panels is presented. In the case a double-panel with internal air cavity model, parameter studies are conducted to compare STL results with different cavity depths in relationship to acoustic wavelengths. Results show that STL is reduced when the wavelength is twice the depth, implying that a strong transmission effect is present associated with the fundamental resonance cavity frequency with zero vibration nodes in the depth direction. Comparisons between single panel, Air-filled Double and Triple Panel structures are studied. As the number of panel layers is increased the thickness of each panel is decreased to have the same total mass. Air-cavity interactions in layered panels play an important role in sound transmission. Results show that more layers of thinner panels have stronger Air-cavity interactions showing stronger Air-cavity resonances in the frequency response for STL. Overall, multilayered panels with the same total mass show increased STL over the range of frequencies studied between 0 and 2000 Hz.
Further studies are conducted to study the effect of connecting the panels with periodic lattice structures. By connecting the panels, the STL is reduced, while significantly increasing the stiffness and strength under other mechanical loads. Air-cavity effects in panels with periodic connections between the panels, while introducing cavity resonances in the structure frequency response, does not significantly alter the Structure-borne sound radiation and overall STL characteristics. This study helps in understanding the challenges in designing structures needed to exhibit good structural rigidity and also has good sound insulation.
Honeycomb sandwich panels exhibit desirable structural properties of high stiffness and low mass. Previous studies have examined the STL characteristics for honeycomb panels interacting with air, up to 1000 Hz and showed that in this frequency range, Auxetic honeycomb with the total mass, which exhibit a negative effective Poisson ratio, gives higher STL compared to Regular honeycomb. In the present work, it is shown that for frequencies between 1000 Hz and 1600 Hz, the STL for Auxetic is reduced below the STL value for Regular honeycomb. Beyond 1600 Hz, the STL for Regular honeycomb is significantly reduced.
Previously studies have not considered the interaction of water with honeycomb panels. In this work, the STL characteristics for the honeycomb panels with water on both sides, and mixed combinations of Air on Incident side and Water on transmitted side and Water on Incident side and Air on transmitted side are given. In the case of water on both sides of the honeycomb panels, the overall STL is significantly reduced compared to air interaction on both sides, and over the entire range up 2000 Hz, Auxetic gave higher STL compared to Regular. In mix-match cases of Air-Water and Water-Air, Regular exhibited higher STL over Auxetic.
In addition to the steady-state analysis discussed above, a transient analysis of acoustic pressure waves propagating and interacting with panels are also discussed and correlations are made with the results of time-harmonic procedures. Two pressure waves are considered, a sinusoidal amplitude driven at 100 Hz, and a Ricker pulse amplitudes spread over a broader range of frequency but centered at 100 Hz.
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