Candidate: Haocheng Yu, School of Computational Science and Engineering; home unit in Aerospace Engineering, Georgia Institute of Technology.
Title: Impact of Leakage due to Ill-fitting Earplugs on the Dissipation of High-Amplitude Sound into Vorticity
Thesis Advisory Committee:
- Prof. Spencer H. Bryngelson (Advisor), School of Computational Science and Engineering
- Prof. Krishan K. Ahuja (Co-Advisor), School of Aerospace Engineering
- Prof. Lakshmi N. Sankar, School of Aerospace Engineering
- Prof. Julien Meaud, School of Mechanical Engineering
- Prof. Qi Tang, School of Computational Science and Engineering
- Prof. Beckett Zhou, School of Aerospace Engineering
Date: Wednesday, July 1, 2026
Time: 11:00 AM – 1:00 PM EST
Location: Coda Building, Room C1315 (Grand Park), Georgia Tech
Online (Microsoft Teams): https://teams.microsoft.com/meet/232516160666016?p=3HkHdR0erDtez6bFab
Abstract: "Personnel exposed to high sound pressure level (SPL) noise rely on earplugs to prevent noise-induced hearing loss, yet non-absorbing earplugs depend on the seal between the earplug body and the ear-canal wall rather than on bulk absorption. The small air gaps that remain when the fit is imperfect can therefore dominate the acoustic performance. At high SPL, an additional dissipation mechanism becomes active in which incident acoustic energy is converted into vorticity at the sharp edges of a small opening and is then dissipated by viscosity, the same mechanism that governs acoustic liners, perforated plates, nozzles, and other small-aperture elements. This dissertation quantifies the acoustic dissipation of small, rigid-walled airborne leak paths under high-SPL excitation and resolves how the incident energy partitions between viscous attenuation and conversion into vorticity.
The work combines three methods. A custom two-sided impedance tube measures acoustic absorption directly using an impulse technique that reaches peak levels above 150 dB at the test article, with O-ring face seals that make it suitable for small leak-path test articles. Direct numerical simulation is performed with the open-source, GPU-accelerated compressible-flow solver MFC, to which this work contributed an immersed-boundary treatment of complex geometries. Spectral modal analysis is then applied to the simulation data to separate the dissipation pathways.
Using these methods, the dissertation first studies leakage in modeled earplug-canal geometries. Experiment and simulation cross-verify on a uniform leak path, reproducing the same spectral shape and SPL ordering of the reflection, transmission, and absorption coefficients. Two- and three-dimensional simulations of a nonuniform ill-fitting earplug then resolve substantial conversion of acoustic energy into vorticity around leak paths at high SPL, with the frequency dependence interpreted through the Strouhal number. To the author's knowledge, this is the first direct numerical evidence that the SPL-dependent vortex-formation mechanism documented for isolated slits and orifices activates under earplug-like leakage conditions. Because the modeled geometry omits soft-tissue compliance, contact mechanics, and the middle-ear termination, these results are interpreted at the mechanism level rather than as predictions for a specific human ear.
The dissertation then resolves the mechanism in a canonical acoustically-driven slit. A direct numerical simulation database spanning incident sound pressure level, Strouhal number, and Reynolds number reveals two distinct absorption regimes: a vortex-dominated regime at high amplitude in which the Strouhal number is the primary controlling parameter, and a viscosity-dominated regime at low SPL in which the Reynolds number plays a comparable role. Spectral proper orthogonal decomposition separates, mode by mode and frequency by frequency, the kinetic-energy and viscous-loss contributions to the dissipation and links each to its coherent spatial structure, which the integrated absorption coefficient cannot resolve. The partition between the two pathways is governed by the coupled effect of the Strouhal number, the Reynolds number, the SPL, and the Keulegan-Carpenter number. A triadic orthogonal decomposition further identifies the nonlinear inter-frequency energy transfer, showing a forward cascade in which the forced fundamental feeds higher harmonics through near-mouth and corner-localized convection before viscous removal.
These contributions trace a single physical picture from a motivating application to its underlying mechanism, quantify how high-SPL sound is dissipated at small rigid openings, and provide a frequency- and space-resolved basis for the design of leak paths, slit resonators, and related small-aperture acoustic elements.”