Noise levels in areas adjacent to high performance jets have increased monotonically in the past few years. When personnel are exposed to such high noise fields, the need for better hearing protection is inevitable. Adequate hearing protection may be achieved through the use of circumaural ear defenders, earplugs or both. This thesis focuses on identifying the dominant physical phenomena, responsible for the low frequency (0 – 300 Hz) acoustic response inside the earmuffs. A large volume earcup is used with the undercut seal for the study. The significance of this research is the use of finite element methods in the area of hearing protection design. The objectives of this research are to identify the dominant physical phenomena responsible for the loss of hearing protection in the lower frequency range, and develop FE models to analyze the effects of structural and acoustic modes on the acoustic pressure response inside the earcup. It is found that there are two phenomena, which are primarily responsible for the lower frequency acoustic response inside the earmuffs. These modes are recognized in this thesis as the piston mode and the Helmholtz mode. The piston mode occurs due to the dynamics of the earcup and seal at 150 Hz, which results in loss of hearing protection. The Helmholtz mode occurs due to the presence of leaks. The resonant frequency of the Helmholtz mode and the pressure response depends on the leak size. / Master of Science