The Hybrid III and NOCSAE headforms are two headforms used in impact testing, though their vibrational characteristics are not well understood. They may have different kinematic responses in various impact scenarios if the impact excites any of their natural frequencies; resonance is especially likely to occur in short-duration impacts with a wider frequency spectrum. The same impact on two headforms that perform similarly in blunt impacts can be much different in shorter-duration projectile impacts due to the vibrational responses. The research presented in this thesis had three objectives: to identify the natural frequencies of the Hybrid III and NOCSAE headforms and compare them with published human head values to determine which has a more biofidelic vibrational response; to quantify the frequency response of different baseball catcher's masks and assess their abilities to limit vibrations transferred to the headforms; and to compare kinematic and frequency responses between headforms in different impact scenarios (high-speed, low-mass projectile impacts vs. low-speed, high-mass pendulum impacts) and see how they are affected by various types of head protection. The results show the importance of considering frequency content in impact testing, suggesting that the NOCSAE headform may be more biofidelic in short-duration impacts since its natural frequencies better align with those seen in the human head. The catcher's masks experienced greater vibrational responses than the headforms, but since the NOCSAE's first natural frequency falls within the bandwidth being excited, resonance was seen in this headform's acceleration responses for the projectile baseball impacts. Lastly, while both headforms had higher peak linear accelerations (PLAs) from the short-duration projectile impacts than the pendulum impacts, the projectile impacts caused high frequencies to be excited in the NOCSAE headform, while only exciting low frequencies in the Hybrid III. These results may not be as relevant for long-duration loadings, as indicated by the similar responses between headforms for both the pendulum and helmeted projectile impacts. However, when a wide range of frequencies are being excited with short-duration impacts, these results are important to consider, since natural frequency excitation can influence head injury risk due to higher accelerations. / Master of Science / The Hybrid III and NOCSAE headforms are dummy headforms used in impact testing, but their vibrational characteristics are poorly understood. They may perform differently in certain loading environments due to structural differences; their frequency responses might differ based on impact characteristics. Short-duration impacts excite a wider range of frequencies than longer-duration (padded) impacts. While headforms generally perform similarly during padded impacts where resonant frequencies are avoided, excitation of these frequencies during short-duration impacts can result in different kinematic measurements between headforms. The research presented in this thesis had three objectives: to identify the natural frequencies of the Hybrid III and NOCSAE headforms and compare them with published human values to determine which better represents the head's vibrational response; to quantify the vibrational characteristics of different baseball catcher's masks and assess their abilities to limit vibrations transferred to the headforms; and to compare kinematic and frequency responses between headforms in different impact scenarios (high-speed, low-mass projectile impacts vs. low-speed, high-mass pendulum impacts) and see how they are affected by various helmets. The results show the importance of considering frequency content in impact testing, suggesting that the NOCSAE headform behaves more like the human head in short-duration impacts. Even though the catcher's masks "rang" more than the headforms, the vibrations from the projectile impact were in the appropriate range to excite the NOCSAE's natural frequencies. Thus, there was still an oscillatory response in this headform even when protected with the mask. Lastly, the projectile impacts caused higher accelerations in both headforms than the pendulum impacts. However, high frequencies were only experienced by the NOCSAE headform due to the projectile impacts; for the same impact, the Hybrid III just had low frequencies excited. These results are not as relevant for long-duration impacts, since there were similar responses in both headforms for both the pendulum and helmeted projectile impacts. However, they are very applicable for the short-duration impacts that excite a wide range of frequencies, since natural frequency excitation can increase the risk of head injury due to higher acceleration magnitudes.