Aircraft crew members and maintenance personnel are subject to significantly high sound pressure levels. Cumulative exposure to such high levels could induce hearing loss. Therefore, choosing the optimal hearing protector is of the utmost importance. The equipment used to measure the sound levels in the aircraft cabin for helmet selection must be subjected to airworthiness regulations to be flown. This thesis documents the selection, implementation and validation of an in-flight data acquisition system. The signal route is characterized with digital signal processing theory and a comparison to LMS Test.LAB acoustic analysis software. Bell 412 in-flight data is presented to validate the measurement method. The main rotor and tail rotor harmonics were found to dominate the low frequency sound pressure levels. The analysis concluded that the SPH 5CF helmet satisfies the Canadian Labour Code Part II for in-flight occupant noise exposure limit of a maximum of 87 dBA for eight hours.