Analysis of Helicopter Sound for the Development of A New Generation Active Headset

Helicopters generate substantial noise levels, especially at low frequency. These noise levels are normally not harmful for rpm Rotation per minute the ear. However, the low frequency content masks the speech . Therefore, pilots tend to set the communication system at maximum level so that the sound levels reach severe amplitudes for the ear. These high sound levels are exposing the ear to fatigue and hearing loss. The low speech intelligibility caused by the background noise is also a safety issue since it is most important that all commands can be understood correctly. The risk of noise induced hearing loss can normally be evaluated by sound pressure level measurements. However, in some cases, the standardized methods for these measurements does not take into account some secondary effects of the frequency distribution of the sound pressure levels. This paper adresses a noise exposure situation where the noise level itself is not harmful for hearing but is still the reason for increased risk for noise induced hearing loss. Military helicopter crews in Sweden have recent shown an increase in noise induced hearing loss. A combined active passive technique has been chosen as a possible solution. Therefore, the dominant sound sources have to be identified and the number of harmonics for each source determined. An investigation of the coupling between the structure borne sound and the air borne sound was also performed. This analysis is the basis for the development of a new generation active headset, and is presented in this paper.