Activities of European Research Laboratories Regarding Helicopter Internal Noise

Although the acoustic comfort in helicopter cabins is not subject to European Recommendations regarding aeronautic environmental noise (ACARE 2020), helicopter manufacturers use many resources to improve internal acoustic comfort. This task is particularly difficult because, on the one hand, passengers are in close proximity to the disturbing sources and, on the other hand, the noise frequency range is located in the domain of high sensitivity of the human ear (500-5000 Hz). These activities are often conducted in conjunction with external laboratories specialized in the aeronautic domain. The purpose of this paper is to describe how different European laboratories (affiliations of the authors), involved in a "Helicopter Garteur Action Group" (AG20), usually address this problem of helicopter internal noise, in particular in terms of design, characterization or active control of vibration applied to helicopter panels, in order to improve acoustic comfort. Typical measurement techniques and applications of simulation methods are presented to illustrate the activities of laboratories, especially the characterization and optimization of the acoustic behavior of an isolated helicopter panel and, secondly, the evaluation of its effect in a cabin mock-up or in flight. In addition, procedures of active (or semi-active) control are described and applied to the vibro-acoustic transmission of an isolated panel, then to an anti-torque plate of a helicopter mock-up and finally in flight, in order to reduce the noise produced by gear-box vibrations.