Session : acoustic Acoustic comfort optimization in a H175 helicopter

摘要

Airbus Helicopters, Inc. offers the most complete range of corporate helicopters. These rotorcrafts combine smooth rides and luxurious interiors with industry-leading safety and low noise signatures, ensuring that operations are neighbor-friendly wherever they are performed. The combination of aerodynamic, acoustic and mechanical noise sources however typically leads to a rich spectrum with potentially very loud and annoying broadband and tonal components especially inside the helicopter. Moreover, strong mechanical coupling and complex noise transmission paths in the helicopter render acoustic optimization a challenging task. To achieve improved cabin comfort and apply efficient noise treatments, it is essential to identify the dominant noise sources and treat them in the right order of priority. Thus, having a tool to easily locate and rank the most contributing panels and sources in the cabin would be extremely beneficial to support quiet helicopter design. Many works have already been published on such noise source separation technique and applications, especially in the automotive industry. Among available technologies, array techniques based on beamforming have been largely developed and used for interior noise analysis but rarely applied to interior aircraft or helicopter noise. This paper presents an interesting study carried out on a H175 helicopter and showing how the interior sound field is analyzed with an acoustic array technique. The tool used in this project has been recently developed by MicrodB and his partner Siemens. It is based on a double layer spherical array placed at the center of the cabin and is able to provide a 360 degree "acoustic image" of the sound field in the cabin. A unique combination of two spherical arrays allows to cover a broad frequency range: one rigid sphere containing 36 microphones distributed on its surface covers the mid and high frequencies, whereas a second larger diameter open sphere with 24 microphones handles the low frequency part. The acoustic array is connected to a portable LMS SCADAS measurement system and the LMS Test.Lab software is used to process the data. Sound source localization results can be readily obtained inflight after just a few seconds of measurements. Alternatively, the measurement system can be used autonomously to record the entire flight for different flight conditions and provide the source localization result at a later stage. As such, this is a revolutionary tool as it is extremely quick, is able to analyze transient phenomena and avoids lengthy and costly in-flight measurements as performed with traditional techniques like acoustic sound intensity measurement or holography. The combination of a dual spherical array and specific processing algorithms such as spherical beamforming and equivalent source method allows to extend the application range of beamforming and to successfully handle noise source identification in cavities. This technique has already proved its efficiency in non-reverberant environments such as automotive interior noise optimization in windtunnels. One challenge of the current project is to apply the technique to the helicopter cabin which is a more reverberant environment. Noise source analysis has been performed in the cabin of the H175 helicopter for various flight conditions and will be illustrated in the paper. It is interesting to see how the noise sources vary as a function of thrust or flight speed. Noise spectrum is analyzed in detail and dominant sources are identified in the cabin for most critical frequencies. Source contribution results will also be shown where sound power is obtained by integrating the acoustic intensity over selected emission areas. This efficient measurement technique allows to get clear indications on possible noise treatments in the cabin to reach improved acoustic comfort. Best practices for such array measurements as well as advantages and limits of this technique will be clearly explained. This study allows to confirm that such acoustic array technique is very well suited for helicopter interior noise analysis.