The Microflown, a Novel Approach to Helicopters Interior Noise Testing

Noise levels recorded in the helicopter cabin are severely affected by the strength and vicinity of noise sources. Jet engines, the gearbox and the rotors can be considered as separated sources-whose spectral content is strongly tonal and rpm dependant-exciting simultaneously the cabin acoustic cavity. The total sound pressure level measurable in cabin can be then considered as a summation of partial contributions generated by each source acting separately. The energy transfer from each source to the cabin can be either structure borne, via the mechanical joints connecting the gearbox to the helicopter\´s ceiling, or airborne, via the sound propagation in the air from the rotors to the cabin. Analysis techniques such as transfer path analysis are largely applied in the automotive industry that allows identifying the main transmission paths and their relative contribution to the total sound pressure level in cabin. Several variants exist for TPA model to be realized, but all are eventually dependent on the experimental NVH data measure in operating conditions. However, measuring acoustic data on helicopters is a difficult matter because of the multiplicity of source, their high correlation (all sources are dependant on the rotational speed) and the high reflectivity of the acoustics field in cabin. Traditional testing techniques as acoustic intensity or near field acoustic holography fail correctly addressing the problem, as they cannot account for the "negative contribution" of reactive intensity or for the complexity of test set-up whose requirement are often colliding with the operating needs of a flight mission. An alternative testing technique that makes use of novel acoustic probes, the Microflown, proves able to deal with harsh environmental testing conditions while improving measurement accuracy and paves the way for a better understanding on how to reduce noise level in helicopter cabins. The Microflown probe couples a miniaturized microphone and a microscop- - ic particle velocity probe. The latter is realized making use of nano-technologies (MEMS) and allows measuring accurately in-situ particle velocity providing a powerful tool to identify noise flow from the sources to the cabin. In the present paper a full description of the novel acoustic probe is provided and an application case is presented that allows realizing noise maps of a helicopter cabin in operating conditions. The noise maps measured with the Microflown point out critical areas that are susceptible of design modifications such as to improve cabin acoustic comfort