Air conditioning system noise measurement and characterization for aircraft ground operations

Abstract

The protection of workers exposed to noise during aircraft Turn Around Time or maintenance operations with engines off, the so-called Ramp Noise, has justified the setting of requirements issued as ICAO noise standards and regulations applicable in a number of countries. In this context, Airbus is pursuing very significant research efforts on the development of methodologies and means to predict ramp noise levels and optimize noise control means integration into Auxiliary Power Unit and aircraft air conditioning systems. The different noise components of interest are the turbomachinery noise generated by fans, compressors and turbines, the jet noise and the flow noise generated by flow distortions (NACA air intake profiles, grids...). In the present article, an additional component called “case radiated noise”, generated by Air Cycle Machine vibrations, air flow ducting, fans... within the belly-fairing casing is investigated. Its propagation can be either aerial or structural through the vibration of the light structure (e.g. composite panels) of the bellyfairing. Noise tests have been performed on an Airbus long range aircraft in order to characterize each source of noise generated when air conditioning systems are activated on ground and to quantify their contribution to noise levels at different servicing locations. To this purpose, innovative silencers have been specifically designed and manufactured to insulate the different inlets and outlets of the two main air conditioning systems. The silencers were tested beforehand to check the related Transmission Loss. The measurements were close to the theoretical results, so that the silencers had the expected efficiency. The tests were successfully performed and have enabled the characterization of the major noise sources coming from air conditioning systems contributing to ramp noise levels, as well as their ranking. In particular, noise due to structure borne excitation has appeared of second order of magnitude compared with airborne propagation. The Environmental Control System airborne propagation through the belly-fairing grids appears as the main contributor in the high frequency range above 1000Hz, the associated leaks significantly decreasing the Transmission Loss of the belly-fairing panels. The noise radiating through the grids being omnidirectional, its impact is significant all around the aircraft, at the servicing locations as well as under the belly-fairing. Moreover, the impact of the grids on external noise being directly linked to the percentage of the grid openings versus the total surface of the belly-fairing panels, it is also possible to roughly assess the impact on noise due to a grid size modification.