Membrane-type acoustic metamaterials for aircraft noise shields

Abstract

In der Arbeit werden akustische Membran-Metamaterialien auf ihre Anwendbarkeit zur effizienten Reduktion von tieffrequenten Tönen untersucht. Es werden analytische Modelle zur Berechnung der Schalldämmung von Schallschutzmaßnahmen mit Membran-Metamaterialien entwickelt. Diese Modelle werden verwendet, um ein Schallschutzschild für einen Flugzeugrumpf zu entwerfen. Die experimentelle Charakterisierung des Schallschutzschildmodells erfolgt unter realitätsähnlichen Einbau- und Anregungsbedingungen an einer Flugzeugrumpfteststruktur.

The reduction of low-frequency noise transmission is a challenging task, especially when mass and size of suitable noise reduction measures are highly constrained. The recently emerged so-called acoustic metamaterials, however, have the potential to provide a solution for this difficulty. This work investigates the applicability of membrane-type acoustic metamaterials (MAMs) in an aircraft noise shield design for the reduction of low-frequency tonal cabin noise generated by counter-rotating open rotor engines. MAMs are thin and lightweight structures which exhibit excellent low-frequency noise reduction properties that cannot be achieved by conventional partitions with equivalent mass. Previous analytical and experimental investigations of MAMs have mostly been performed under greatly simplified conditions, such as normal incidence or small-scale dimensions. Since these conditions are rarely found in practice, the acoustic performance of MAMs needs to be evaluated in more realistic environments. In this work, computationally efficient analytical models for the prediction of the acoustical properties of MAM unit cells, multi-celled MAM arrays, and multi-layered structures containing MAMs under oblique plane wave excitation are developed. These models are verified and validated using numerical simulations and experiments, respectively. Parameter studies are performed to identify the most important parameters for the design of an aircraft noise shield with MAMs. These results are used in the design of a realistically sized experimental MAM noise shield model attached to an acoustic fuselage demonstrator. This demonstrator represents a full-scale single aisle aircraft type fuselage section in a rather simplified construction that can be acoustically excited by a loudspeaker array generating realistic engine sound fields. The analytical results show that the acoustic performance of MAMs under non-orthogonal incidence and within finitesized multi-celled arrays is not significantly altered as long as the MAM unit cell is smaller than the acoustic wavelength. Furthermore, it is shown that the low-frequency sound transmission through multi-layered structures (e.g. double walls) can be greatly enhanced by integrating MAM layers inside the air gaps. Finally, the noise shield demonstrator measurement results confirm the basic effectiveness of MAMs even in a much more complex and realistic environment. However, these results also reveal that the performance of the MAMs inside the noise shield is greatly affected by airborne flanking sound paths and the spatial characteristics of the excitation sound field.