## 2 Potential Applications of Aeroacoustic Metamaterials
### Reflection
For example, when the desired behavior of the metasurface is to act like an inclined wall in the deflection of sound, the metasurface should be effective over a wide frequency range, provided that each frequency has a different reflection angle or a different virtual inclination of the wall. While this concept has proven to be effective in a purely acoustic domain, deeper investigation is needed when the hosting fluid is moving, i.e., a background aerodynamic flow is present. In the virtually scarfed nacelle example, the metasurface would be grazed by the flow, and likely, a new \"corrected\" phase shift profile should be defined, taking into account, among other factors, the flow Mach number. Near-total reflecting metasurfaces can also be exploited to achieve high transmission losses for cabin noise reduction. Although take-off and landing are the loudest moments, cruise is clearly the longest section of a civil aviation aircraft mission, and hence, the most interesting for passenger comfort improvements by noise reduction treatments. In this flight condition, excluding internal sources like air-conditioning systems, the main external cabin noise contributions are from the engines and the turbulent boundary layer excitations that produce vibrations that transmit to the fuselage stringers and skins that radiate noise inside the cabin. With the increasing emphasis on noxious emission reduction and fuel consumption efficiency, there has been a renewed interest in different engine concepts beyond turbofans, like propfans, i.e., open rotor engines, to obtain the speed and performance of a turbofan with the intended fuel economy of a turboprop (for instance, the Airbus has patented aircraft designs with twin rear-mounted, contra-rotating propfans [53] and Boeing does as well [54]). One of the main outcomes that has so far discouraged the usage of propfans is the excessive cabin noise produced by this type of engine compared to the classic turbofan. The spectrum of noise measured inside the cabin would show discrete intense peaks at low-frequencies, i.e., in the 50-1500 Hz range, correlated with the engine rotators being at their fundamental and higher harmonics. Most kinds of treatment for cabin noise reduction are difficult to implement due to the small space available in the cabin sidewalls and the strict limits on the overall aircraft weight. Several works have employed resonant membranes as an effective way of suppressing the low frequency noise inside the cabin. Metasurfaces composed by membranes with added masses [55] can be attached to aluminum panels to guarantee a huge effective dynamic mass at resonance frequency, suppressing the bending vibrations of the structure and inducing radiation cancellation from the surface due to the anti-phase motions at anti-resonance working frequencies. Furthermore, a double wall design involving resonant-decorated membranes can be adopted to improve the noise shielding efficiency at low frequencies, keeping the overall structure lightweight [56; 57; 58; 59]. Lightweight structures, showing up to 50 dB in transmission loss at low frequencies that may also exhibit good mechanical properties have also been presented [60; 61; 62], in which a honeycomb frame is used as a core, covered by very thin rubber membranes. Multilayer sandwiches have shown very good acoustic insulation over the 50-1500 Hz range. This kind of layout seems to be the most promising due to its ability to yield the desired acoustic properties with a typically strong structure like a honeycomb without the need to add mass onto the membranes, hence respecting the prerequisite of lightness and not negatively affecting the aircraft's efficiency or maximum payload.