# Smart Materials for Green(er) Cities, a Short Review
## 2 Smart Materials for Green(er) Cities
### Acoustic Metasurfaces for Green Acoustic Isolation  
Metamaterials form a new class of materials whose physical properties derive essentially from their micro- or meso-structure, and not from their nature [93]. In particular, it is possible to manufacture metamaterials with properties unknown in nature, such as materials with a negative optical index [94]. In addition, the properties do not depend on the selected material (e.g., polymer, metal, ceramics).  
Figure 6: PCM in ceiling + manual ventilation (i.e., open windows, during the night) on the 23 rd of August.  
Figure 7: PCM in ceiling + mechanical ventilation (during the night) on the 4 th of June.  
Acoustic metasurfaces form a sub-class of metamaterials. Their ability to manipulate acoustic waves (passively) by means of structured plates that are very thin in relation to the wavelength gives them very interesting properties for \"green acoustic\" applications. They could indeed replace standard acoustic panels, often made of a wooden panel with holes masking a thick layer of absorbent porous material. Metasurface panels can attenuate sound (even at low frequency) without the need for this absorbent layer, often made from materials that are difficult to recycle, such as glass wool or polyurethane [95]. They could thus provide a green acoustic insulation solution for tomorrow's buildings. Metasurfaces are designed to control the characteristics of reflected and transmitted waves, while having a sub-wavelength thickness [96]. Research in this field has progressed considerably in recent years, thanks in particular to the development of rapid prototyping techniques for complex geometries using additive manufacturing.  
Acoustic metasurfaces can be used to manipulate the angles of reflection and transmission of incident waves, making it possible, for example, to generate large sound-free areas [97; 98; 99; 100]. These surfaces are made up of geometric patterns designed to interact with incident waves. However, the patterns must have a characteristic size close to a quarter of  
Figure 8: PCM in ceiling and on the walls + manual ventilation (i.e., open windows, during the night) on the 23 rd of August.  
Figure 9: PCM in ceiling and on the walls + mechanical ventilation (during the night) on the 28 th of June.  
a wavelength, which limits the application to low frequencies only. Other metasurfaces modify wave reflection and transmission coefficients at their interface [99] or directly absorb incident waves. Some of these absorbing metasurfaces are based on soft and flexible resonating membranes that can damp low-frequency acoustic waves [100, 101]. However, these membranes are often fragile, which limits their range of application. Another category of absorbing metasurfaces is based on the Helmholtz resonator principle [102, 103, 104, 105]. These resonators preferentially absorb and dissipate acoustic energy around their resonance frequency. This resonance frequency is inversely proportional to the volume of the resonant cavity, which limits the applications to low frequencies. However, it is possible to circumvent this issue by using \"flattened\" resonators where thin but wide cavities are used. A thin but long spiral waveguide (a coiled Helmholtz resonator) can also act as a resonant cavity. For illustration purposes, we present in Figure 10 an example of a flat, 3D-printed spiral waveguide. Such designs make it possible to absorb low frequencies down to 50 Hz, using structured plates with thicknesses significantly smaller than the wavelength [106, 107, 108]. One issue, however, is that absorption occurs only in a very narrow frequency range. Absorption in a wider frequency range requires the combination of several metasurface plates tuned to different frequencies (by adjusting their geometry). Despite this limitation, such metasurfaces could already be used to damp specific resonance modes, which can occur in concert halls, large offices or home studios.