# A Metamaterial Solution for Soundproofing on Board Ship
## 6 Conclusions  
In this paper, an eco-sustainable solution to replace traditionally adopted solutions on board ships to reduce transmitted noise through cabin ceilings was presented. The Metasolution investigated, being made only with steel parts, is a valid replacement and a reasonable green alternative to noise-absorbing panels mostly made of mineral woods.  
The systematic FE analysis performed to design the Metasolution identified the optimal solution described by the parameters given in Table 3. This particular configuration, tested in the double chamber, shows an advantage of 6 dB in terms of Rw, compared with the benchmark panel. Moreover, Figure 10 shows that the Metasolution has a better performance than the benchmark solution in terms of STL, in the overall frequency range investigated (100-5000 Hz), with the exception of the 100-160 Hz interval.  
Other authors have studied similar solutions [19; 20; 21] numerically and theoretically in the aeronautical field; however, the Metasolution, the first application of metamaterials in the naval field, was also analyzed experimentally in the laboratory and tested under real conditions, proving that it conforms with normative ISO [22; 23; 24; 25] and IMO [28] standards. With respect to the core for sandwich panels proposed by other authors, the Metasolution responds to the need for an easier industrialization and production alternative. Unlike the panels studied in the aeronautical field, mainly designed to guarantee high stiffness, the design of the Metasolution has been studied to obtain a trade-off between stiffness (to ensure self-standing capacity) and acoustic performance in the medium-low frequency range.  
The production cost for the Metasolution panel is aligned with the cost of traditional panels mounted on board ships. Moreover, the use of a compact integrated panel of 40 mm height leads to saving space and to a straightforward installation procedure, not impaired by the 2.5 kg/m2 weight increase.  
The validity of the Metasolution as an alternative method of soundproofing was proven on a mock-up of a ship's deck structure simulating the operational environment. The results showed that the calculated Rw was comparable with that obtainable with a traditional under-deck ceiling system.  
The fire tests performed according to the FTP Code showed that the proposed solution entirely respects the safety regulations for installation on board ships, as explained in Table 4.  
To conclude, the panel has suitable characteristics to be a valid eco-sustainable solution for noise control on board ships, as an alternative to traditional mineral wool products. This type of solution could pave the way for the substitution of traditional solutions for vibration and noise control in other configurations in the marine field.  
Conceptualization, L.D., S.C. and G.K.O.D.; methodology, S.C. and L.D.; software, S.C. and L.D.; data curation S.C. and L.D.; writing--original draft preparation, S.C., G.R., and G.K.O.D.; writing--review and editing S.C., G.K.O.D. and G.R.; supervision, L.D. and M.B.; project administration, L.D. and M.B. All authors have read and agreed to the published version of the manuscript.  
The research was performed inside the project \"Silent Ceiling\" financed by Fincantieri SpA for the study of ceiling acoustic insulation systems for public areas onboard cruise ships. The project was carried out in collaboration with the companies CSNI Scarl and Phone Vibes.  
Not applicable.  
The authors would like to thanks Eng. [PERSON], supervisor of the project for Fincantieri, and Eng. [PERSON], coordinator of the project for CSNI Scarl, for sharing the data and the obtained results.  
The authors declare no conflict of interest.