A wideband triple-layer microperforated panel sound absorber
Microperforated panels are light, clean and tunable sound absorbers, which fundamentals were already established in the previous century. They consist of a distribution of minute holes of diameter d on a panel of thickness t with perforation ratio ϕ in front of an air layer of thickness D. Therefore, the frequency band of absorption of a single-layer microperforated panel depends on (d, t, ϕ, D). In noise control applications, such a simple structure provides absorption in the frequency range of interest if holes are of sub-millimetric diameter. The former single-layer microperforated panels had two main weaknesses: narrow frequency band of absorption and a high manufacturing cost. However, nowadays these deficiencies can be easily surpassed. The absorption frequency band can be drastically increased by means of multiple-layer structures. The manufacturing cost may be reduced with the application of techniques currently used in the advanced circuitry industry. This is illustrated by the design of a triple-layer microperforated panel yielding absorption in almost four and a half octaves. The parameters of the triple-layer microperforated panel are optimised by Simulated Annealing to provide maximum absorption within a prescribed frequency band.