Passive control: design and optimization of microperforated panel (MPPs)

P. Cobo

Microperforated panels (MPPs) were proposed to provide sound absorption without any porous materials. They are specially recommended in these situations where the traditional fibrous absorbers produce health concern, such as in hospitals and the food, pharmaceutical and microelectronic industries. Sound absorption in MPPs is produced by visco-thermal losses in the large amount of submillimetric orifices made on the panel. They are tunable absorbers whose absorption curve depends on their constitutive parameters (d, the orifice diameter, t, the panel thickness, and Ø, the perforation ratio, or porosity). To provide absorption, a surface absorber must have input impedance matched to that of the air, Z0. Whilst it is possible to find a combination of parameters of the MPP affording an acoustic resistance close to Z0, within some frequency band, to decrease its mass reactance it is necessary to introduce the negative imaginary impedance of an air cavity. Therefore, a MPP absorber comprises a microperforated panel in front of a rigid wall with an air gap of thickness D in between. The performance of MPP absorbers has been widely proven.

It has been showed that there exists a tradeoff between the peak and the bandwidth of the MPP absorption curve. Its bandwidth can be increased at the cost of decreasing its peak. To avoid this tradeoff, multiple layer MPP (ML-MPP) absorbers were proposed. These ML-MPP structures are able to afford absorption both in a wide band with a large peak. Since the performance of a SL-MPP depends on four parameters (d,t, Ø,D), the design of an N-layer MPP will involve the tuning of 4N parameters.

A Virtual Instrument has been developed to facilitate the design and optimization of SL- and ML-MPPs.

Graphical User Interface (GUI) for the design and optimization of MPP absorbers

The Simulation part allows analysing single and multiple-layer MPPs.

The GUI allows to choose single-layer o multiple-layer MPPs

Normal incidence (left) and random incidence (right) behaviour of a DL-MPP/P absorber

From an engineering point of view, it would be attractive to know the combination of constitutive parameters within given variation ranges that provides the maximum mean absorption for a prescribed frequency band. This is a typical optimization problem. More specifically, it consists of looking for a global maximum in a solutions space where there may be a number of local maxima. Simulated annealing (SA) is an optimization method especially appropriate for such kind of problems.

Graphical User interface for the optimization of MPP absorbers

The Virtual Instrument allows to obtain the parameters of the system that provides the maximum average absorption within some prescribed ranges.

Optimal parameters of a DL MPP/P absorber

Nowadays, it is relatively easy to design a MPP for providing sound absorption in a frequency band of a couple of octaves. However, the widespread use of MPPs as sound absorbers in the noise control industry is far from occurring. Likely, it is due to its high manufacturing cost. For instance, a MPP providing sound absorption of two octaves around 1400 Hz should require roughly 500.000 holes/m2 of diameter 0.2 mm. Since such a minute holes are usually done by laser technology, it cost is rather dissuasive.

Some proposals have been done to reduce the manufacturing cost of MPPs. Perforating a plate in slits rather than in circular holes is cheaper. Also, combining a perforated plate (with millimetric holes) with a commercial filtering mesh (with micrometric holes) to provide a panel absorber with acoustical performance similar to that of a MPP (MIU). Although these proposals reduce the manufacturing cost of this kind of sound absorbers to about 50-100 €/m2, it is still not cheap enough to catch the attention of the sound absorbing manufacturing industry. Some recent techniques allowing to cheapen the manufacturing of MPPs are shown in the below Figure.