Micro perforated plate

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A Micro Perforated Plate (MPP) is a relative new kind of absorber. Traditional absorbers are porous materials e.g. mineral wool, glass-fibre or polyester fibres, which are impossible to use in harsh environments like in an engine compartment. Traditional absorbers have many drawbacks like pollution, fire hazard and lifetime issues. There are also allergenic and esthetical concerns. As an alternative to a porous material one can use a micro perforated plate, which is a plane plate in various materials with sub-milometer punched holes into it. An MPP is normally 0.5 - 2 mm thick. The degree of perforation is normally in the region 0.5 - 2 percent depending on the application and the environment where the MPP is to be mounted.

For a while, perforated metal panels with holes in the 1 - 10 mm range have been used as a cage for sound-absorbing glass-fibre bats where large holes let the sound waves reach into the absorbent fibre. Another use has been the creation of narrowband Helmholtz absorbers which can be tuned by hole size and the dimensions of the hole distance and air gap behind the panel. However, when the hole dimensions are in the region of 0.05 - 0.5 mm, it proves that narrow absorption peaks become much wider, making the additional fibre absorber more or less unnecessary, while still maintaining a very high absorption factor. By varying geometrical and material parameters, the acoustical performance can be tailored to meet a multitude of specifications in various applications.

The main reason why Micro Perforates has become so popular among acousticians is that they have a good absorption performance but without the disadvantages of a porous material. Furthermore, an MPP is also preferable from an esthetical point of view.

[edit] How does the MPP work?

The main idea with an absorber is to convert acoustical energy in to heat. In a traditional absorber the sound wave propagates in to the absorber. Because of the proximity of the porous material the oscillating air molecules inside the absorber looses its acoustical energy due to friction. A MPP works almost in the same way. When the oscillating air-molecules penetrate the MPP, the friction between the air in motion and the surface of the MPP causes dissipation of acoustical energy.

[edit] History

The idea of using micro perforated plates as sound absorbers originates from Professor Daa-You Maa who in the 1970's derived the first equations modelling the acoustical impedance of an MPP. Further possibilities aiming to improve the accuracy of Maa’s original model are currently being investigated. One other major phenomenon that currently being investigated is the nonlinear effect i.e. an MPP behaves differently depending on the magnitude of the incident sound wave.

[edit] References

Daa-You Maa "Potentials of micro perforated absorbers" Journal of the Acoustical Society of America Vol 104 (1975) No. 5 pp. 2868 – 2866

Helmut V. Fuchs, Xueqin Zha "Micro Perforated Structures as Sound Absorbers – A Review and Outlook" Acta Acoustica united with Acoustica Vol 92 (2006) pp. 139 - 146

Hans Bodén, Ying Guo, Hüseyin Bora Tözün "Experimantal Investigation of Nonlinear Acouatic Properties for Perforates" 12th AIAA Aero Acoustic Conference 8 - 10 May 2006 Cambridge, Massachusetts.