Micro perforated plate

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A Micro Perforated Plate (MPP) is a relative new kind of sound absorber. Traditional sound 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-millimeter 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.


Microperforation refers to the placement of holes or apatures in a material. Microperforation can be inserted in a web with needles, pins, laser, high pressure air, open flame or high pressure water jet.

Packaging films are often microperforated in order for the packaged contents to breathe. Other applications include medical to allow wounds to breathe, sound and vapor barriers and other applications.

The most simple and cost effective method of microperforation is with pinned rotary tooling.

Image:http://upload.wikimedia.org/wikipedia/en/3/36/3rollers.gif

These high precision tools can be integrated into an existing line of machinery, or built as a stand alone unit. Pins and needles can create two types of microperforations, hot and cold. Cold perforation simply refers to a cold pin that penetrates a material. In materials that have elasticity, this can result in a "volcano" hole that is preferred in many applications. In more brittle films, cold perforation can cause slitting rather than creating a round hole, which can jepordize the materials integrity under pressure. The solution to this issue is often Hot needle perforation. Hot needle perforation melts a hole in the material, causing a re-enforced ring around the hole. This ring of molten material aids in the integral stregnth of the final product. Pinned perforation rollers can be manufactured to house up to 1250 pins per square inch, with hole sizes down to 70 micrometres or so. Pinned sleeves can be interchanged to achieve different perforations. Pinned rollers or pinned plates can be also used to microperforate materials.

There are few companies in the world that manufacture high caliber rotary microperforation tooling, Micro perforation equipment, and micro perforation machinery. One of these manufacturers are in the United States: Stewarts of America Pins & Pinned Products, Inc.

Laser perforation is a much more expensive version of perforation. Laser perforation simply burns a hole in the material being processed. One laser system offers flexible hole sizes and patterns. This is an example manufacturer in the USA: ORCA


Contents

[edit] How the MPP works

The main idea with an absorber is to convert acoustical energy into 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 lose their 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 1970s derived the first equations modeling 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

  • 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 "Experimental Investigation of Nonlinear Acouatic Properties for Perforates" 12th AIAA Aero Acoustic Conference 8 - 10 May 2006 Cambridge, Massachusetts.

[edit] Useful Links