Predicting the absorption coefficient of micro-perforated plates

Aluminium Honeycomb perforated panel

Aluminium Honeycomb perforated panel

I was playing around with the topic of this blog post on my laptop in the train a while ago (while reading the paper "Predicting the absorption of open weave textiles and micro-perforated membranes backed by an air space" by Kang et al), and thought it might make a good topic for a blog post.

Micro-perforated plates

What are micro-perforated plates/membranes? I'll just quote Wikipedia's concise explanation: a thin [...] plate, made from one of several different materials, with small holes [...] in it. An MPP offers an alternative to traditional sound absorbers made from porous materials.

The basic idea is this: because the holes are so tiny, sound gets absorbed while passing through the holes due to friction. A nice example of micro-perforated plates are transparent membranes, which you can see through while they still offer sound absorbing qualities.

Designing micro-perforated panels

I wrote up some equations from the paper in JavaScript. The equations give the absorption coefficients for normal incident sound and a diffuse sound field. Note that the following equations are only meant for designing flat, micro-perforated plates with a relatively small closed air space behind them. Also, I take no responsibility for any errors in the calculations, although I noticed that the calculations did seem to agree with the plots in the paper.

This is currently for non-metallic substances only (as given by the paper), let me know if someone out there wants me to implement metallic membranes as well (or equations given by some other paper)!

If you're at a loss as to what to try, you can try the following values: hole width: 0.3 mm, hole spacing: 3.3 mm, cavity depth: 0.1 m, mass: 0.19 kg/m2, thickness: 0.17 mm.

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