Absorption designer

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Permalink Reply by Arthur van der Harten on October 12, 2023 at 9:37pm

Hi Catherine,

That result is technically correct. The result of the absorption designer, or the underlying Transfer Matrix Method is 'ideal'. The assumption made is that the bottom layer is an infinitely rigid baffle with infinite impedance, or zero admittance. If you were to apply anything but a porous absorber to that, then the resulting impedance is also infinite.

The implication is that you took a solid plate, or perforated plate, and glued and screwed it to the infinitely massive layer in the most ideal way possible, so that they move (or not) as one. This is a condition that we acousticians try to get in a concert hall if wood is the specified finish. We then try (sometimes in vain) to get the contractor to fully glue (for 100% of the area) the wood to a masonry or concrete wall, and then screw it down, so it will never move again. The hope is that the wood will not absorb low frequency energy.

So yes, this is technically correct, but ideal in an unrealistic way. Note also that the ideal Transfer Matrix Method in that case will have no porosity anywhere, which is also not a realistic condition for many kinds of finishes. Sometimes it helps to add just a very thin layer of porous absorption to the top layer. How much, though - that's a judgement call.

In your pictures, make sure you know where the incident side and rigid backing are. They are labeled.

Thank you for the very good question.

Arthur