online absorber calculator (acousticmodelling.com)-Acoustics Forum-Acoustic Fields Forum

Hello everyone

I found an online calculator for several absorber types: http://www.acousticmodelling.com/

It claims to be capable of calculating the absorption -coefficient for multilayer absorbers. I am interested in porous absorption of low frequencies since it is the easiest to DIY. The materials I want to use are rock wool (6 kPa*s/m2 flow resistivity) and glass wool (5 kPa*s/m2 flow resistivity). So i did a little trial and error with this calculator and I came up with these parameters:

Layer 1: 120mm porous absorbent, 6000 Pa.s/m2
Layer 2: 540mm air
Layer 3: 240mm porous absorbent, 5000 Pa.s/m2
Layer 4: not used (Rigid backing)

Using the Allard & Champoux model for porous absorbers (which should be the most accurate from what I’ve read in several articles) I got a result that showed me an absorption coefficient of over 0.8 at 50 Hz, about 0.67 at 30 Hz, and 0.5 at 20 Hz (!). Of course this construction would require 90cm (35,5”) in depth, which is more than twice the depth of the ACDA -12, but could compete at LF absorption and would be a lot cheaper.

So my question is: Is this amount of LF-absorption even possible with porous absorbers (are there even references)?

Thank you for your time :)

Cheers
Matt