The previous post made me think a bit (https://www.avforums.co.za/index.php/topic,90866.0.html), I found this calculator that can graph the effectiveness of a porous absorber over the audible frequency spectrum. I found the flow resistivity for the 100mm of 48kg/m3 fibreglass I use, it was 8140, and added the air gap of 24mm. (Got the panels from Isover)
If you can / have space, the air gap will increase your absorption to a point. The gap should not be more than the actual panel, then you'll get a dip in the midrange absorption. If you add a 100mm gap behind a 100mm panel, it acts as a 200mm panel effectively, so you can double your panel thickness "for free". Even the 24mm makes a difference. I also plotted a "bass trap", if you go 300mm thick with 300mm air gap, it's about optimal itv bass absorption.
Here is the graph (As you can see such a panel is very effective down to the room transition frequency) :
https://imgur.com/a/LH9ziSv
And the calculator :
http://www.acousticmodelling.com/porous.php
And a link to how my panels look, before I added a gap. With some room measurement graphs (note - the Freq response is VERY smoothed, it was for me to look at the shape of the house curve. I also tuned it down a lot below 20Hz, it goes flat to 15Hz, but it was too much low freq energy for me) :
https://imgur.com/a/CQeWjfU
If you can / have space, the air gap will increase your absorption to a point. The gap should not be more than the actual panel, then you'll get a dip in the midrange absorption. If you add a 100mm gap behind a 100mm panel, it acts as a 200mm panel effectively, so you can double your panel thickness "for free". Even the 24mm makes a difference. I also plotted a "bass trap", if you go 300mm thick with 300mm air gap, it's about optimal itv bass absorption.
Here is the graph (As you can see such a panel is very effective down to the room transition frequency) :
https://imgur.com/a/LH9ziSv
And the calculator :
http://www.acousticmodelling.com/porous.php
And a link to how my panels look, before I added a gap. With some room measurement graphs (note - the Freq response is VERY smoothed, it was for me to look at the shape of the house curve. I also tuned it down a lot below 20Hz, it goes flat to 15Hz, but it was too much low freq energy for me) :
https://imgur.com/a/CQeWjfU
