25mm of absorption is so thin that it will only absorb the high frequencies and sound the same as turning down the treble knob on your receiver.
If you are going to absorb, then you should make the absorbers broadband (absorb equally at all frequencies down to about 200Hz). This way, the panels won't act like a tone control.
To get broadband absorption, the panels need to be at least 150mm thick. Or, if you want to save some money, make them 100mm thick and mount them with a 100mm air gap between the wall.
I would agree to disagree!, broadband absorbers and bass traps are 2 completely different things with different functions and purpose.
While bass traps are used for absorbing low frequencies from 20hz - 120hz(With Membrane inside) , the broadband absorbers are used for absorbing high and mid range frequencies.
With a 50mm insulation you can make a broadband absorber and with 100mm insulation u can possibly achieve those 250hz above frequencies.
To absorb them 40hz - 120hz frequencies you would have to build a bass trap with a membrane inside and a vacuum box also leaving a cavity in between the bass trap and the wall of around 50mm depending on the room size and /or if its a tri trap or a regular bass trap.
And honestly there is no difference between 100mm or 150mm cause u definitely need the membrane inside the bass trap, 150mm insulation would do exactly what the 100mm insulation would and i speak here from experience.
Also im talking about at least 96kg/m3 density of glass wool only, not rock wool! (Preferably Owens Corning 705's)
Besides, these days there are a lot of new synthetic absorptive material that one can sandwich between the traps and absorbers but you still wont be able to achieve the results to equal up to a bass trap with a membrane.
The bad side of DIY traps and absorbers, one would never be able to test them NRC for the build. The tests itself cost about 30k per test, you can always Mode test your room before the installation and after to check the absorbed frequencies but then again how many here have the knowledge or the time for that matter.
People spend lacs in pro audio gear, but little did they know about room acoustics? You can buy that denon 300k amp for your home theater but if your room is not acoustically right then what good is it?
So i would suggest to find a professional in room acoustics who would charge you a bomb or find some one like these guys cueacoustics.in who manufacture ready to use acoustical products or go the DIY way and buy that expensive home theater system and save on the acoustics, the choice is yours.
Sound is perceived, its a perception of a human mind.
P.S. : I have no affiliation with the manufacturer i mentioned earlier, it was for info purpose only.