HiFi room acoustic treatment

From Europe alone, native language is not English to me. I pray for understanding.
I joined the acoustic treatment of the ceiling and the wall behind the loudspeaker. On the ceiling go Rigips Rigiton 8-15-20 gypsum boards, and above them stone wool.

On the wall behind the speaker goes 15 cm of stone wool, gypsum panel with 8% perforation, and another 5 cm of stone wool.

I am concerned about the final layer of the wall. I would put 2 cm wide strips, with a distance of 2 cm (50% coverage).

However, I can not find a stone wool with the finish in the appropriate color. Can I put a thin microfibre fabric over a stone wool?
1. you are on the right track

2. yes, you could - but why?
if a diffusor it needs some depth; if an absorber, it also needs some depth

- the Master handbook of Acoustics is an xlnt English language tome or bible on what to do

what is your native language?
my native language is Croatian (Croatia is European Union country, east of Italy, on the Adriatic sea). Google translator can help, but this translation often leads to confusion, especially in more complex sentences.
I plan to cover the whole wall with wooden panels with wooden slats covering 50% of the surface. Below this I would install a 5 cm thick stone wool. If I did not want to see the stone wool between the wooden slats, I would put a thin microfiber fabric. I hope the fabric will not prevent the absorption of high tones?
Throw what I wrote, under that layer of stone wool I would put across the wall of the bass trap. Gypsum board panels with perforations 8%, and 15 cm below that of stone wool. The wall is made of clay brick 30 cm wide.
I hope it is clearer now.
btw.  Can I buy an online edition of the Master Handbook of Acoustics? In that case I could use an online translator.

well, not sure if anyone here is fluent in Croatian, or if you can buy that book - why not start checking libraries to see if it has been translated, or if something similar is published in Croatian

here are some helpful sites:

only for a rectangular room:


and here is one from Canada (which features the treatments they use in their igloos and snow caves):

Thanx Randy!