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):

The acoustic treatment panel has been designed and tested in acoustics laboratory, so the sound absorption curves for each panel thickness are very predictable. The result is a series of products that can improve the acoustic performance of any public space. Visit here for details: https://www.fabricacousticpanels.com/acoustic-treatment/
By using a thicker fabric panel or by incorporating air gaps between the panel and the ceiling, up to 100% of the noise can be absorbed at a particular frequency.  
I treated the walls of my shoppe in Florida, behind the listening position, with a treatment similar to what you're describing.   Don't know what color you're looking for, but: I found that simple burlap, over 2" of rock wool, stapled to the strips of wood, eliminated all the slap echo in the room.    Various(but limited) colors are available:  http://www.joann.com/burlap-fabric/prd23682.html
High nrc perforated acoustic panels for walls and ceilingsPerforated ceiling panels are easy to install, easy to disassemble, and easy to use. Exposing to the grid system creates an attractive ceiling.
You can use perforated ceiling tiles for perfect sounding systems and rooms. So I thought about that I found a useful link to buy quality products for that
you can goto perforated acoustic panels

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The perforated acoustic panels are made of good material and look good. This is will make your project attractive and these panels are exported from countries like Australia, Singapore and the UK.

By installing the perforated acoustic panels at the right place they will work at their best.