First: Acoustic panels are used to make the room sound better by reducing or eliminating reflections and excess energy that muddy and smear what you are listening to. They aren't for soundproofing - that's something entirely different. You'd use these in a home theater or music room to improve the experience.
As the car guys say: "There's no replacement for displacement!"
Thin panels only work on higher frequency sound and don't even put a dent in the real problems that exist in all small to mid-sized rooms. These are 5-1/2" (140MM) thick and are made to be spaced off the wall by that same amount. That gap between the panel and the wall makes the panel more effective, especially for lower frequency sound.
The interesting part of this build is how I fastened the cloth using the grooves and splines. I didn't want to use a billion staples to attach the cloth to separate frames and then try to figure out how to attach the frames to the boxes to make it all look neat. The splines worked perfectly and greatly simplified the process.
The thumbnail shows what I want to do on the front of these - a black and white rendition of 6 of my favourite albums. I haven't done that yet - I need to make (or have them made) stencils to paint the cloth.
You can watch this video with narration here:
   • How To Make Real Acoustic Panels | In...  
It also has measurements from the room these panels were used in to show how much of a difference they made. Of course listening it is the real test and I can tell you that the improvement in the room is easy to hear.

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