I understand the physics of putting the equivalent mass in the gap, but it might be a logistical problem. The gap is 7cm wide and it would be hard to pour new concrete in, the blocks you are mentioning, would have to be custom made, to fit in the gap, also there would be problem attaching it.
But i was thinking if it would also be of any benefit, to try to fill the gap as much as possible with small strips of PB? its 12,5mm and 2,5 m. cuts easily and i could attach it to the existing concrete wall with mason mud, and the rest i could srew them on top of each other?
next step/question would be, what do you suggest about closing the window opening all together?
I have three windows in the room i want to put up an iso booth, (atttached is just a sketch for now. The greyed out window is in question. 220cm high, 73cm wide, 27cm deep.)
in two of them i will install heavier windows as suggested in the previous comments.
That leaves one opening that would be closed with iso booth anyway.
Brick and mortar for that as well? In case i would once leave the offices and would want to avoid removing the bricks, is it an option to use double or triple PB.
Between the window and the PB i would fill with insulation. I could even sort wasted PB chunks (cut to size) on top of each other?
Briliant.. that way i could screw the final frontal Pb directly into the plywood. No side joists necesary.
Given the height of the window opening, would you suggest having 2 chambers/ compartments for sand filing?