Not sure what that black product is, but the absorbers aren't broadband...
From Harman's paper about the
Multichannel Listening Room, top of Page 5 [edit: I see that youngho already posted this]:
"The inner walls and ceiling of the double-wall IAC shell are made of heavy gauge steel panels separated 10 cm and filled with fiberglass. The inner surfaces are perforated with 2.34-mm openings
to provide substantial sound absorption inside the room. The inner walls are entirely floated and separated from the outer wall of the shell by a 10 cm space to minimize mechanical and acoustic transmission of noise." [emphasis Duke's]
That sounds to me like the walls and ceiling of the room are (broadband?) absorbers in the Multi-Channel Listening Room.
Harman's reference listening rooms are built exactly the same with same treatment at different locations in the world...[emphasis Duke's]
So I take it, then, that the double-blind-speaker-shuffler room has the same (presumably broad-band absorptive) wall and ceiling construction?
From Harman's paper again, Page 7:
"The average Tm value for the MLL is about 0.23 s, which is slightly below the calculated ITU and EBU optimal value of 0.29 s. However, the curve
falls within the minimum recommended value, and is quite uniform with frequency, only rising slightly below 125 Hz."
Whatever function the fiberglass-filled perforated walls and ceilings perform, apparently a remarkable degree of broadband absorption has been achieved in that room, and presumably in any others "built exactly the same."