Not to defend Fremer, but walls of records, edge on, are a damned good start for treatment of the generic room.
To much may not be a good thing.
Cardboard can absorb sound waves. To much cardboard can absorb a lot of sound waves.
I remember several years ago being in a medical records room with several rows of medical records stored in cardboard folders. Just going from memory each row was about 40ft long.
I was stranding at one end of a row and a fellow worker at the other end of the same row. I asked the worker a question at a slightly elevated speaking voice due to the length, distance, we were apart from one another. I could see his lips moving but did not hear anything. To make a long story short when hollering at one another neither of us could hear the other. The sound waves of our voices were being absorbed by the cardboard file holders.
What frequencies are most easily affected?
.