We recently converted an apartment space in a mixed use building to an office. It was very loft-like with 18ft exposed steal ceilings and mostly glass and concrete walls. It sounded like a gymnasium when I lived in it as an apartment. You could hear the neighbor walking around his living room and could barely understand someone speaking normally across the room. An acoustics expert recommended a number of changes but for a rental space acoustic panels like Armstrong Soundsoak and area rugs on the wood floors (with high quality padding below) are two things we did that are removable and not as expensive as you might think. They made up about half of the dampening effect while a drop ceiling did the rest. It now sounds like a library and we can no longer hear the neighbor at all (nor can he hear us).

I hadn't thought of the Armstrong panels. How do they install? I always assumed they were permanent (or nearly so), glued to the walls.

Ours are held on with strapping and few sheetrock screws since they are way overhead but they are very light weight. Picture hangers[1] would work for a small lowdown install or maybe some sort of double stick something... you would have to repaint and patch some holes/paint but its a huge difference. While we where waiting for them to be installed we just had them leaning against the walls like paintings waiting to be hanged. Even that was amazingly different.

http://www.caseyswood.com/shoppingcart/zen-cart/images/Sawto...

Offically! you do this with a thing called "Impaling Clips"

http://www.armstrong.com/pdbupimages/209751.pdf

https://acousticalsolutions.com/product/impaling-clip/

My new favorite product name