What would be the cheapest way to reduce echo in an empty room?
I'll be having an empty room in my apartment for perhaps a few months and right now, there's nothing I could put in there, except perhaps for some empty cardboard boxes.
The room has an open entrance to other rooms, so whenever someone is doing something in most other rooms (even not so noisy activities), this creates a surprisingly loud and actually really annoying echo that can be heard everywhere.
The room is about 23 feet long and 13 feet wide, the entrance is about 6 feet wide.
I wouldn't really want to try blocking the entrance, because you need to be able to go through that room to get to the balcony. There's also a lot of daylight coming through that open entrance, blocking it would make other rooms significantly darker.
Assuming I would put some cardboard boxes in there, where exactly should I put them for most efficiency?
What other, cheap options are there and how would I do it?