I knew a ballet group that was meeting in a large warehouse room -
they covered the walls with large grey cardboard egg flats - you may
have seen them, they carry more eggs than the traditional cartons,
though those can also be used. I think one of their members/friends
had a chicken ranch....Not sure where you can get them, or how much
they cost, but they do absorb sound and keep it from reflecting all
This is what can happen when you improvise with sound deadening material:
Cardboard? Cardboard!? CARDBOARD!!!