Try your speaker with a test mounting in a large rigid cardboard baffle or box. You should hear a significant change in speaker tone and base response. Obviously not the same as your actual ceiling joist cavity, but should be an audible difference.

If your ceiling speakers are open back, consider packing some accoustic wool ( Rockwool (née Roxul) mineral wool type material) into the joist cavity, and perhaps boxing in the cavity with vertical material to create a proper box of sorts up there.