Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 587…
“ Concerning those who stand outside, if they hear the sound of the shofar, they fulfill their obligation. If they hear the echo, they do not fulfill their obligation. Similarly, if one sounds the shofar into a large barrel or something similar, those who hear the sound of the shofar fulfill their obligation and those who hear the echo do not. ”
