1. A POROUS ACOUSTICAL TILE CONSISTING ESSENTIALLY OF AN EXPANDED MINERAL AGGREGATE IN PARTICULATE FORM BONDED TOGETHER BY A FIRED THIN CLAY COATING ON THE PARTICLES LEAVING INTERCONNECTED VOIDS IN THE TILE, THE CLAY COATING INCLUDING BENTONITE GROUP AND KAOLINITE GROUP CLAYS IN WHICH TILE THE PROPORTIONS BY WEIGHT ARE APPROXIMATELY FROM ABOUT 60 TO 80% PERLITE, 13 TO 26% KAOLINITE GROUPCLAY, AND 6 TO 14% BENTONITE GROUP CLAY.