AtlasIED BIA-100 Bi-Axial Horn Speaker with Twin Reflex Air ColumnModel BIA 100 is the original reflex horn featuring a bi axial design with twin air columns and exponential bell for wide angle coverage with a linear frequency response. The horn provides excellent acoustic loading maintained to a 350 Hz low frequency roll off and optimizes delivery of high and mid range signal levels. BIA 100 is designed for use with AtlasIED PD Series compression drivers and models having 13 8" 18 male threads and 0. 707" dia.