Auralex 4" Cornerfill
"CornerFills are sections of flat-cut Studiofoam corner acoustic absorbers that are beneficial in two ways. Aesthetically, they give you smooth, clean edges against which to butt your acoustic wall treatments and bass traps for a more professional appearance. Acoustically, they smooth out the excess low frequency energy that congregates in room corners. 4x4x24 : 9 per box (18 lineal ft.). Available colors: Charcoal, Purple, Plum, Blue, Beige, Brown, Burgundy, Red, Orange, Kelly Green, Forest Green and Teal."