If a wall is a node, then the nearest other node at any frequency will be 1/2(wavelength) away from the wall. Given this, the antinode is midway between those two points, or 1/4(wavelength) away from the wall, for any given frequency. So, for example, if you want to filter out 60Hz, divide 18’ (the wavelength of 60Hz) by 4 = 4’6″ and hang a thin layer of frictional material (such as fiberglass or acoustic foam) at that distance from the wall(s) and floor and ceiling.