Using the D.A.S. Audio LX-218A self-powered subwoofers, the system tech team chose a cardioid arrangement, and whilst solving the problem of L/R cancellation “alleys” by arranging the subs in a line, this creates a second problem: narrowing the overall field, so bass cancellation moves form the middle of the audience to the edges. This is, in turn, solved by creating a “virtual arc” – delaying the subs that are furthest away. The various stacks are then processed in groups and with some careful measurements, phase can be adjusted so the summing is perfect. Of course, a similar process is then required for the cardioid array to prevent a build up of bass frequencies on the stage.
Festival sound systems