Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:The two focused ultrasound beams A and B are excited at the same source pressure and at the angular frequencies xa = x0 Dx/2 and xb = x0 + Dx/2, where x0 = 2pf0 and Dx are the center and modulation angular frequencies, respectively, such that Dx x0 (f0 being the center frequency) [3,4,11].
Category: