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:where I(r, z, t) is the dynamic component of the acoustic intensity due to the interaction of the pressure waves [3–5], vi = pi/q0c0, i = a, b, describes the particle velocity for each beam -q0 being the ambient density of the medium – and * denotes the complex conjugate.
Category: