Results for the interference pattern in a reverberant sound field have been derived by means of an eigenmode analysis and compared with results obtained from the free‐wave model. It is found that the incoherent or random nature of the sound field assumed in the free‐wave model can only be achieved if the room is excited by random noise. For single frequency excitation an equivalent incoherent field can be approached by additional averages of results over many different source locations. The analytical solutions are supported by a computer study based on the eigenmode model. A detailed discussion about the correction factor due to boundary interferences in relation to sound power measurement is also given.