The Lin-Shu dispersion relation is applicable in the (asymptotic) case of tight spirals (large wave numberk _R). Here we reconsider the various steps leading to the Lin-Shu dispersion relation in higher approximation, under the assumption that the wave numberk _R is not large [(k _Rr) =O(1)], and derive a new dispersion relation. This is valid for open spiral waves and bars. We prove that this dispersion relation is the appropriate limit of the nonlinear self-consistency condition in the case where the linear theory is applicable.