While I am studying the wave propagation in fluids, the amplitude modulation seems to be governed by the Nonlinear Schrodinger (NLS) equation. In some of the journal papers the nonlinearity parameter, N seems to be of high value (N≈O(104)) and so on. I understand that weak nonlinearity assumption is used in the paper. With N>>1 is the weak nonlinearity assumption valid? Paper: A.H. Nayfeh. "Nonlinear propagation of a wave packet in a hard‐walled circular duct." The Journal of the Acoustical Society of America 57.4 (1975): 803-809.