When a single higher order model is generated in a rigid rectangular or narrow annular duct, at high enough intensities, generation of higher harmonics is observed. In the case of multimode inputs, the author and his collegues have shown theoretical results indicating the existence of strong acoustic—acoustic interactions at the first order of the perturbation scheme. This interaction would lead to energy being fed into both higher and lower harmonics. In this paper experimental results are presented, using a spinning mode synthesizer, which varify the theoretical predictions. Implications of these results for some of the practical problems have been discussed.