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In order to assess the ultrasound dual frequency effects, sonochemical degradation of p-nitrophenol (p-NP) in an aqueous solution has been carried out with ultrasound at three operating frequencies, i.e., at 25, 40 kHz each independently, and the combination of two frequencies (25+40 kHz) simultaneously. Based on the rates of degradation, a kinetic study has been performed which leads to the evaluation of apparent kinetic rate constants for the degradation of p-NP. The influence of various parameters including initial solution pH, bulk solution temperature, on the degradation of p-NP was studied for these three frequency modes (25, 40 and 25+40 kHz) in order to investigate the temporal behaviour of this reaction, especially when it was operated in combined mode (25+40 kHz). The energy efficiency in the case of dual frequency mode is much better than single frequency modes. Modelling and cavity dynamics simulations have also been carried out to explain the observed effects. During combined mode operation, an improvement in the rate of degradation has been observed. The variation in the rate constants has been explained based on the difference in the acoustic pressure field in different systems including ultrasonic bath and dual frequency processor.