A very strong dependence of sonochemical reactivity as a function of the transducer design is put into evidence. Moreover, this dependence is completely different when considering homogeneous, heterogeneous liquid-liquid or heterogeneous liquid-solid reactions. These surprising results, that show the complexity of sonochemical reactors, can be understood if a precise description of the reactor is undertaken. Different experimental tools were used and proved to be very useful in this way. Because of that complexity, the design and optimisation of sonochemical reactors is still an open problem and a very promising model that predict the acoustic field and the acoustic streaming is proposed and validated.