Solid-liquid mixing is widely used for many reactions and crystallization. In this operation, particle abrasion and damage to the apparatus due to particle collisions become an important issue. Therefore accurate prediction of collision frequency is indispensable for the optimization of operational conditions and equipment design. This presentation will discuss a correlation equation for particle collision frequency with an impeller. The influence on the collision frequency of operational conditions such as impeller speed, particle size, solid-liquid density difference and impeller form was examined based on a collision sound detection method using wavelet analysis. It was found that the particle often collided with the impeller at a time interval of several tens of milliseconds. The mechanism of this important phenomenon for estimating collision frequency was clarified. The correlation equation was derived as a function of collision energy by experimentally establishing a model in which collision sound pressure has a log-normal distribution independently of operational conditions.