The investigation of biological dynamics involves many interesting aspects such as abstraction, modelling and simulation, but a very important aspect to consider is the representation of such dynamics. A correct, flexible and formal representation mechanism of interesting behaviours is the right starting point for all successive modelling steps and it can be helpful in capturing the essence of the system under investigation. In particular, provided a formal representation of the relevant traits of biological systems behaviours, the application of P systems to their modelling can become more and more fruitful. For this reason, here we present a formalism allowing us to represent biological oscillations by means of strings composed by parametric words that we call beats.