In this work we present a simple and low-cost setup to illustrate the dependence of the behaviour of a standing wave in a guitar string with the initial conditions. To do so, we impose two kinds of initial conditions; in the first instance, the initial shape of the string is varied. Secondly, different nodes are imposed on the string. This dependence was studied using the Fourier analysis of the sound produced by the vibration of the string with a smartphone. The simplicity of the proposed activity makes it suitable to be implemented in any classroom to illustrate the concept of normal modes and as an example of Fourier series in a real system that is also familiar for the students.