In this work, the change of frequency of a sound wave due to the Doppler effect has been measured using a smartphone. For this purpose, a speaker at rest and a smartphone placed on a cart on an air track were used. The change in frequency was measured by using an application for AndroidTM, `Frequency Analyzer¿, which was developed by us speci¿cally for this work. This made it possible to analyze four types of mechanical motions: uniform linear motion, uniform accelerated linear motion, harmonic oscillations and damped harmonic oscillations. These experiments are suitable for undergraduate students. The main novelty of this work was the possibility of measuring the instantaneous frequency as a function of time with high precision. The results were compared with alternative measurements yielding good agreement. ; Gómez-Tejedor, JA.; Castro Palacio, JC.; Monsoriu Serra, JA. (2014). The acoustic Doppler effect applied to the study of linear motions. European Journal of Physics. 35(2):25006-25015. doi:10.1088/0143-0807/35/2/025006. ; Senia ; 25006 ; 25015 ; 35 ; 2