The Pound-Rebka experiment is a famous experiment to test the theory of general relativity and is taken as paradigm for probing that Einstein's relativity is true. In this experiment there are two Doppler effects involved, namely the gravitational Doppler effect predicted by GR (General Relativity) and the inertial Doppler effect predicted by SR (Special Relativity). Each kind of effect is modelled by its own equations. In this experiment, the aim was to balance both effects in order to attain a null effective Doopler effect, so electromagnetic frequency resulted to yield the same measured value as the original emitted frequency. That implied that if photons were emitted from the top of the tower towards the ground detector, the gravitational Doppler effect would be a Doppler blue shitf, it is to say, a frequency increase. But, if those photons were emitted fron the ground towards the top of the tower, the detector sited in that top would measure a lower frequency for the same gravitacional Doppler effect. In order to achieve the relative inertial movement required by SR, the emission source of photons was placed in the center of a loudspeaker cone, so by vibrating the speaker cone the source moved with varying speed, thus creating varying Doppler shifts.