So in the case where the wave source is moving towards a stationary observer, the observer will observe a frequency f' where f'=f(v/(v-vs)) f= frequency of wave v=velocity of wave vs=velocity of source moving towards observer I understand that the increase in observed frequency is caused because the wavelength of the wave is shortened since the source is moving toward the observer. In other words, more waves can be crowded into the area between the source and observer than if the the source was at rest. My question is this, why does the velocity of the waves emitted not change when the source is moving? Why does the observed frequency increase as a result of decreased wavelength and not as a result of increased velocity? Please tell me if my question is not clear so I can rephrase.