In the Doppler effect, we see that the frequency of sound that the observer hears changes according to the motion of the observer and source. For example, if the source is moving towards the observer, who is stationary, the frequency appears to increase. But if the frequency increases, shouldn't the energy associated with the wave also increase? Where does this energy come from?
According to the sound energy equation, the energy varies with the square of the velocity of the particles ( https://en.m.wikipedia.org/wiki/Sound_energy ). But doesn't that mean that the energy will increase no matter what direction the the particle is moving in?
Isn't this contradictory to the frequency given by Doppler's equation, which says frequency(and hence te energy) decreases if the source is moving away from the observer.
As a possible solution to this, can we consider the Doppler effect to be caused not by frequency changes, but by changes in perceived intensity of sound because of distance between source and observer?