Sound: The Doppler Effect

What happens when sources and/or observers of waves move?

If you have ever heard a speeding amulance siren passing you on the road, you have already experienced the Doppler effect! The Austrian physicist Christian Doppler first explained what happens when sources and/or observers of waves move in 1842. The wavelength of a wave changes when its a source is moving relative to the medium in which the wave travels. Since the wavelength changes while the wave speed does not, and the frequency of the wave will also change.

Experiment with the simulation below to investigate the Doppler effect. Gradually increase the speed of the airplane, keeping track of the wavelengths of the waves traveling ahead and behind it. Since the airplane moves to the right before it emits the next wavefront, it will have moved toward the wavefront that was emitted in the forward direction and away from the wavefront that was emitted in the backward direction. As a result the spacing between adjacent wavefronts will be smaller in the forward direction and larger in the backward direction. Notice that once the wavefron leaves the airplane, it travels at the wavespeed in air, which is completely independent of the airplane's speed. Imagine walking on a bridge over a very still lake, dropping pebbles in the water at a constant frequency as you walk. The ripples move at a constant speed away from where each pebble hits the water, regardless of how fast you are walking!

  What happens if you increase the airplane's speed to the point where it is traveling faster that the wavespeed? When airplanes travel faster than the speed of sound, they outrun the sound waves that are emitted directly ahead of them, resulting in a shock wave where the wavefronts are strongly compressed. What happens to the shockwave angle as the airplane speed inceases even further? Since the wavespeed of water waves is on the order of several meters per second, it does not take much speed to outrun water waves, and similar shockwaves can be easily seen coming off the bows of ships or boats, or even fast human swimmers!

Use the above simulation to answer the following questions?