Trumpeter A holds a B-flat note on the trumpet for a long time. Person C is running towards the trumpeter at a constant velocity. Person B is running away from the trumpeter at the same rate. Person D is standing still the whole time. Which person hears a frequency that is lower than the B-flat?
Person B hears a note that is lower than a B-flat. This is because as the runner is going away from the stationary trumpeter, the sound waves become elongated, or stretched out. This increases the wavelength and decreases the frequency.
