You are playing middle A (440 Hz) on the saxophone on a street corner. A trombone is playing middle A (440 Hz) in the open bed of a pickup truck as it approaches you. You hear a beat frequency of 4 Hz. What frequency do you hear from the trombone and what property of sound waves is being demonstrated in this problem? The frequency is either 444 or 436 Hz, and the property is the Doppler Effect, Fundamental Frequency, Harmonics, Speed of Sound in Air. (I think it's Doppler Effect.)