If you are playing your saxophone at middle A (440 Hz) and a trombone also playing middle A (440 Hz) in the open bed of a pickup truck is approaching you. The beat frequency you hear is 4 Hz. What frequency do you hear from the trombone? The formula I know I am supposed to use is: fbeat=f1-f2 but for my question what is f1 and f2. two answers i get depending on me putting 440 for f1 and f2 and the fbeat of 4 is 440 and 436. Which one is right?