Angel is setting off fireworks on New Year's Eve. The fireworks are plugged into the ground. Fireworks shoot upwards at a speed of 20 m/s and emit a sound with a frequency of 936 Hz. After moving 1.2 seconds, the fireworks explode and produce a sound with a lower frequency of 609 Hz, but with an amplitude 10 times the previous sound. Just before the fireworks shot upwards, Angel retreated at 5 m/s, then stopped as the fireworks began to slide. It is known that the speed of sound in air is 340 m/s. a. How did Angel hear the fireworks for the first second?

b. What frequency does Angel hear when the fireworks explode?