I drive my car at VL= 18m/s down a road. Two motorcycles are also driving down the road, both at 20 m/s. When one motorcycle is ahead of me, and one behind me, I hear a beat frequency. The motorcycles' engines both produce a frequency of 400 Hz. The speed of sound is 344 m/s. When a motorcycle is 3m away, the intensity of the sound from the engine is 0.6W/m^2.
1. What is the sound intensity level (in decibels) of a single motorcycle engine at 3m.
2. If both motorcycles are 3m away, what is the sound intensity level (in decibels) of the sound from both of their engines combined?