The equation of progressive wave is given by $$Y = \sin \left[ {x\left( {{t \over 5} - {x \over 9}} \right) + {\pi \over 6}} \right]$$ cm. Which one of the following is correct?

Two cars approach a stationary observer from opposite sides as shown in the figure.

The observer hears no beats. If the frequency of the horn of the car B is 504 Hz, then the frequency of the horn of the car A will be