Two cars have identical horns, each emitting a frequency of fs = 406 Hz. One of the cars is moving with a speed of 10.5 m/s toward a bystander waiting at a corner, and the other car is parked. The speed of sound is 343 m/s. What is the beat frequency heard by the bystander?

Explanation:

Let's start finding the frequency heard by the bystander due to the moving car. We need to use Doppler effect here:

v is the speed of sound (v = 343 m/s)  

So, we have:

Now, the beat frequency heard by the bystander is the combine of the frequencies, it means the difference between them. Therefore the equation is given by:

I hope it helps you!