Two automobiles are equipped with the same singlefrequency horn. When one is at rest and the other is moving toward the first at 20 m/s , the driver at rest hears a beat frequency of 9.0 Hz. Requried:
What is the frequency the horns emit?

f ≈ 8.5Hz

Explanation: The phenomenon known as Doppler Shift is characterized as a change in frequency when one observer is stationary and the source emitting the frequency is moving or when both observer and source are moving.

For a source moving and a stationary observer, to determine the frequency:

where:

is frequency of observer;

is frequency of source;

c is the constant speed of sound c = 340m/s;

is velocity of source;

Rearraging for frequency of source:

Replacing and calculating:

8.5

Frequency the horns emit is 8.5Hz.