# doppler effect and sound waves

• May 20th 2008, 01:44 PM
checkmarks
doppler effect and sound waves
A sound wave of frequency 300Hz is emitted towards an appproaching car. The wave is reflected from the car and then received back at the emitter at a frequency of 315 Hz. If the speed of sound is 240 m/s, what is the velocity of the car?
• May 20th 2008, 01:54 PM
CaptainBlack
Quote:

Originally Posted by checkmarks
A sound wave of frequency 300Hz is emitted towards an appproaching car. The wave is reflected from the car and then received back at the emitter at a frequency of 315 Hz. If the speed of sound is 240 m/s, what is the velocity of the car?

If the range rate of the car is \$\displaystyle v\$, then the recieved frequency will be \$\displaystyle f_0 (1-2v/c) \$, where \$\displaystyle c\$ is the speed of sound and \$\displaystyle f_0\$ is the source frequency, the \$\displaystyle 2\$ is because this is a two way doppler.

Now an approaching car has negative range rate, so \$\displaystyle v=-v_{car}\$ , where \$\displaystyle v_{car}\$ is the car speed.

RonL