The Doppler Effect
|fs||frequency of source||Hz|
|v||speed of sound||m/s|
|vo||velocity of observer||m/s|
|vs||velocity of source||m/s|
Tips to Remember
- In this formula, the key to success is following the sign conventions. The direction that the sound travels is always the positive direction; set the other velocities accordingly. For example, consider a problem in which a person and a barking dog run toward each other as shown at the right. The sound travels to the left, so everything moving left must have positive velocity, and vice versa. Therefore v is positive (as always); vo is negative since the human obersever is moving right, and vs is positive because the dog (the sound source) is running left.
- It’s always a good idea to make sure your answers make sense. Remember that when the source and observer are getting closer together, you expect the frequency to increase, i.e., fo > fs . Conversely, when the source recedes from the observer, expect a decreasing frequency, or fo < fs.