The Doppler Effect
Key Formula
| fo | observed frequency | Hz |
---|---|---|---|
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.
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