1. The Doppler Effect
THE LAST LECTURE

2. What is the Doppler Effect?

3. The Doppler Effect
The relative motion between the sound source and the receiver causes the Doppler Effect – resulting in different frequencies being heard

4. The Doppler Effect
If the source is moving away from the receiver, the received frequency is lower than the source (more distance between sound waves)
If the source is moving toward the receiver, the received frequency is higher than the source (smaller distance between sound waves)
REMEMBER: Sound frequency relates to pitch (Hz)
Sound amplitude (energy) relates to loudness

5. The Doppler Effect
Therefore a siren on an emergency vehicle or a whistle on a train - has a high pitch as it moves toward you, and drops to a low pitch as it passes you
App

6. Calculations
The following equation is used to calculate the apparent frequency of the source:
vo is: + if moving towards the source, - if moving away from the source
vs is: – if moving towards the receiver, + if moving away from the receiver

7. Sample Problem
A car is travelling 29 m/s toward a stationary sound (whistle) that has a frequency of 625 Hz. If the speed of sound is 337 m/s, what is the apparent frequency of the sound as heard by the driver of the car?

8. Sample Problem
If the car in Eg.1 was travelling away from the stationary sound, what would be the apparent frequency of the sound heard by the driver of the car?

9. Homework
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