1. D OPPLER E FFECT

2. R ECAP FROM L AST WEEK … Intensity Level (a.k.a., loudness) Sound Intensity

3. E XAMPLE

4. T HE DOPPLER EFFECT Why does a siren of a police car sound higher pitched as it is coming toward you, but drops to a lower pitch as it moves away from you?

5. T HE DOPPLER EFFECT This phenomenon was first identified in 1842 by Austrian physicist Christian Doppler as is called the Doppler effect. It involves the concepts of velocity (of an object), wavelength and frequency (of sound)

6. T HE DOPPLER EFFECT When the firetruck is at rest, the sound waves emitted by the truck are symmetrical so a listener in front of the truck will hear the same frequency (same number of condensations per second) as a listener behind the truck.

7. T HE DOPPLER EFFECT When the firetruck begins to move, the condensations in front of the truck are closer together The sound waves “bunch up” because the truck gains ground on an already emitted condensation before emitting the next one.

8. T HE DOPPLER EFFECT - MOVING SOURCE The sound waves in front of the truck have a smaller wavelength (and higher frequency) = higher pitch The sound waves behind the truck have a larger wavelength (and lower frequency) = lower pitch Notice the wavelength of the sound perceived by the observer changes, so the observer hears a different frequency

9. T HE DOPPLER EFFECT – M OVING S OURCE Frequency heard by observer Frequency emitted by source Speed of source Speed of sound For a source moving TOWARD a stationary observer

10. T HE DOPPLER EFFECT - MOVING SOURCE Frequency heard by observer Frequency emitted by source Speed of source Speed of sound For a source moving AWAY from a stationary observer

11. E XAMPLE A high speed train is traveling at a speed of 44.7 m/s when the engineer sounds the 415 Hz warning horn. The speed of sound is 343 m/s. What are the frequency and wavelength of the sound, as perceived by a person standing at a crossing when the train is (a) approaching and (b) leaving the crossing

12. E XAMPLE - S OLUTION

13. T HE DOPPLER EFFECT – M OVING O BSERVER If the sound source is stationary, and the observer is moving, there is a Doppler shift. The observer will intercept more wave condensations per unit time than a stationary observer would. The observer is detecting a different frequency, f o.

14. T HE DOPPLER EFFECT – M OVING OBSERVER Frequency heard by observer Frequency emitted by source Speed of observer Speed of sound For an observer moving TOWARD a stationary source

15. T HE DOPPLER EFFECT – MOVING OBSERVER Frequency heard by observer Frequency emitted by source Speed of observer Speed of sound For an observer moving TOWARD a stationary source

16. A SSIGNMENT P. 504 #63-69 (pick 4) P. 504 #76-80 (pick 4) Due Thursday. Final exam 6/9 will cover waves and sound (chapters 16 & 17)