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Doppler Effect: Sound Frequency Shifts

Explore the Doppler effect, its impact on sound waves, and how to calculate frequency changes. Includes Doppler ball demos and calculation examples. Enhance your knowledge with activity sheets and practice questions.

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Doppler Effect: Sound Frequency Shifts

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  1. Key Areas covered • The Doppler effect is observed in sound and light • The Doppler effect causes shifts in wavelengths of sound and light

  2. What we will do today • State what is meant by the Doppler effect • Describe how the Doppler effect affects sound and carry out an experimental demonstration to show this • Carry out calculations on the above

  3. The Doppler Effect

  4. Original Doppler Experiment • Doppler Shift : Twig • Your teacher will now also show you a demonstration with a Doppler ball

  5. What is the Doppler effect? • The Doppler effect is the change in frequency you notice when a source of sound waves is moving relative to you. • When the source moves towards you, more waves reach you per second and the frequency is increased. • If the source moves away from you, fewer waves reach you per second and the frequency is decreased. • Doppler Shift Demonstrator

  6. Moving source • As you can see from the simulation (and teacher demo) as you move closer to a source, the frequency you, the observer, receives increases. • As you move away from a source, the frequency you receive decreases.

  7. Calculating the frequency • When a source produces a sound of frequency fs, we can calculate the observed frequency, fo, using a formula. • The formula changes slightly depending on whether you move towards the source or move away from the source.

  8. Calculating the frequency Moving towardsthe source • The observed frequency, fo, is higher: • fo = fs v . (v - vs) • fs = frequency of source • v = speed of sound (approx 340ms-1) • vs = speed of source • Towards = Take away

  9. Calculating the frequency Moving awayfrom the source • The observed frequency, fo, is lower: • fo = fs v . (v + vs) • Away = Add

  10. Example 1 • What is the frequency heard by a person driving at 15 ms-1toward a blowing factory whistle (f = 800 hz) if the speed of sound in air is 340 ms-1? • fo = fs v . (v - vs) = 800 340 . (340-15) = 800 x 1.04 fo = 837 Hz

  11. Example 2 • What frequency would he hear after passing the factory if he continues at the same speed? • fo = fs v . (v + vs) = 800 340 . (340+15) = 800 x 0.958 fo = 766 Hz

  12. Questions • Activity sheets: • The expanding Universe • You should now be able to answer questions 1-18 in your class jotter

  13. 1. A = higher; B= lower; C = Doppler; D = effect 2. (a) and (b) Teacher Check 3. (a) 412 Hz (b) 389 Hz (c) 800 Hz (d) 1035 Hz (e) 20 ms−1 (f) 60 ms−1 4. Teacher Check 5. (a) 1063 Hz (b) 944 Hz 6. (a) 216 Hz (b) 186 Hz 7. 14·5 ms−1 8. 466 Hz 9. 556 Hz, 525 Hz 10. (a) 1·07 m (b) 1·2 m 11. (a) 971 Hz (b) 1030 Hz 12. 37·8 ms−1 13. (a) 354 kHz (b) Decrease – denominator is larger (c) 345 kHz 14. (a) Teacher Check (b) 1569 Hz (c) 1437 Hz 15. 21·3 ms−1 16. (a) Teacher Check (b) 2106 Hz (c) 2302 Hz 17. 154 s 18. 120 ms−1 19. A = longer; B = red; C = shorter; D = blue; E = away 20. (a) 1·01 × 10−1 (b) 5·07 × 10−2 (c) 525 nm (d) 682 nm (e) 434 nm (f) 365 nm Answers

  14. 2013 Revised Higher C

  15. CfE Higher Specimen Paper B

  16. 2015 CfE Higher E

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