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Sound Waves: Types, Speed, Intensity, Doppler Effect, and Doppler Shift in Light

Explore sound waves, speed calculations, intensity measurements, logarithmic scales, and the Doppler effect. Discover how Doppler shift applies to light and its relevance in astronomy.

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Sound Waves: Types, Speed, Intensity, Doppler Effect, and Doppler Shift in Light

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  1. Announcements 10/8/12 • Prayer • Exam 1 – one problem left to grade Pearls Before Swine

  2. From warmup Extra time on? (nothing in particular) Other comments? I dreamed that we all failed our tests and you were driving a mechanical claw machine and picking us up with the claws and throwing us off the SWKT.....?......?

  3. Sound Waves What type of wave? What is waving? Demo: Sound in a vacuum Demo: tuning fork Demo: Singing rod Sinusoidal? Not necessarily. Demo: musical disk

  4. Speed of sound Speed of sound… in gases: ~300-1200 m/s in liquids: ~1000-1900 m/s in solids: ~2000-6000 m/s v = sqrt(B/r) compare to v = sqrt(T/m) Speed of sound in air 343 m/s for air at 20C Dependence on temperature (eqn in book and also given on exam)

  5. Intensity • Intensity: power/area • “Spherical waves” • Non-spherical waves? • Question: you measure the sound intensity produced by a spherically-emitting speaker to be 10 W/m2 at a distance of 2 meters. What will be the intensity at 8 meters away? • Question: What is the total sound power (watts) being produced by the speaker?

  6. From warmup If the "sound level" of a noise changes by 20 dB, by how much did the intensity change? 20 dB = a factor of 100 change in the intensity. I ~ 10^(dB/10) (only ~6 got this right) add 10 to b 10 to I β = 10 log( I / Io ) I0 = ? Solve for I?

  7. Decibels Threshold of hearing 0 dB 10-12 W/m2 Whisper 30 dB 10-9 W/m2 Vacuum cleaner 70 dB 10-5 W/m2 Rock Concert 120 dB 1 W/m2 Nearby jet airplane 150 dB 1000 W/m2

  8. Logarithm Review • Log10(x) is the inverse of 10y → if x = 10y then y = log10(x) • I.e. “10 to the what equals 22?” answer: 1.3424 calculator: log10(22) • Review of “Laws of Logs”: • 1. log(ab) = log(a) + log(b) • 2. log(an) = n log(a) • log10(100) = ? Translation: 10 to what equals 100? • ln(100) = ? (“ln” = loge = log2.71828…) Translation: e to what number =100? (4.605…) • Ambiguity: “log(100)”…could be either log10 or ln • Question: log10(1,000,000) = ? • Question: If log(3) = 0.477, what is log(300)?

  9. Other Power and Intensity Scales Power or Intensity • sound • dB β = 10 log(I/I0) I0 = 10-12 W/m2 • microwaves/rf • dBm β = 10 log(P/P0) P0 = 1 mW • electronics/electrical circuits • dB β = 10 log(P2/P1) (ratio only)

  10. Clicker question: • A +3 dB increase is just about a factor of 2 in intensity. How many dB represents a factor of 4 increase in intensity? • +4 • +6 • +8 • +9 • +10

  11. Clicker question: dBm: β = 10 log(P/P0) P0 = 1 mW • How much power is -30 dBm? • 0.001 mW • 0.003 mW • 0.030 mW • 0.100 mW • You can’t have negative dBm because you can’t take the log of a negative number

  12. Clicker question: • If 30 dBm is 1 mW, how much power is 33 dBm? • 1003 mW • 1006 mW • 1100 mW • 2000 mW • 3000 mW

  13. Question: • You have a 1 volt amplitude sine wave. You want to go up 3 dB in power. How many volts do you need? (Recall: Power ~ amplitude2; true for voltages, sound, and light waves as well as waves on a string)

  14. Doppler Effect • Demo: Doppler Speaker • What happens if the source is moving? • What happens if the observer is moving? • Key point: Frequency is _______________when the source and observer approach each other, ______________ when they go away from each other • Stokes Come, Come, Ye Saints recording • http://stokes.byu.edu/bells.wav (0:32)

  15. The Pie Factory vbelt • Spacing between pies = ? •  = v/f • vs source speed • vo observer speed • v speed of sound (vbelt) • If observermoves toward source (pie maker), she would measure the same ___________ but the pies are coming at her at a faster ________ • If sourcemoves toward observer, the __________ shrinks, but the pie _______ doesn’t change f = vnew/lold f = vold/lnew lnew=(vbelt-vs)/fs wavelength speed wavelength speed

  16. Doppler, cont. • Combine both effects: • What does  mean? • Stokes Flash video • http://stokes.byu.edu/teaching_resources/doppler_script_flash.html (1:50)

  17. Doppler shift of light • Distant light source • Traveling toward you • Traveling away from you • See HW 19.4 for equation

  18. Astronomy • Edwin Hubble, 1929: Distance to galaxies is proportional to their speed • Distance measured through Cepheid variable star observations, “standard candle” • How did he measure speed? • Doppler shift of spectral lines! • That’s now a standard technique for today’s astronomers when they want to measure distance to far away objects… just measure Doppler shift. • Hubble’s Law and the Big Bang • (Yes, it’s OK for LDS to believe in the Big Bang…)

  19. From warmup Section 17.4 discusses both the Doppler effect and shock waves. How/why are they related? Hint: compare Fig. 17.10 to Fig. 17.11 With both sound waves and shock waves if the source has a velocity then the side towards which the wave is moving, will experience a condensing of the waves versus having the waves being in the exact center of each wave on all sides. The side in the same direction of the velocity will have less distance between waves than the opposite side. Fig 17.10 Fig 17.11

  20. Sonic Boom • http://stokes.byu.edu/teaching_resources/boom_script_flash.html (2:47) • http://stokes.byu.edu/teaching_resources/boomray_script_flash.html (2:53) • Happens with all types of waves whenever the sources is traveling faster than the speed of the wave… …so, what is “sonic boom” of water waves?

  21. θ Sonic Boom Sonic boom manifested by condensation of water in air sinq = vsound/vsource = 1/“Mach number”

  22. Sonic Boom Sonic boom of bullet in flight (holographic interferometry) q • How fast is the bullet traveling? • Mach # = 1/sinq

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