1 / 27

Sound waves

Sound waves. You can think of a sound wave as an oscillating pattern of compression and Expansion ( D P), or as an oscillating position for small packets of Air [s(x,t), which leads to the above picture]. REMEMBER this is a longitudinal wave!. D p m = rw s m v s.

lincoln-hatcher
Télécharger la présentation

Sound waves

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Sound waves You can think of a sound wave as an oscillating pattern of compression and Expansion (DP), or as an oscillating position for small packets of Air [s(x,t), which leads to the above picture]. REMEMBER this is a longitudinal wave! Dpm = rwsmvs I = ½ rw2sm2vs = ½(Dpm)2/vsr

  2. WAVES II--SOUND

  3. Chapter 17 Problems NOTE: r=1.21 kg/m3 v=343 m/s At T=20oC FIRST: What is the ratio of the Intensities between the two cases? e). What is the pressure difference that corresponds to each of these intensities?

  4. Extra Office HoursExam Week Monday (28 April): 1:30 to 3:30 Tuesday (29 April): 11:00 to 12:00 and 1:30 to 3:30 Final Exam is at 8:00-10:00 on Wednesday 30 April 2008 in SW 007

  5. Interference with Sound waves Fig. 17.8 Suppose the difference L2-L1 is 1.70 m and S1 and S2 emit sound at a frequency of 100Hz. How will the intensity at P change if the phase offset between S2 and S1 is changed from zero to p? (assume v=340m/s)

  6. Chapter 17 Problems

  7. Reflections at a Boundary

  8. Standing waves: Pipes The resonance frequencies of pipes depend on the conditions at the two ends. A closed end needs a NODE, and an open end needs an ANTINODE. The book gives you formulae for the two cases that you can remember, OR you can just remember these two conditions and draw pictures! (I find this way of doing it MUCH easier.) Question: Why do we not consider the case of both ends closed? What would be the condition in that case?

  9. Chapter 17 Problems

  10. If the amplitude of a sound wave is doubled, by what number of dB does the intensity of that sound wave increase? As usual, please provide a brief explanation for your answer • Since Intensity and amplitude are related by the equation I = 0.5pv(w^2)(s^2), if the amplitude increased by a factor of 2, then I will increase by the square root of two. (8 or so made various mistakes such as this) • It increases by a factor of four because it is squared according to the equation for amplitude (7 answered this way) • If the area does not change any, then the intensity would increase by a factor of four. BUT the sound level in db is waul to 10 db log I/I0 which would be an increase in 6 db. (8 answered like this)

  11. Standing waves: Pipes The resonance frequencies of pipes depend on the conditions at the two ends. A closed end needs a NODE, and an open end needs an ANTINODE. The book gives you formulae for the two cases that you can remember, OR you can just remember these two conditions and draw pictures! (I find this way of doing it MUCH easier.)

  12. Chapter 17 Problems

  13. E.G. Indy Car passing: http://www.youtube.com/watch?v=fndBi4OKA5M Doppler Effect The frequency shifts up if the source and observer are getting closer to each other, and down if they are receding from each other. Think of old “murder on the train movies”, or the sound of an Indy car as it goes by. NOTE: this is the phenomenon that is used to measure the changes in the velocity of stars so accurately that extra-solar planets can be detected (e.g. problem 13-52); it works for light and all kinds of waves, not just sound!

  14. Two identical loud speakers are emitting sounds at a frequency of 130 Hz, but one of the two is on the ground and the other is on a flat bed rail car moving at a speed of 10 m/s what beat frequency is heard by an observer on the ground who views the rail car approaching him? (take the velocity of sound to be 343 m/s). Please give a brief explanation of how you got your answer. (29 no answers, 6 confused; 14 got the number right, but some for the wrong reason) • 126 Hz. f'=f(v+vd/v+vs) since the object is moving towards the person, the equation is plus instead of minus. (BE CAREFUL with this equation, ask yourself if the frequency would be greater or smaller as a result of the motion; also this does not address the beat freq. question; 5 went this route). • f' = f(v/v-vs) f' = 130(343/343-10) = 134 Hz 134Hz-130Hz = 4Hz (NOTE the sign in the denominator.)

  15. BEATS If the frequencies are not matched, then the interference changes from constructive to destructive periodically in time as the higher-frequency wave picks up an extra p phase shift. The “Beats” show a maximum every time there is a 2p phase shift (i.e. the higher frequency wave picks up a whole cycle on the lower frequency wave). Beat frequency is just the difference in the frequencies of the two waves.

  16. Chapter 17 Problems

  17. Sound waves You can think of a sound wave as an oscillating pattern of compression and Expansion (DP), or as an oscillating position for small packets of Air [s(x,t), which leads to the above picture]. REMEMBER this is a longitudinal wave! Dpm = rwsmvs I = ½ rw2sm2vs = ½(Dpm)2/vsr

  18. Chapter 17 Problems NOTE: in many problems this same idea will be expressed more like:“what values of L give a minimum of intensity when listening to the Two beams simultaneously?”

  19. What is the fundamental definition of temperature? • Temperature is on the Kelvin scale. It has no upper limit, but has a lower limit of 0 Kelvin. [3; so the book says, but what does this mean?] • Temperature is an SI base quantity related to our sense of hot and cold. [10 responses] • The measurement of the kinetic energy of a material/atoms/molecules [13 responses]

  20. What physical phenomenon is used to determine the temperature in a common alcohol (or mercury) thermometer. • Ideal Gas law [3 responses] • Thermal expansion: As an object heats up the object expands. [18 responses] • Volume expansion [7 responses] • What I wanted you to notice is that it is, in fact, volume expansion NOT linear expansion; you let the volume expand into a narrow channel making the change easier to read! ALL thermometers rely on some MEASUREABLE physical change.

  21. Chapter 18 Problems NOTE: the cross sectional area changes! In Thermal expansion, all linear dimensions change (length of edges, sides of holes etc.) by the same fraction for a given DT See next page for solution

  22. Solution to 18-83 This much to raise to 0 C This much to melt 0.7kg of ice 233.1kJ left over, how far can that warm up the water? Note that the authors chose the initial temperature and mass carefully to have each part of the process take exactly 233.1kJ

  23. Extra Office HoursExam Week Monday (4 May): Aditi: 10:00 to 12:00 (NOTE: now in SW250) DVB: 1:00 to 2:00 Tuesday (5 May): Aditi: 10:00 to 12:00. (NOTE: now in SW250) DVB: 3:00 – 5:00 Final Exam is at 8:00-10:00 AM!! on Wednesday 6 May 2009 in SW 007

  24. Since Exam III • Waves: • Transverse and Longitudinal • Harmonic • Travelling; Standing/resonances • Velocity, frequency, wavelength, phase angle • Power and intensity and “sound level” • Interference • Doppler and Beats • Thermodynamics • Temperature/Thermal Expansion • Heat Capacity/Specific Heat • Latent heat

  25. Final exam Will be comprehensive! ~1/4 - 1/3 of the questions will be on the new stuff (since exam III) Will be out of approximately 126 points (i.e. about 50% longer than the midterms but you have more than twice as long). ~6 multiple choice 5 multi-part questions.

  26. Review requests for Final • New stuff/oscillations/waves (23 requests) • Ang. momentum/torque etc. (4 requests) • Fluids (2 requests) • Friction (2 requests) • All else, no more than 1 request

  27. Thanks for a great semester!!

More Related