2. IB Physics Waves and Sound

3. The 5 Properties of Waves  Rectilinear Propagation: waves propagate (move or spread out) in straight lines in all directions away from the source.  Reflection: change in direction of a wave at an interface between two different media so that the wave returns into the medium from which it originated.  Refraction: change in direction of a wave due to a change in its speed. This is observed when a wave passes from one medium to another.

4.  Diffraction: bending of waves around small obstacles and the spreading out of waves passing through small openings.  Interference: the addition (superposition) of two or more waves that results in a new wave pattern. The 5 Properties of Waves

5. Any Wave: is a traveling disturbance that carries energy. Web Link: The WaveThe Wave Two types of waves: 1) Transverse – the disturbance is perpendicular to the direction the wave is traveling 2) Longitudinal – the disturbance is parallel to the direction the wave is traveling Web Links: Longitudinal & Transverse, LongitudinalLongitudinal & TransverseLongitudinal

6. ? Consider a transverse wave on a string, frozen in time:AA A = Amplitude x y = wavelength Now consider the position of one point on the string: time y AA T = period f = frequency = 1/T

7. Frequency (f), period (T), wavelength (), and wave speed (v) are all related: If the wave crests are 12 m apart, and you see one pass by you every 3 seconds, how fast are they going? v = /T v = f

8. Ex: Radio Waves Radio waves travel at the speed of light in a vacuum, 3.00 x 10 8 m/s. Find the wavelength of radio waves from “Light Rock” 106.7 FM. How long do you think the wavelengths of radio waves are ? Radio waves are a type of Electromagnetic Waves

9. Electromagnetic Waves These don’t need a medium (substance to travel in).

10. The speed of a wave depends on the medium. v = f That means that these two are inversely proportional f f All other waves require a medium (air, water, string, etc.)

11. Waves on a String We can develop a formula for the speed of waves on a string: v What qualities of the string do you think determine this speed??

12. Ex: The rope has a length of 1.2 m and a mass of 2.0 kg. If he shakes the end 4.0 times a second, and it yields the pattern shown, what is the tension in the rope?

13. Sound Waves *any longitudinal waves Sound waves in air consist of vibrating air molecules Web Link: LoudspeakerLoudspeaker

14. Compare a longitudinal wave in a slinky to a sound wave in a tube:

15.  A = pressure amplitude = max. change in pressure very small for common sounds ( 3x10 -2 Pa)  Loudness is related to pressure amplitude

16. Frequency of Sound Waves: number of cycles per second Healthy young ears can hear frequencies between: 20 Hz20,000 Hz Which end do we lose with age? UltrasonicInfrasonic  Pure tone – a single frequency sound (most sounds are composed of many frequencies)

17. Ex: Lightning & Thunder Speed of Sound depends on the medium (gas, liquid or solid) v sound in air = 343 m/s (767 mi/hr) How can you tell how far away the storm is ??

18. Speed of sound in an ideal gas k = 1.38 x 10 -23 J/K (Boltzmann const.) T = Kelvin temperature m = molecular mass  = C P /C V

19. Ex: Verify the speed of sound in air if the average air molecule mass is 28.9 u, and  = 1.40 for air.

20. Speed of sound in a liquid Bulk Modulus (adiabatic) Density of liquid Ex: Water The speed of sound in water is around 1500 m/s (that’s over 4 times as fast as it travels in air!)

21. Speed of sound in solid bars Young’s Modulus Ex:Train Tracks

22. Sound Intensity of the sound wave  to the wave SI unit = Sound Intensity =

23. Spherically Uniform Sound: Area = 4r 2 How much less is the sound intensity for the person that is farther away? 600 m 200 m Ex: Fireworks

24. To compare two intensities (I and I 0 ):  (in decibels) = 10 log (I/I 0 ) Notes on Sound Intensity  Intensity is related to loudness (but not directly proportional)  10 -12 W/m 2 = lower threshold of human hearing  1 W/m 2 = enough to cause ear damage  Different sound intensities are compared using: decibels (dB) “Intensity Level” (unitless) For sound meters, I 0 = threshold = 10 -12 W/m 2

25. Ex: If a sound meter picks up an intensity of 2 x 10 -11 W/m 2, what will be the intensity level reading?

26. Ex: If you bring a decibel meter to a rock concert, at what decibel reading should you start worrying about ear damage? (James Taylor would never play this loud……)

27. Ex: Standing 3 times as far from a uniform sound source will decrease the intensity level by how many decibels?

28. Some interesting applications of Sound Waves Sonar: Ex: Dolphins Ex: Web Link: Ocean floor mappingOcean floor mapping Ultrasound: Web Link: How does Ultrasound work?How does Ultrasound work? Cancer treatment: Web Link: Local HyperthermiaLocal Hyperthermia

29. The Doppler Effect- the change in wave frequency resulting from motion of the source or observer Consider the waves from a sound source: Now, consider the waves from a moving source: You hear a lower frequency as it moves away from you You hear a higher frequency as it moves toward you

30. Web Links: Doppler Effect I and IIDoppler Effect I and II Doppler Effect (1 source)

31. Here’s how to calculate the frequency heard by the observer: f’ = perceived frequency f = actual frequency v s = speed of the source v = speed of the wave Source moves toward still observer Source moves away from still observer

32. Ex: If the fire engine has a speed of 30 m/s, and its siren has a frequency of 500 Hz, what frequency does the pedestrian hear as it moves a)toward him ? b)away from him ?

33. Now, if the observer is moving at speed v O ….. toward the source:away from the source:

34. Web Link: Doppler effect in acousticsDoppler effect in acoustics

35. Now, if both observer and source are moving… vsvs vovo (signs determined as before)