1. Warm-Up: February 29, 2016
Write down everything you know about sound.

2. Homework Questions?

3. Think-Pair-Share
If a tree falls in the woods and no one is around to hear it, does it make a sound?
In space, can anyone hear you scream?

4. Chapter 15 (OpenStax Chapter 17)
Sound
Chapter 15
(OpenStax Chapter 17)

5. Sound and Hearing
Sound is a disturbance of matter that is transmitted outward from its source.
Sound is a wave.
Hearing is the perception of sound, just as vision is the perception of light.

6. Vibrating String
As a string vibrates, a small portion of its energy goes into compressing and expanding the surrounding air.
These compressions (high pressure regions) and rarefactions (low pressure regions) move out as longitudinal waves having the same frequency as the string.

7. Sound Waves Longitudinal in air and most other fluids.
In solids, can be both transverse and longitudinal.
Amplitude of sound waves decrease with distance from its source.
Spread out over larger and larger areas.
Absorbed by objects (including eardrums).
Energy lost as heat.
Wavelength, frequency, amplitude, and speed are important, as they are for all waves.

8. Frequency Same as for other waves.
Perception of frequency is called pitch.

9. Still true

10. You-Try #2
What frequency sound has a 0.10 m wavelength when the speed of sound is 340 m/s?

11. Wavelength
Distance between adjacent identical parts.

12. Solids (longitudinal)
Speed of Sound
Depends on temperature.
Higher temperature  faster particle motion  faster sound waves
Nearly independent on frequency or wavelength
Sample speeds, in m/s
Gases at 0°C
Liquids at 20°C
Solids (longitudinal)
Air
331
Ethanol
1160
Polyethalene
920
CO2
259
Mercury
1450
Marble
3810
Oxygen
316
Fresh water
1480
Glass (Pyrex)
5640
Helium
965
Sea water
1540
Lead
1960
Hydrogen
1290
Human tissue
Steel
5960

13. Speed of Sound Table 15-1, Page 405 (Need for homework)
(Do not need to memorize – a table will be provided on your test)

15. Warm-Up: March 1, 2016
A sonar echo returns to a submarine 1.20 s after being emitted. What is the distance to the object creating the echo? Assume the submarine is stationary in the ocean.

16. Reminder
Units Quiz next class
All or nothing

17. Changing Medium
The medium is the matter through which a sound wave travels.
When a sound wave passes from one medium to another, frequency and period stay constant.
Wave speed is determined by the medium (and temperature).
Wavelength is determined by

18. Think-Pair-Share
You observe two musical instruments that you cannot identify. One plays high-pitch sounds and the other plays low-pitch sounds. How could you determine which is which without hearing either of them play?
Solution: Compare their sizes. High-pitch instruments are generally smaller than low-pitch instruments because they generate a smaller wavelength.

19. You-Try #10
A physicist at a fireworks display times the lag between seeing an explosion and hearing the sound, and finds it to be s. His thermometer shows an air temperature of 20.0°C. Estimate the distance between the physicist and the explosion.

20. Sound Intensity and Sound Level
OpenStax Section 17.3

21. Sound Intensity
The “loudness” of a sound depends on its sound intensity, 𝐼.
Intensity= Power Area = ∆pressure 2 2𝜌 𝑣 𝑤 (Do not need to memorize)
SI unit is 𝑊 𝑚 2
Sound intensity level, 𝛽, is measured in decibels
The bel, upon which the decibel is based, is named for Alexander Graham Bell
𝛽 dB =10 log 10 𝐼 (Do not need to memorize)
Logarithmic scale

22. Sound Intensity Level, 𝜷 (dB)
Intensity, 𝑰 (W/m2)
Example/effect
1× 10 −12
Threshold of hearing at 1000 Hz 10
1× 10 −11
Rustle of leaves 20
1× 10 −10
Whisper at 1 m distance 30
1× 10 −9
Quiet home 40
1× 10 −8
Average home 50
1× 10 −7
Average office, soft music 60
1× 10 −6
Normal conversation 70
1× 10 −5
Noisy office, busy traffic 80
1× 10 −4
Loud radio, classroom lecture 90
1× 10 −3
Inside a heavy truck; Damage from prolonged exposure
100
1× 10 −2
Noisy factory, siren at 30 m; Damage from 8 h/day exposure
110
1× 10 −1
Damage from 30 min/day exposure
120 1
Loud rock concert; Threshold of pain
140
1× 10 2
Jet airplane at 30 m; Severe pain, damage in seconds
160
1× 10 4
Bursting of eardrums

23. Relationships between 𝐼 and 𝛽
Double 𝐼  𝛽 increases by 3 dB
Multiply 𝐼 by 5  𝛽 increases by 7 dB
Multiply 𝐼 by 10  𝛽 increases by 10 dB

24. Doppler Effect and Sonic Booms
OpenStax Section 17.4

25. Doppler Effect
The Doppler Effect is an alteration of the observed frequency of a sound due to motion of either the source or the observer.
The change in frequency due to relative motion is called a Doppler shift.
Named after Austrian Christian Johann Doppler ( )

26. Stationary Source, Stationary Observer

27. Moving Source, Stationary Observer

28. Stationary Source, Moving Observer

29. Doppler Effect
Occurs for all waves, including sound, light, and water waves.
Used to determine relative velocities of galaxies.
(See Page 408 of your textbook for special cases of the equation.)

30. You-Try 17.4
Suppose a train that has a 150. Hz horn is moving at 35.0 m/s in still air when the speed of sound is 340. m/s.
What frequencies are observed by a stationary person at the side of the tracks as the train approaches and after it passes?
What frequency is observed by the train’s engineer traveling on the train?

31. Suppose a train that has a 150. Hz horn is moving at 35
Suppose a train that has a 150. Hz horn is moving at 35.0 m/s in still air when the speed of sound is 340. m/s.
What frequencies are observed by a stationary person at the side of the tracks as the train approaches and after it passes?
What frequency is observed by the train’s engineer traveling on the train?

33. Warm-Up: March 2/3, 2016
The frequency of the middle C musical note is Hz. A cello plays a middle C inside in a house where the air temperature is exactly 20℃.
What is the period of the sound wave?
What is the wavelength of the sound wave?
The sound wave travels out of the house through an open window. If the temperature of the air outside is exactly 0℃, what are the frequency, period, and wavelength of the sound wave outside?

34. Homework Questions?

35. Sonic Booms As 𝑣 source approaches 𝑣 sound , 𝑓 obs approaches infinity
When 𝑣 source = 𝑣 sound , the front edge of each successive wave is superimposed on the previous one. The observer gets them all at the same instant, so 𝑓=∞, but only if the observer is directly in the path of the object.
Not considered a sonic boom.
A sonic boom is created when 𝑣 source > 𝑣 sound , where maximum superposition of sound waves is along a cone with the moving object at the tip of the cone

36. Sonic Boom

37. Sonic Boom Object traveling faster than speed of sound.
No sound is received by the observer until after the object passes.
Sound waves from approaching source are superimposed with sound waves from the receding object.
Constructive interference creates a cone-shaped shock wave, called a sonic boom.

38. Sonic Boom

39. Two sonic booms from one airplane

40. Mach Number

41. Bow Wakes
A bow wake is created when the wave source moves faster than the wave propagation speed.
A sonic boom is an example of a bow wake.

42. Think-Pair-Share
What are some applications of the Doppler Effect?

43. Applications of Doppler Shift
Ultrasound – measure blood velocity
Police radar – measure car velocities
Meteorology – track storm clouds
Astronomy – relative speeds of galaxies

44. You-Try 36
Two eagles fly directly toward one another, the first at 15.0 m/s and the second at 20.0 m/s. Both screech, the first one emitting a frequency of Hz, and the second one emitting a frequency of 3800 Hz. What frequencies do they receive if the speed of sound is 330 m/s?

45. Assignment Read Section 15.1 Optional: Read OpenStax 17.1-17.4
Page 405 #1-5; Page 409 #6-10; Page 410 #12-17
Read Section 15.2

46. Units Quiz!
Clear everything off of your desk except writing utensils and erasers.
8 minute time limit
If you appear to be talking, looking at anyone else’s quiz, allowing another student to look at your quiz, or using any unauthorized aide, you will receive a zero.
If any electronic device is seen or heard, it goes to the office and you will receive a zero.

47. The Physics of Music
Section 15.2

48. Sound Interference and Resonance: Standing Waves in Air Columns
OpenStax Section 17.5

49. Sound Interference
All waves exhibit constructive and destructive interference.
Demonstrating interference proves something is a wave.
Noise-cancelling technology uses destructive interference to reduce noise levels by 30 dB or more.
Sound resonance, such as in musical instruments, is due to interference.

50. Beats
When two sound waves of similar frequencies interfere, beats are generated.
𝑓 𝐵 = 𝑓 2 − 𝑓 1

51. You-Try 40
What beat frequencies result if a piano hammer hits three strings that emit frequencies of 127.8, 128.1, and Hz?

52. Resonance in Tubes
Resonant frequencies interfere constructively. Other frequencies interfere destructively and are absent.
Half-open tube (open at one end, closed at the other)
Sound wave enters the open end of the tube.
Sound wave travels down the tube.
Sound wave reflects off the closed end.
Sound wave return to the open end.
If the wavelength of the sound wave and the length of the tube are correct, the exiting sound wave constructively interferes with the incoming sound wave, and a standing wave is created.
An antinode (maximum displacement) at the open end
A node (no displacement) at the closed end

53. Figure 17.23
Resonance of air in a tube closed at one end, caused by a tuning fork. A disturbance moves down the tube.

54. Figure 17.24
Resonance of air in a tube closed at one end, caused by a tuning fork. The disturbance reflects from the closed end of the tube.

55. Figure 17.25
Resonance of air in a tube closed at one end, caused by a tuning fork. If the length of the tube L is just right, the disturbance gets back to the tuning fork half a cycle later and interferes constructively with the continuing sound from the tuning fork. This interference forms a standing wave, and the air column resonates.

56. Figure 17.26
Resonance of air in a tube closed at one end, caused by a tuning fork. A graph of air displacement along the length of the tube shows none at the closed end, where the motion is constrained, and a maximum at the open end. This standing wave has one-fourth of its wavelength in the tube, so that λ = 4L .

57. Fundamental Frequency
Any wavelength that produces a node at the closed end and an antinode at the open end will produce a standing wave.
The longest possible wavelength is 𝜆=4𝐿, where 𝐿 is the length of the pipe. This creates the lowest possible frequency, called the fundamental frequency.

58. Overtones
Instead of one-fourth of a wavelength fitting inside the tube, 3/4 or 5/4 or 7/4, etc. could fit inside the tube.
These shorter wavelengths create higher frequency standing waves called overtones.

59. Harmonics
Harmonics include the fundamental frequency and all overtones.
The first harmonic is the fundamental frequency
The second harmonic is the first overtone
The third harmonic is the second overtone

60. Harmonics
The fundamental and overtones can be present in a variety of combinations. This is why middle C sounds different on different instruments.
The fundamental frequency is the same, but the overtones and their mix of intensities are different.

63. Warm-Up: March 4, 2016
A rock is dropped down a well on a warm summer day. A splash is heard 2.35 seconds later. How far below the top of the well is the water? Use 𝑔=9.80 m/s2.

64. Tube closed at one end
Resonant frequencies of a tube closed at one end are
𝑓 1 is the fundamental
𝑓 3 is the first overtone …

65. You-Try 17.5
What length should a tube closed at one end have on a day when the air temperature is 22.0℃ 𝑣 𝑤 =344 m s , if its fundamental frequency is to be 128 Hz (C below middle C)?
What is the frequency of its fourth overtone?

66. What length should a tube closed at one end have on a day when the air temperature is 22.0℃ 𝑣 𝑤 =344 m/s , if its fundamental frequency is to be 128 Hz?
What is the frequency of its fourth overtone?

67. You-Try 48a
Find the length of an organ pipe closed at one end that produces a fundamental frequency of 256 Hz when the speed of sound is 342 m/s.

68. Tube open at both ends
Resonance is created with antinodes (maximum displacement) at both ends.
Resonant frequencies of a tube open at both ends are:
𝑓 1 is the fundamental
𝑓 2 is the first overtone
𝑓 3 is the second overtone

69. You-Try 44
What are the first three overtones of a bassoon that has a fundamental frequency of 90.0 Hz? It is open at both ends.
(The overtones of a real bassoon are more complex than this, because of its double reed.)

70. Assignment Read Section 15.2 Optional: Read OpenStax Section 17.5
Page 424 #30-39 all, odd, odd