1. Exam 2 corrections – due today Today’s lecture
Announcements
Exam 2 corrections – due today
Today’s lecture
Comments of some previous HW problems
Summary of the physics of waves
The physics of sound
11/7/2018
PHY Lecture 18

2. Pressure due to water: P1=0 P2=rg(2d) P3=rg(3d) 11/7/2018
PHY Lecture 18

3. Solutions: y(x,t) = f (x ± vt) Examples: pulse wave periodic wave
The wave equation:
Solutions: y(x,t) = f (x ± vt)
Examples:
pulse wave
periodic wave
velocity
function of any shape
11/7/2018
PHY Lecture 18

4. 11/7/2018
PHY Lecture 18

5. Longitudinal waves propagating in a fluid or solid
Sound waves
Longitudinal waves propagating in a fluid or solid
11/7/2018
PHY Lecture 18

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7. Sound intensity: (energy/(unit time × unit area))
Periodic sound wave
In terms of pressure:
Sound intensity: (energy/(unit time × unit area))
Decibel scale:
11/7/2018
PHY Lecture 18

8. Source b (dB) Lawn mower 100 Normal conversation 70 Mosquito buzzing
Some representative values
Source
b (dB)
Lawn mower
100
Normal conversation 70
Mosquito buzzing 40
Threshold of hearing
11/7/2018
PHY Lecture 18

9. Peer instruction question
Suppose that you are trying to sleep but your roommate has the stereo on at a sound level of b= In trying to diplomatically and accurately achieve an appropriate sound reduction, which of the following is better to request:
Please reduce the stereo intensity by half.
Please reduce the stereo decibel level by half.
Please reduce the pressure amplitude of the sound wave produced by the stereo by half.
11/7/2018
PHY Lecture 18

10. String instruments (Guitar, violin, etc.)
The sound of music
String instruments (Guitar, violin, etc.)
(no sound yet.....)
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PHY Lecture 18

11. coupling to air
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PHY Lecture 18

12. Peer instruction question
Suppose you pluck the “A” guitar string whose fundamental frequency is f=440 cycles/s. The string is 0.5 m long so the wavelength of the standing wave on the string is l=1m. Assuming the speed of sound is 343 m/s, what is the wavelength of the sound wave which is produced?
(A) 1m (B) 0.78 m (C) 1.28 m
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PHY Lecture 18

13. “Wind” instruments (standing waves in air)
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15. The “Doppler” effect v=sound velocity
observer moving, source stationary
observer
source
vS=0 d
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16. The “Doppler” effect v=sound velocity
observer stationary, source moving d
toward
away
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17. Example: toward away Velocity of sound: v = 343 m/s 11/7/2018
PHY Lecture 18

18. Peer instruction question
Is Doppler radar described by the equations given above for sound Doppler?
yes (B) no
Is “ultra sound” subject to the sound form of the Doppler effect?
(A) yes (B) no
11/7/2018
PHY Lecture 18

19. Relative velocity of source toward observer
away
Relative velocity of source toward observer
11/7/2018
PHY Lecture 18