《大学物理》英文课件17 Waves-II
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Waves – II
Dr. Huaqiu Deng
E-mail: hqdeng@scut.edu.cn
Sound Waves
What is a sound wave? - Sound wave is of any
longitudinal wave that
requires a material medium to exist.
Solution: (a) L0
(b) L1.5
(c) N6
Intensity
Inte:nIsiP A t y1 2v2sm 2
P: time rate of energy transfer of the sound wave A: area of the surface intercepting the sound.
The Doppler Effect (1842 by Austrian Physicist
Johann Christ源自文库an Doppler)
The Doppler effect: Motion-related frequency change. Holds for all kinds of waves
The applications of Doppler Effect
Sample Problem – Double Doppler shift in the
echoes used by bats
Problem: bats navigate and search out prey by emitting, and then detecting reflection of, ultrasonic waves, which are sound waves with frequencies greater than can be heard by a human. Suppose a bat emits ultrasound at frequency fbe = 82.52kHz while flying with velocity vb = 9.00m/s as it chases a moth that flies with velocity vm = 8.00m/s. What frequency fmd does the moth detect? What frequency fbd does the bat detect in the returning echo from the moth?
f'(v tvtDt)/vvD
f'vvD f vvD
v/ f
v
f ' f vvD v
Source Moving; Detector Stationary
The motion of S changes wavelength.
f'v v f v
' vTvST vvS
f ' f v vvS
General Doppler Effect Equation
f ' f vvD vvS
f': detectef :demitted
v:sousnpdereed lattia ovier
vD:detectvoS:rsource
Toward means shift up; Away means shift down
Both Detector and Source are Stationary
Interference
LL2 L1
2 1 2 L L2
fully constructive interference L
m (2),m 0 ,1 ,2 ...
0,1,2...
ful ly(d2 em str u1 c)tiv,e inm te rf0 er,1 e,n2 c.e ..L0.5,1.5,2.5...
n
f vn4vL, n1,3,5,
Beats
f f f Beat frequency: beat 1 2
Musicians use the beat phenomenon in tuning instruments. If the beat disappear, the instrument is in tune with that standard.
The frequency detected by detector D is the rate at which D intercepts wavefronts.
f 'vt/v f t
Detector Moving; Source Stationary.
The motion of detector changes the intercepted number of wavelengths.
For a pipe of length L with two open
ends:
Wavelength :
2L,
n
n1,2,3,
Frequency : f vn2vL, n1,2,3,
n : Harmonic number
For a pipe of length L with one open
ends: 4L, n1,3,5,
Sound Level in Decibels
Sound level: 10logII0 (dB)
Reference intensity level: I0 1012W/m 2
Sources of musical Sound – Standing wave
Sources of musical Sound – Standing wave
Sample Problem
Problem: two point sources S1 and S2, which are in phase and separated by D = 1.5λ, emit identical sound waves of wavelength λ. (a) What is the path length difference of the waves from S1and S2, at point P1, which lies on the perpendicular bisector of distance D, at a distance greater than D from S1 and S2. (b) What are the path length difference and type of interference at P2. © What is the number of points N around this circle at which the interference is fully constructive?
General Doppler Effect Equation: replace f (frequency of source) with f’ (associated with motion of detector).
f ' f vvD v
f ' f v vvS
f ' f vvD vvS
The applications of Doppler Effect
Dr. Huaqiu Deng
E-mail: hqdeng@scut.edu.cn
Sound Waves
What is a sound wave? - Sound wave is of any
longitudinal wave that
requires a material medium to exist.
Solution: (a) L0
(b) L1.5
(c) N6
Intensity
Inte:nIsiP A t y1 2v2sm 2
P: time rate of energy transfer of the sound wave A: area of the surface intercepting the sound.
The Doppler Effect (1842 by Austrian Physicist
Johann Christ源自文库an Doppler)
The Doppler effect: Motion-related frequency change. Holds for all kinds of waves
The applications of Doppler Effect
Sample Problem – Double Doppler shift in the
echoes used by bats
Problem: bats navigate and search out prey by emitting, and then detecting reflection of, ultrasonic waves, which are sound waves with frequencies greater than can be heard by a human. Suppose a bat emits ultrasound at frequency fbe = 82.52kHz while flying with velocity vb = 9.00m/s as it chases a moth that flies with velocity vm = 8.00m/s. What frequency fmd does the moth detect? What frequency fbd does the bat detect in the returning echo from the moth?
f'(v tvtDt)/vvD
f'vvD f vvD
v/ f
v
f ' f vvD v
Source Moving; Detector Stationary
The motion of S changes wavelength.
f'v v f v
' vTvST vvS
f ' f v vvS
General Doppler Effect Equation
f ' f vvD vvS
f': detectef :demitted
v:sousnpdereed lattia ovier
vD:detectvoS:rsource
Toward means shift up; Away means shift down
Both Detector and Source are Stationary
Interference
LL2 L1
2 1 2 L L2
fully constructive interference L
m (2),m 0 ,1 ,2 ...
0,1,2...
ful ly(d2 em str u1 c)tiv,e inm te rf0 er,1 e,n2 c.e ..L0.5,1.5,2.5...
n
f vn4vL, n1,3,5,
Beats
f f f Beat frequency: beat 1 2
Musicians use the beat phenomenon in tuning instruments. If the beat disappear, the instrument is in tune with that standard.
The frequency detected by detector D is the rate at which D intercepts wavefronts.
f 'vt/v f t
Detector Moving; Source Stationary.
The motion of detector changes the intercepted number of wavelengths.
For a pipe of length L with two open
ends:
Wavelength :
2L,
n
n1,2,3,
Frequency : f vn2vL, n1,2,3,
n : Harmonic number
For a pipe of length L with one open
ends: 4L, n1,3,5,
Sound Level in Decibels
Sound level: 10logII0 (dB)
Reference intensity level: I0 1012W/m 2
Sources of musical Sound – Standing wave
Sources of musical Sound – Standing wave
Sample Problem
Problem: two point sources S1 and S2, which are in phase and separated by D = 1.5λ, emit identical sound waves of wavelength λ. (a) What is the path length difference of the waves from S1and S2, at point P1, which lies on the perpendicular bisector of distance D, at a distance greater than D from S1 and S2. (b) What are the path length difference and type of interference at P2. © What is the number of points N around this circle at which the interference is fully constructive?
General Doppler Effect Equation: replace f (frequency of source) with f’ (associated with motion of detector).
f ' f vvD v
f ' f v vvS
f ' f vvD vvS
The applications of Doppler Effect