PH 105 Dr. Cecilia Vogel Lecture 6. OUTLINE Natural or Normal Modes Driving force Resonance ...
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Transcript of PH 105 Dr. Cecilia Vogel Lecture 6. OUTLINE Natural or Normal Modes Driving force Resonance ...
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PH 105
Dr. Cecilia VogelLecture 6
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OUTLINE
Natural or Normal Modes Driving force Resonance
Helmholtz resonator Standing Waves
Strings and tubes Longitudinal vs transverse waves
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SuperpositionWhen two disturbances (or waves)are at the same place at the same time,total disturbance is the sum of the two.
watch impulsive waves
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InterferenceBecause of superposition,Waves, when they meet
can add or interfere constructively so the total is
periodic waves, when they meetcan cancel or interfere destructively so the total is
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BeatsTwo waves with slightly different frequency (period) go in and out of phase
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InterferenceWaves from two source,will have places where they interfere constructively
what does it sound like with sound?what does it look like with light?
and other places where they interfere destructively
what does it sound like with sound?what does it look like with light?
video
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Normal Modes
A normal or natural mode isa way the system behaves when left to move naturally.
How does a pendulum behave naturally?How does mass on a spring behave naturally? How does string vibrate naturally?
Some systems have multiple normal modes
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Driving ForceYou can apply a periodic driving forcea force that pushes the system periodically
Period of driving force =
Example: pushing a swing
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Sympathetic VibrationsA driving force will often cause the driven system to vibrate
with the same period as the driving force.If the driving vibrator is vibrating naturally, these vibrations are called sympathetic vibrations.
Listen to the tuning fork;listen again when on box
box driven by tuning fork.both emit sound
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ResonanceWhen the frequency of the driving force matches a natural frequency,
the driving force hasthe vibrator is resonating
Why push a swing each time it swings?Observe spring on and off resonance.
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Helmholtz ResonatorA bottle with a neck is analogous to a mass on a spring.
the air in the neck is the mass which oscillates the volume of air in the bottle acts as a spring
Called a Helmholtz resonator f=resonant frequencyV = volume of bottlebigger bottle, _____ r freq (pitch)a = neck area, l= neck length long, skinny neck, _____ freq
2
v af
Vl
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Closed Tube Resonances If tube is closed at both ends
the pressure has no there is a pressure antinode at ends
Observe slinky “pressure” hi & lo at fixed end observe that a pressure antinode is a
displacement (motion) node!
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Closed Tube Resonances How can there be antinodes at both
ends?
If etc
L = L = L =
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Resonant Frequencies of Closed Tube L = n/2
n = 1, 2, 3, 4, 5, ….
Since f = v
2
vf n
L
n shows there are many resonant frequencies
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Resonances of Open Tube
If tube is open at both ends, it has a pressure node at both ends displacement __________ analysis is similar
2
vf n
L 1f nf
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Tube with One Closed End
If tube is closed at one end there is a pressure _________ at that
end _______ at the other end
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Closed Tube Resonances How?
If etc
L = L = L =
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Resonant Frequencies L = n/4
n = 1, 3, 5, 7, 9…. (only odd!)
Since f = v
4
vf n
L
n odd
1f nf
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Standing Wave in String String is generally fixed at both
ends node at analysis like
2
vf n
L 1f nf
L = n/2 n = 1, 2, 3, 4, 5, ….
Were measured resonantfrequencies integer times f1?
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Standing Wave in String Combine
Can change resonant freq’s by changing
Tv
2
vf n
L
2
n Tf
L
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Impedance and Resonance A reflection can occur any time there is a
change in impedance. Acoustic Impedance means difficulty of air
flow observe wave machines
There can be resonance in each part of a complex tube:
L1
L2
L3
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SummaryInterference is the addition of waves at point where they meet
constructive interference destructive interference
Normal modes are natural behaviorsometimes multiple natural frequencies
At resonancedriving frequency matches natural freqdriving force has a huge effect
Resonance of Helmholtz resonator, open and closed tubes, strings