physics - 6 - waves - sound · frequency of 262 hz. if these sound waves have a wavelength in the...
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WAVES - SOUNDIB PHYSICS | UNIT 6 | WAVES - SOUND
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Simple Harmonic MotionIB PHYSICS | UNIT 6 | WAVES - SOUND
6.1
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Warm up
What words would you use to describe the motion of a bobble head doll?
• Oscillating
• Back and Forth
• Repeating
• Etc.
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Consider This…
Time (sec)
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Consider This…
Hooke’s Law: F = -kΔx
+Δx
-Δx
-F
+F
-a
+a
Force is opposite to the displacement
F = maAcceleration is in the same
direction as the force
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Why the Negative Sign??
a -x“Acceleration is proportional to the opposite of the displacement”
Proportional to
+Δx
-Δx
-a
+a
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Where is the Greatest…
C
B
A
Displacement Velocity Acceleration
X X
X
X X
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Dis
pla
cem
ent
Time
Vel
oci
ty
Time
Acc
ele
rati
on
Time
A
B
C
Slope of d vs t
Slope of v vs t
Opposite of disp
+Δx
-Δx
A
B
C
A
B
C
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A B C
_____ vs Displacement
Velocity Acceleration
Displacement
A B C
Displacement
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Properties of SHM
Property What is it? Symbol Unit
Period Time per cycle T [s]
Frequency Cycles per second f [Hz]
Dis
pla
cem
ent
Time
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Period is related to Frequency
Period = 1 / Frequency
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Try this…
You are standing on the beach with your feet in the water and notice that a new wave comes crashing in every 4 seconds, what is the frequency of these waves?
𝑇 = 4 s
𝑓 =1
𝑇=1
4= 𝟎. 𝟐𝟓 𝐇𝐳
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Try this…
Taylor Swift’s song Shake it Off has a tempo of 160 beats per minute how many seconds are in between each beat (the period)
𝑇 =1
𝑓=
1
2.67= 𝟎. 𝟑𝟕𝟓 𝒔
𝑓 =160 𝑏𝑒𝑎𝑡𝑠
1 𝑚𝑖𝑛×1 𝑚𝑖𝑛
60 𝑠𝑒𝑐= 2.67 Hz
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Energy for SHMEn
ergy
Time
PE
KE
Total Energy
A B C B A B C
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Properties of Traveling WavesIB PHYSICS | UNIT 6 | WAVES - SOUND
6.2
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Warm Up
a -x
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What is a Wave?
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What is a Wave?
A wave is a disturbance that carries __________ through
matter or space
matter through which a wave travels
energy
medium
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Vibrations and Waves
Most waves are caused by a vibrating object
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Is the Medium Moving?
The medium particles
oscillate back and forth
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Two Types of Waves
Transverse Longitudinal
Particles move perpendicularto the wave’s motion
Particles move parallelto the wave’s motion
Examples:• Ripples in a pond• Light Waves
Examples:• Sound Waves• Earthquake Waves
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Properties of a Wave
Property Symbol Unit
Amplitude A [m]
Wavelength λ [m]
Period T [s]
Frequency f [Hz] or [s-1]
distance time
A
A
λ
λ
λ
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Properties of a Wave
compression rarefaction
peak/crest trough
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How Many Waves?
1.5
1
1
0.5
0.5
1.5
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Draw a Wave #1
Amplitude = 2 boxes | Wavelength = 4 boxes
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Draw a Wave #2
Amplitude = 4 boxes | Wavelength = 8 boxes
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Waves and Energy
↑ Amplitude = ⃝ Energy
↓ Amplitude = ⃝ Energy
↑ Wavelength = ⃝ Energy
↓ Wavelength = ⃝ Energy
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Wave Speed Equation
Speed = Frequency × Wavelength
Sym
bo
lsU
nit
s
= ×
= ×
v λf
[m s-1] [m][Hz]
[s-1]
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IB Physics Data Booklet
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Try this…
A piano string vibrates with a frequency of 262 Hz. If these sound waves have a wavelength in the air of 1.30 m, what is the speed of sound?
𝑣 = 𝑓𝜆 = 262 (1.3)
= 𝟑𝟒𝟏𝐦 𝐬−𝟏
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Read a Wave #1
# of Waves
Period
Amplitude
Frequency
1
2
3
3
2
1
0
Met
ers
Seconds
0 1 2 3 4 5 6 7 8 9 10 11 12
3
4 s
2 m
0.25 Hz
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Read a Wave #2
1
2
3
3
2
1
0
Met
ers
Seconds
0 1 2 3 4 5 6 7 8 9 10 11 12
# of Waves
Period
Amplitude
Frequency
1.5
8 s
3 m
0.125 Hz
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Sound is Pressure
Vibrations pressurize the air molecules and those pressure waves cause our ears to vibrate too!
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Pitch is Related to Frequency
High pitched sounds have _________ frequencies
Low pitched sounds have _________ frequencies
high
low
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Sensing Pitch
Sadly, the range of frequencies that we can hear diminishes with age…
Frequency Yes No
8,000 Hz
10,000 Hz
12,000 Hz
14,000 Hz
15,000 Hz
16,000 Hz
17,000 Hz
18,000 Hz
19,000 Hz
20,000 Hz
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How do you Compare?
You Elephant Dog Dolphin
Min Frequency 6 Hz 40 Hz 70 Hz
Max Frequency 12,000 Hz 46,000 Hz 150,000 Hz
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Exit Ticket
The crests of waves passing into a harbor are 2.1 m apart and have an amplitude of 60 cm. Ten waves pass an observer every minute.
What is their frequency? What is their speed?
10 𝑤𝑎𝑣𝑒𝑠
1 𝑚𝑖𝑛×1 𝑚𝑖𝑛
60 𝑠= 0.167 𝑤𝑎𝑣𝑒𝑠
𝑠
𝑓 = 𝟎. 𝟏𝟔𝟕 𝐇𝐳
𝑣 = 𝑓𝜆
= (0.167)(2.1)
= 𝟎. 𝟑𝟓𝒎 𝒔−𝟏
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Standing Waves and SoundIB PHYSICS | UNIT 6 | WAVES - SOUND
6.3
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Standing Waves
Nodes Antinodes
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Standing Waves
# of Standing Waves # of Wavelengths Wavelength (m)
1 0.5 24
2 1 12
3 1.5 8
4 2 6
12 m
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Microwave Ovens Use Standing Waves
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What do you notice from the video?
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Harmonics
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Open Pipe Resonance
Antinode Antinode
L =32𝜆
1𝜆
12𝜆
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Closed Pipe Resonance
Node Antinode
L =54𝜆
34𝜆
14𝜆
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Strings make sound too!
_________________changes depending
on the string tension
Two ways to increase frequency in string:
wave speed
increase
tension
decrease
length
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String Resonance
L =32𝜆
1𝜆
12𝜆
Node Node
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Review of End Conditions
Closed Pipe Node Antinode
Open Pipe Antinode Antinode
String Node Node
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All the Harmonics!
Open Closed String
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Remember Pitch and Frequency
High pitched sounds have high frequencies
Low pitched sounds have low frequencies
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Making Different Pitches
The lengths are designed for the fundamental frequency
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Calculating Frequency | Open Pipes
A open organ pipe is 2.1 m long and the speed of sound in the pipe is 341 m/s. What is the fundamental frequency of the pipe?
𝐿 = 12𝜆 𝜆 = 2𝐿 = 2 2.1 = 4.2 𝑚
𝜆 = 4.2 𝑚𝑣 = 341 𝑚 𝑠−1𝑓 = ?
𝑣 = 𝑓𝜆 𝑓 =𝑣
𝜆=341
4.2
= 𝟖𝟏. 𝟐 𝐇𝐳
L
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Resonant String Practice
The note produced on a violin string of length 40 cm produces a wave speed of 250 m/s. What is the first harmonic of this note?
𝐿 = 12𝜆 𝜆 = 2𝐿 = 2 0.4 = 0.8 𝑚
𝜆 = 0.8 𝑚𝑣 = 250 𝑚 𝑠−1𝑓 = ?
𝑣 = 𝑓𝜆𝑓 =
𝑣
𝜆=250
0.8
= 𝟑𝟏𝟐. 𝟓 𝐇𝐳
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Finding Resonance
Water surface
Tuning Fork
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Calculating Frequency | Closed Pipes
You found an unmarked tuning fork in your collection. You notice that the smallest length for resonance is 12 cm. If the speed of sound is 345 m/s, what is the tuning fork frequency?
What should the length of the tube be for the 2nd resonant position?
𝐿 = 14𝜆 𝜆 = 4𝐿 = 4 0.12 = 0.48 𝑚
𝑓 =𝑣
𝜆=345
0.48= 𝟕𝟏𝟖. 𝟕𝟓 𝐇𝐳
𝐿 = 34𝜆 = 3
40.48 = 𝟎. 𝟑𝟔 𝒎
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Speed of Soundand Wave InterferenceIB PHYSICS | UNIT 6 | WAVES - SOUND
6.4
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Speed of Sound Depends on Medium
Air (25 °C)760 mph0.21 miles/sec
Speed of Sound for Air (at any temp)
v = 331 m s-1 + 0.6 × (Temp in °C)
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Speed of Sound Depends on Medium
Why does Medium Affect Speed?
molecule spacing
air
iron
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Using the Speed of Sound
You see lightning strike and immediately start counting, once you get to 7 seconds, you hear the boom of thunder. How far away is the storm?
Air (25 °C)760 mph0.21 miles/sec
𝑑 = 𝑣𝑡 = 0.21 7
= 𝟏. 𝟒𝟕 𝐦𝐢𝐥𝐞𝐬
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Shortcut for Clocking a Storm
As soon as you see lightning strike, start counting…
One one thousand, Two one thousand…
Stop counting as soon as you hear the thunder from that bolt of lightning
Distance in Miles = Time / 5
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ECHO…. Echo…. Echo….
When you hear an echo, you are hearing the sound after it has reflected off of an object and returned to your ear
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Calculating Distance from an Echo
A submarine send out a signal in water (v = 1,530 m/s) and hears that signal return 0.8 seconds later. How far away is the island?
𝑣 =𝑑
𝑡
𝑑 = 𝑣𝑡 = 1530 0.4 = 𝟔𝟏𝟐𝒎
Only half the time to go one way
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How do we locate sounds?
source
Sound reaches one ear before the other. It also sounds
different from different locations due to the shape of our ears.
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IB Physics Data Booklet
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Interference
When several waves are in the same location, they combine to produce a new wave that is different from the original waves.
After waves pass by one another continue on _____________unchanged
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Name that Interference
_________ Interference _________ InterferenceConstructive Destructive
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Constructive Interference
What is the resulting amplitude when
these waves meet?
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Destructive Interference
What is the resulting
amplitude when these
waves meet?
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Wav
e A
Time
Wav
e B
Time
Sup
erp
osi
tio
n
Time
Superposition of Waves
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IB Sample Question
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IB Sample Question
Both the waves below are moving at 0.5 m s-1 towards each other. What is the displacement at a distance of 1 m, after 4 s has passed?
(+3) + (-2) = +1
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Noise Canceling Headphones
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More Interference
DestructiveConstructive
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More Interference
A
B
C
B
A
C
Constructive:Path Difference = nλ
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More Interference
Destructive:Path Difference = (n+½)λ
D
E
E
D
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IB Physics Data Booklet
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Try This
Two coherent point sources S1 and S2 emit spherical waves.
Which of the following best describes the intensity of the waves at P and Q?
P Q
A Maximum Minimum
B Minimum Maximum
C Maximum Maximum
D Minimum MinimumMax
Min
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Try this #1
Two speakers are separated by a distance of 5 meters, if they emit a coherent sound signal of 850 Hz. If the speed of sound is 340 m s-1, is this person in a maximum or minimum location?
5 m
2.1 m 2.9 m
Path Difference
2.9 – 2.1 = 0.8 m
𝑣 = 𝑓𝜆
𝜆 =𝑣
𝑓=340
850= 0.4 𝑚
0.8
0.4= 𝟐
Maximum because result
is a perfect integer
Path Difference = (___) × λ
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Try This #2
4 m
3 m
If these speakers are playing a note with a frequency of 680 Hz, is this person standing at a maximum or minimum spot? Assume a speed of sound of 340 m s-2
What frequency would result in the opposite effect?
Path Difference
5 - 3= 1 m
𝜆 =𝑣
𝑓=340
680= 0.5 𝑚
𝟒. 𝟓 =2 m
𝜆(Could be anything
that ends in .5)
5 m 2
0.5= 𝟒
Maximum because result
is a perfect integer
𝜆 = 0.44 m
𝑓 =𝑣
𝜆=340
0.44= 𝟕𝟕𝟑 𝐇𝐳
Path Diff. = (___) × λ