Cell Phones silent Clickers on - Physics · CLICKER 5.1 When sound travels from a source to a...
Transcript of Cell Phones silent Clickers on - Physics · CLICKER 5.1 When sound travels from a source to a...
This is PHYS 1240 - Sound and Music
Lecture 5
Professor Patricia Rankin
Cell Phones silent
Clickers on
Remember - Learning Team – you can email/skype/facetime/zoom in virtual office
hours
Graduate student Teaching Assistant
Tyler C Mcmaken - online 10-11am Friday
Undergraduate Learning Assistants
Madeline Karr Online 6-7pm MondayRishi Mayekar Online 11-12noon ThurMiles Warnke Online 3-4pm Wed
Professor
Patricia Rankin Online 2-3pm Wed
Physics 1240 Lecture 5
Today: Shock waves, Doppler Effect, Beats, Propagation
of Sound, Interference, Superposition
Next time: Characteristics of sound, Waves in
times/space, waves in 2d, 3d, Standing Waves
physicscourses.colorado.edu/phys1240
Canvas Site: assignments, administration, grades
Homework – HW2 Due Wed Jan 29th 5pm
Homelabs – Hlab2 Due Monday Feb 10th 5pm
Debrief – last class
Normal or natural modes
Q – Damping
number of cycles before amplitude
drops to 4.32% of starting value
Q – Resonance – peak frequency/bandwidth that excites
motion at least half peak amplitude
Estimating distance using sound – echoes, lighteningAfter you see lightning, start counting to 30 (30s). If you hear thunder before you reach 30, go indoors. Suspend activities for at least 30 minutes after the last clap of thunder.
𝜈 =𝜆
𝑇= 𝜆𝑓
Wavelength
PeriodFrequency
Tutorial
How can you improve piano key estimate?
What other info might help?
#of pianos sold in a year
#of piano tuners
#of violins
#of piano students
Socioeconomic status
Period/frequency
Dimensional Analysis
CLICKER 5.1
When sound travels from a source to a listener,
what do air molecules never do?
a) Travel from the source to the listener at 344 m/s
b) Move back and forth with the frequency of the sound wave
c) Bump into other air molecules
d) Move in the direction of wave propagation
e) Air molecules do all of the above
CLICKER 5.1A
When sound travels from a source to a listener,
what do air molecules never do?
a) Travel from the source to the listener at 344 m/s
b) Move back and forth with the frequency of the sound wave
c) Bump into other air molecules
d) Move in the direction of wave propagation
e) Air molecules do all of the above
I grab the left end of a LONG string and
wiggle it up and down. There is a purple
painted spot somewhere on the string.
What choice below best shows the motion
of the purple spot?
A) (up and down)
B) (pushed steadily to the right)
C) (left and right)
D) (no motion)
E) (circular path)
5-4
Clicker 5.2
I grab the left end of a LONG string and
wiggle it up and down. There is a purple
painted spot somewhere on the string.
What choice below best shows the motion
of the purple spot?
A) (up and down)
B) (pushed steadily to the right)
C) (left and right)
D) (no motion)
E) (circular path)
5-4
Clicker 5.2 A
A dust particle is located in front of a
speaker. The speaker moves back and forth at a
constant rate, creating a loud sound wave.
Which choice below shows the motion of the
dust particle?
speakerdust
5-3
A) (up and down)
B) (pushed steadily to the right)
C) (left and right)
D) (no motion)
E) (circular path)
Clicker 5.3
A dust particle is located in front of a
speaker. The speaker moves back and forth at a
constant rate, creating a loud sound wave.
Which choice below shows the motion of the
dust particle?
speakerdust
5-3
A) (up and down)
B) (pushed steadily to the right)
C) (left and right)
D) (no motion)
E) (circular path)
Clicker 5.3 C
Shock waves
• “Sonic boom” occurs when object travels faster than the
local speed of sound (waves can’t get out of the way fast
enough)
• Bullets
• Whip Crack
• Apatosaurus tail
Boat wakes
If you toss a pebble in a pond, little waves will form
in concentric circles and propagate away from the point
of impact. If a boat travels through the pond at 3 to 5 miles
per hour, little waves will propagate in the same way both
ahead of and behind the boat, and the boat will travel
through them.
if a boat travels faster than the waves can propagate
through water, then the waves "can't get out of the way" of
the boat fast enough, and they form a wake. A wake is a
larger single wave. It is formed out of all the little waves
that would have propagated ahead of the boat but could
not.
When an airplane travels through the air, it produces sound waves. If the plane
is traveling slower than the speed of sound (the speed of sound varies, but 700
mph is typical through air), then sound waves can propagate ahead of the
plane. If the plane breaks the sound barrier and flies faster than the speed of
sound, it produces a sonic boom when it flies past. The boom is the "wake" of
the plane's sound waves. All of the sound waves that would have normally
propagated ahead of the plane are combined together so at first you hear
nothing, and then you hear the boom they create.
Doppler Effect
• Doppler effect: the shift in frequency of a wave where
the source and the observer are moving relative to one
another (higher frequency if moving toward each other)
∆𝑣
𝑣sound≅ 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛𝑎𝑙 change in 𝑓 =
𝑓1−𝑓0
𝑓0
∆𝑣 = source velocity – observer velocity
𝑓0=emitted frequency
𝑓1=received frequency
Clicker 5.4
A fire truck travels away from you at 67 mph (30 m/s). If the speed of sound
is 343 m/s, do you hear a lower- or higher-pitched note from the siren than
the driver? What’s the percent change in the frequency?
A) You hear 8.75% lower
B) You hear 8.75% higher
C) You hear 19.5% lower
D) You hear 19.5% higher
E) You hear the same pitch as the driver
Clicker 5.4 A
A fire truck travels away from you at 67 mph (30 m/s). If the speed of sound
is 343 m/s, do you hear a lower- or higher-pitched note from the siren than
the driver? What’s the percent change in the frequency?
A) You hear 8.75% lower
B) You hear 8.75% higher
C) You hear 19.5% lower
D) You hear 19.5% higher
E) You hear the same pitch as the driver
% change in 𝑓 ≅∆𝑣
𝑣sound=
30m/s
343m/s≅ 0.0875
What happens when sound
wave encounters an
obstacle?
Provides basis for acoustics
Sound Propagation – Possibilities
• Reflection (diffuse or spectral)
• Absorption (greater for softer surfaces)
• Refraction (when speed of sound changes)
• Diffraction (greater for larger wavelengths)
Acoustic Shadows
Regions where sound has failed to
propagate
Refraction
Light
Why straws look bent in water
Refraction:
bending due to a change in the
speed of sound (change in medium)
Diffraction
Allows sound to get around corners
Depends on wavelength of wave/size of gap
Diffraction – single slit (ripple
tank)
Diffraction Double Slit
peaktrough
Adding/Superimposing
• Waves just add
• Adding two waves can be constructive or destructive
Interference/Superposition
Constructive Interference
• For ∆𝐿 = 𝐿2 − 𝐿1 = difference between your distance from one source
and your distance from a second source:
• Constructive: ∆𝐿 = 𝑛λ (where 𝑛 is an integer)
Destructive Interference
• For ∆𝐿 = 𝐿2 − 𝐿1 = difference between your distance from one source
and your distance from a second source:
• Destructive: ∆𝐿 = (𝑛 + 1/2)λ2
(where 𝑛 is an integer)
Noise Cancellation
Clicker 5.5
If you are in a room with two speakers each producing sine waves with a
wavelength of 2 meters, where should you stand if you don’t want to hear any
sound?
A) 2 meters from one speaker and 2 meters from the other
B) 2 meters from one speaker and 4 meters from the other
C) 2 meters from one speaker and 3 meters from the other
D) 3 meters from one speaker and 5 meters from the other
E) 1 meter from one speaker and 0.5 meters from the other
Clicker 5.5 C
If you are in a room with two speakers each producing sine waves with a
wavelength of 2 meters, where should you stand if you don’t want to hear any
sound?
A) 2 meters from one speaker and 2 meters from the other
B) 2 meters from one speaker and 4 meters from the other
C) 2 meters from one speaker and 3 meters from the other
D) 3 meters from one speaker and 5 meters from the other
E) 1 meter from one speaker and 0.5 meters from the other
Can also add waves of different
frequencies