Sound waves traveling out
Higher P
Lower P
If the speaker vibrates back and forth at 200 Hz how much time
passes between each time it produces a maximum in
pressure?
a. 0.2 seconds
c. 0.005 seconds
e. 0.05 seconds
b. 200 seconds
d. 0.02 seconds
Today’s song
• “Ode to Joy”, composed by Beethoven
• Played on the power drill instrument
Phys 1240 Announcements
www.colorado.edu/physics/phys1240
LAST TIME: waves and oscillations
TODAY: waves
NEXT TIME: simple harmonic motion
READ: None. Ch. 3.2-3.3 for next Thursday
• Homework 2 and Reading Question 2 due tonight.
– Make sure CAPA is working for you!
• Homework 3 downstairs; Reading Question 3 will
be live later today.
• Exam 1 is two weeks from today (Thursday 2/16).
– Practice exam posted on course website!
– Exam review in class Tuesday 2/14
Wave on a string
PhET simulation
Review: speed, frequency, and
wavelength
Transverse and longitudinal waves
A wave is a traveling disturbance of some
material (the medium)
Which way does the material wiggle?
(Transverse or longitudinal?)
• Animations
• Animation of longitudinal waves:
Clicker question
Is the “wave on string” demo and simulation
showing
A) A transverse wave
B) A longitudinal wave
C) A mixed wave
D) Not sure…
Clicker question
speaker
dust
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?
A)
(up and down)
B)
(pushed steadily to the right)
C)
(left and right)
D)
E)
(no motion)
(circular path)
Clicker question
Which one of the following is most likely to be
impossible?
A:
B:
C:
D:
E:
Transverse waves in a gas
Longitudinal waves in a gas
Transverse waves in a solid
Longitudinal waves in a solid
They all seem perfectly possible
Wave calculations
• We’ve covered the basics of waves:
– Amplitude (loudness – increased by an
amplifier)
– Period and frequency (pitch)
– Wavelength
– Wave speed
– Longitudinal and transverse waves
• Let’s review & apply these ideas in a
bunch of clicker questions
Clicker question
What is the period of this wave?
Amount
0
t1
A.
B.
C.
D.
E.
t2
t1
t2
t2-t1
t3-t1
None of the above
t3
time
Clicker question
The wavelength, λ, is 10 m. What is the speed
of this wave?
Time (sec)
1
A) 1 m/s
B) just under 7 m/s
C) 10 m/s
D) 15 m/s
E) None of the above/not enough info/not sure
Clicker question
You are standing outside on a pleasant day, playing a
recorder. You play A4 (“concert A”, also known as
A440) i.e. during one second 440 compressions of air
move outwards. (Recall: the speed of sound is 344 m/s)
What is the approximate wavelength of the sound
wave?
A. A couple of mm
B. A couple of cm
C. A little under one meter
D. A little over one meter
E. More than a km
Wavelength, frequency, and
instruments preview
• Wind instruments (flute, oboe, organ
pipes, recorder)
– Produce notes with a fixed wavelength
– Based on how waves fit into the pipe
• String instruments (violin, cello, guitar,
bass)
– Produce notes with a fixed frequency
– Based on how the string wiggles
• We will figure out in detail later why this is true.
For now, let’s think about the consequences!
Clicker question
The air warms up by 20o C. The recorder (like most
wind instruments) produces waves of a particular
wavelength. So, changing the temperature will not
change the wavelength of sound waves produced
(noticeably.) What will change?
A. Just the frequency
B. Just the speed of sound
C. Both frequency and speed of sound
D. Neither frequency nor speed of sound
E. Not enough information to tell
Clicker question
You are playing a guitar. The string vibrates 440
times every sec. The air temperature increases by 20
C. Note: the STRING continues to vibrate at the same
440 /sec independent of temperature.
Will the wavelength of the produced sound wave
change?
A. Yes, it goes up
B. Yes, it goes down
C. No, it stays the same
D. Don’t know.