Standing Waves, Resonance,
and The Doppler Effect
Supplemental Instruction
Iowa State University
Conceptual Review
Leader:
Course:
Instructor:
Date:
Matt T
Physics 221
Whisnant/Ho
4/10/13
True/False
 If you turn the volume down so that it sounds approximately half as loud, the amount of
energy being transferred to your ears will be halved as well.
 The motion of waves can be transverse or longitudinal, but not both.
 The speed of a wave depends only on the medium.
 Cases of complete constructive and destructive interference require that the different
waves have identical wavelengths.
 Beats also require that the wavelengths be equal.
 Power in waves varies as distance squared.
 The mathematics behind the Doppler Effect doesn’t depend on whether the moving body
is the source or listener
 In Doppler Effect problems, relative motion towards the source or listener always
increases the frequency of the wave.
Short Answer
1. Your roommate bought a pair of 14” subs (large speakers that produce waves with low
frequencies) and found that while large amounts of power are supplied to them, different
frequencies shake the house different amounts. In particular, one frequency seems to
shake the house more than any others. What is this frequency called?
2. Two alarms with identical frequencies go off at the same. Without putting anything in
between you and the alarms, explain how you can stop hearing the noise.
3. Below is a table that lists some important physical properties of selected metals. Through
which metal will a sound wave propagate the fastest?
Material
Steel (85% C)
Gold
Brass
Cast Iron
Density (kg/m3)
7,800
19,300
8,500
7,300
Elastic Modulus (GPa)
200
80
100
100
4. Which change will increase the average power of a wave in a cable the most?
a. Doubling the tension
b. Doubling the propagation speed
c. Doubling the height of the wave
5. In class, we discussed how simultaneous waves of slightly different frequencies create
“beats,” or constructive waves that sound dissonant to the musical ear. Explain how
chords, which include many different notes and different frequencies, sound good.
Supplemental Instruction
1060 Hixson-Lied Student Success Center  294-6624  www.si.iastate.edu
Problems
1. For those who want to be amazed: Recall that intensity I = Pavg/A. Calculate the rate of
energy transfer to your ears (area 7e-5 m2) from an uncomfortably loud 100 dB signal.
2. Follow-up: You pick up a cheap pair of earplugs that claim to cut 25 dB from loud
noises. At what rate do they absorb energy if used in the previous problem?
3. Written Homework 12.2: In which of these scenarios do you hear a higher-pitched
sound? Explain.
a. You approach a stationary police siren with velocity v0.
b. You are stationary and the same police siren speeds by you.
4. Find the frequencies for the previous problem if v0 = 90 mph. Recall that the speed of
sound is roughly 344 m/s in air at 20 °C.
5. Two identical violins strings are tuned to exactly 440 Hz.
a. If the tension of one string is increased by 1% and the two strings are plucked at
once, what frequency will be heard?
b. If a finger is placed as to reduce the length of one violin by 1%, what frequency
will be heard?
6. Written Homework 12.4 (time-permitting): Use the back of this sheet for space.