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Exam II - Physics 1240 – Spring, 2010 – version 1
1. Suppose you pluck a guitar string and listen to the sound produced, then put your finger down
halfway along the string and pluck it again. What happens to the sound produced?
A)
B)
C)
D)
E)
The period is halved.
The frequency is doubled.
The pitch increases by one octave.
Both A and B.
A, B, and C.
2. What is the frequency of the note which is three octaves above 100 Hz?
A) 103 Hz
D) 600 Hz
B) 200 Hz
E) 800 Hz
C) 400 Hz
3. A sound intensity level (SIL) of 80 dB corresponds to what sound intensity, in W/m2?
A) 10-8 W/m2
B) 10-6 W/m2
C) 10-4 W/m2
D) 10-2 W/m2
E) 104 W/m2
4. What is a key difference between a flute and a recorder?
A) One of them makes use of an edgetone and the other does not.
B) One of them has a double reed and one has a single reed.
C) One of them has a reed and the other does not.
D) Something else.
5. One trumpet produces a sound 10 dB louder than one flute. How many flutes would have to play
together to produce the same sound intensity as the trumpet?
A) 2
D) 10
B) 3
E) 20
C) 7
6. Professor Betterton is playing music in the area behind the
lecture hall. The door is wide open. It is an ordinary (human
sized) door – the real door in our lecture hall! Assume all the
walls absorb sound very strongly, so that echoes off the walls
can be neglected (perhaps we covered all surfaces with
specially designed, strongly sound-absorbing carpets).
Which pitches of sound can be heard by people in the room
who are not in front of the door? That is, which sounds will be
heard in all parts of the room, because they spread out from the
door?
speaker
Absorbing walls
?
Open
door
walls
listener
A) Predominantly low pitches will spread out and be heard by the listener.
B) Predominantly high pitches will spread out and be heard by the listener.
C) All audible pitches will spread out and be heard by the listener, the frequency makes no difference.
D) No audible pitches will spread out and be heard by the listener, the frequency makes no difference.
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7. Suppose there are two tones playing simultaneously from two stationary instruments. The first tone
is a steady 440 Hz, yet you hear beats once every 2 seconds. What can you conclude must be true
about the second tone?
A) The second tone must be either louder or softer, but we can't easily tell which.
B) The second tone must be 0.5 Hz higher.
C) The second tone must be 2 Hz lower.
D) The second tone must be 2 Hz different, but could be higher or lower, we can't easily tell which.
E) The second tone must be 0.5 Hz different, but could be higher or lower, we can't easily tell which.
8. If you turn up the dial on your stereo from 30 dB to 60 dB, what happens to the intensity of sound
coming from the speakers into your room, as measured in Watt/m2? (Assume ideal conditions, with no
echoes or losses.)
A)
B)
C)
D)
E)
It increases by a factor of 10,000.
It increases by a factor of 1,000.
It increases by a factor of 100.
It increases by a factor of 30.
It increases by a factor of 2 (doubles).
9. You are listening to two speakers. The distance from you to one of the speakers is precisely half a
wavelength longer than the distance to the other one. Assume the speakers are wired to produce
sound in synch (in phase) with one another. What do you hear when the speakers play the same pure
tone?
A) A very soft or no sound (that is, it’s much softer than the loudness of either speaker alone).
B) A sound that beats (that is, it gets louder and softer, louder and softer, with time).
C) A sound louder than either speaker alone would produce.
D) A sound of distinctly different frequency than either speaker alone would produce.
E) A sound the same loudness as either speaker alone.
10. A light bulb has a power of 60 J/s (also called 60 Watts). After being on for 5 seconds how much
energy has the bulb released?
A) 60 J
D) 300 J
B) 100 J
E) 600 J
C) 200 J
11. I play a pure tone of frequency 100 Hz, which reflects off the wall I am facing. The bumps and
irregularities on the wall are about 30 cm wide (or smaller). What will happen and why?
A)
B)
C)
D)
E)
The sound will largely reflect diffusely because the surface is rough.
The sound will largely reflect diffusely because the surface is smooth.
The sound will largely reflect specularly because the surface is rough.
The sound will largely reflect specularly because the surface is smooth.
The sound will largely reflect specularly, it doesn’t matter whether the surface is smooth or rough.
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12. How will the sound of a wind instrument change if you alter it (e.g., by sliding a pipe, or closing
holes) in such a way that the tube gets longer by a factor of 4?
A) The pitch goes up by 1 octave.
B) The pitch goes up by 2 octaves.
C) The pitch goes down by 1 octave.
D) The pitch goes down by 2 octaves.
E) The pitch stays pretty much the same, tube length does not play a big role in determining pitch of a
wind instrument.
13. What happens, in the previous question, if instead
you swap instruments for one of the same length, but
with 2 times wider tube diameter? (See the figure.)
Length
A) The pitch goes up by 1 octave.
Diameter
B) The pitch goes up by 2 octaves.
C) The pitch goes down by 1 octave.
D) The pitch goes down by 2 octaves.
E) The pitch stays pretty much the same, tube diameter does not play a big role in determining pitch of
a wind instrument.
14. A speaker in the stadium is emitting a loud, steady sound which spreads out and reaches you.
You have two “receiver microphones” to pick up the sound. Microphone #2 is bigger, it has twice the
receiving area as microphone #1. Both microphones listen to the same sound at the same spot for the
same amount of time. Consider the following two statements and decide if each is true or false:
i) The intensity of sound at the location of each microphone is the same.
ii) The energy absorbed by microphone #2 is twice the energy absorbed by microphone #1.
A) Both i and ii are true.
B) Both i and ii are false.
C) Statement i is true, but ii is false.
D) Statement i is false, but ii is true.
15. Suppose Alex plucks a guitar string gently, so it produces a soft sound; then Betty plucks a guitar
string more vigorously, so the sound intensity Betty produces is twice as large as what Alex produces.
How many decibels louder is Betty’s sound?
A) 1 dB
B) 2 dB
C) 3 dB
D) 5 dB
E) 10 dB
16. The atmosphere on Mars is mainly carbon dioxide, and the weather there is a bit cold. Both
changes tend to make the speed of sound lower; let's say it happens to be 172 m/s there (half what it is
here on Earth). Some Martians visited Earth and have carried home with them a guitar and a flute, on
which they learned to play melodies in unison [same note from both instruments] on Earth. If they try
the same thing on Mars, will there be any change in the notes produced?
A)
B)
C)
D)
E)
The two instruments will sound an octave lower than they do on earth (but will still be in unison)
The two instruments will sound an octave higher than they do on earth (but will still be in unison)
The two instruments will sound just the same as they do on earth.
The flute will sound the same as it does on earth, but the guitar will sound an octave lower.
The guitar will sound the same as it does on earth, but the flute will sound an octave lower.
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17. Which pair of tones would have a slow very distinct beat? (Assume both are equally loud initially.)
A)
B)
C)
D)
E)
10 Hz and 20 Hz
10 Hz and 200 Hz
100 Hz and 200 Hz
100 Hz and 120 Hz
100 Hz and 102 Hz
18. The picture shows a traveling car that is emitting a
sound. The lines show the points of high pressure
(pressure peaks) in the sound wave. There are listeners
at points A, B, C, and D. Who hears the highest pitch?
A) Person A
B) Person B
C) Person C
D) Person D
E) They all hear the same pitch.
19. Which of the following is the best explanation for the effect described in the previous question?
A) Because sound drops off with the square of distance, so you will perceive a steadily changing effect
as the car approaches and then recedes.
B) Because the car is “catching up” to sound waves ahead of it, but is “running away” from the sound
waves behind it, so the time between peaks is different as the car approaches and recedes.
C) Because sound travels through air, and the air is not moving, so the wavelength must be the same
everywhere, and therefore so is the frequency.
D) Because sound waves must pile up in front of a moving car, but will spread out behind it, creating a
different wavelength of sound in the stationary air on the two sides of the moving car.
E) Because the car seems louder and louder as it approaches, and softer and softer as it recedes, which
means the relative frequencies are correspondingly affected.
20. Standing 10 meters from the speakers at an outdoor concert, the SIL is 100 dB. Neglecting
reflections and friction (so there are no “losses”), how loud is it 20 meters from the speakers?
A) 50 dB
B) 60 dB
C) 74 dB
D) 80 dB
E) 94 dB
21. I have a speaker with a diameter of 34.4 cm. Which sounds will head out primarily in the forward
direction from the speaker, and which will spread out in all directions?
A)
B)
C)
D)
E)
Frequencies below 10 Hz will head forward, and frequencies above 10 Hz will spread out.
Frequencies above 10 Hz will head forward, and frequencies below 10 Hz will spread out.
Frequencies below 1000 Hz will head forward, and frequencies above 1000 Hz will spread out.
Frequencies above 1000 Hz will head forward, and frequencies below 1000 Hz will spread out.
None of the above is a good description of which sounds will head forward and which will spread.
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22. All semester I have argued that sound is a wave. But how do we know? In particular, how do you
know sound is not a stream of some sort of “sound particles” emanating and spreading out in all
directions? Of the five particular experiments listed below, only one of them most directly and
convincingly demonstrates that sound must be a wave phenomenon. Which one?
A) For the first homework this term, you went outside and clapped in front of a wall, and were able to
hear a clear and distinct echo.
B) If you listen to two speakers playing the same tone, and move your head around, you find “silent
spots” in a simple pattern in the room, spots where the sound canceled itself out.
C) If you measure the intensity of sound from a small source, you clearly measure that intensity drops
as the square of the distance.
D) When you listen to a pure tone generated by a function generator, you find that doubling the
frequency produces a tone which sounds similar to the original tone.
E) As you change the medium (either the temperature of the air in the room, or the relative humidity
level) the measured speed of sound changes too – it is not just some fixed, constant value.
23. The amplitude of sound wave A is one half the amplitude of sound wave B. How do the intensities
of the two waves compare?
A)
B)
C)
D)
E)
The intensity of wave A is one quarter the intensity of wave B.
The intensity of wave A is one half the intensity of wave B.
The intensity of wave A is equal to the intensity of wave B.
The intensity of wave A is twice the intensity of wave B.
The intensity of wave A is four times the intensity of wave B.
24. I am listening to two speakers. The distance from me to one of the speakers is exactly the same as
the distance to the other one. Assume the speakers are wired to produce sound in synch (in phase)
with one another. What do I hear when the speakers play the same pure tone?
A) A very soft or no sound (that is, it’s much softer than the loudness of either speaker alone).
B) A sound that beats (that is, it gets louder and softer, louder and softer, with time).
C) A sound louder than either speaker alone would produce.
D) A sound of distinctly different frequency than either speaker alone would produce.
E) A sound the same loudness as either speaker alone.
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25. Two waves head towards one another on an ideal string.
The figure on the right shows you what the pulses look like at
time t=0 sec. Look carefully, note the right wave is smaller and
skinnier. They each travel at 1 m/s.
1 m/s
3
2
1
1 m/s
0
Look carefully, note the right wave is smaller and skinnier, try
not to guess, work it out, use the “scratch” spot if that helps.
1
2
3
4
5
6
meters
1
2
3
4
5
6
meters
3
2
1
0
(For "scratch" work)
Which of the figures below shows the shape of the string at t=2 seconds?
A)
3
2
1
0
1
2
3
0
5
0
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
D)
3
2
1
E) None of these is correct.
Answer: B
4
C)
3
2
1
B)
3
2
1
0