Sound MCQ
Preamble
Here is a group of questions designed to check your understanding of the processes of sound generation and sound travel.
Question 1: Thunder and lightning
You have probably noticed that during a thunderstorm you see a lightning flash some time before you hear the thunder.
That is because
A: the thunder is generated only after the lightning has stopped.
B: lightning and thunder are unrelated events.
C*: light travels a lot faster than sound.
?: I don't know.
Feedback:
Although it is true that thunder can be understood as a consequence of lightning, the substantial delays between lightning and thunder are best explained in terms of the significant travel time required for the sound.
Question 2: Speed of sound
Let's follow up on the different speeds for sound and light.
The speed of sound in air is
A: noticeably greater than the speed of light in air
B: exactly the same as the speed of light in air.
C: about the same as the speed of light in air.
D: a few percent of the speed of light in air.
E*: very much less than a few percent of the speed of light in air.
?: Don't know.
Feedback:
This is something that you know or you don't, but you should be able to rule out the first three alternatives using the answer to the previous question. The speed of light in air is about 300 million metres per second and the speed of sound is typically about 340 metres per second.
Question 3: Medium for sound or light
We have established that sound waves travel through air, but can they travel through other materials or a vacuum? In fact sound waves can travel in
A: air only.
B: any kind of gas but not a liquid or a solid.
C*: gas, liquid or solid.
D: vacuum, gas, liquid or solid.
?: Don't know.
Feedback:
Although we normally experience sound travelling through air, it can travel through any material. People are usually less aware of sound in media other than air because it can be quite difficult to get the sound into them - it gets reflected at boundaries between materials.
Question 4: What carries light?
We have established that sound has to be carried by a material medium. How does that compare with light?
Light waves can travel in
A: air only.
B: any kind of gas but not a liquid or a solid.
C: gas, liquid or solid.
D*: vacuum, gas, liquid or solid.
?: Don't know.
Feedback:
This is a crucial difference between sound and light. Sound involves vibrations of a material medium but light can be modelled as electromagnetic waves which consist of electric and magnetic fields. Although the fields are affected by materials they can also exist in "empty" space (empty except for fields that is). Some materials affect light so much that it is all absorbed before it gets very far. So light travels most readily through space and less readily through many materials but sound can exist only in material medium.
Question 5: Sound from a vibrating string
A steel guitar string is stretched between supports that are 60 cm apart and the tension is adjusted so that the speed of transverse waves on the string is 33 m.s-1. The string is then plucked so that it vibrates in its fundamental mode.
Estimate the wavelength of the sound in the air produced by this vibration of the string given that the speed of sound in air is about 330 m.s-1.
A: 30 cm
B: 60 cm.
C: 120 cm.
D: 300 cm
C: 600 cm.
E*: 1200 cm.
?: Don't know.
Feedback:
The key to this little problem is the realisation that the wire makes the air vibrate with a frequency that matches its own frequency. Although the frequencies of the wave on the wire and the sound wave are equal their speeds are different, so their wavelengths must be different. Since wave speed must be equal to the product of frequency and wavelength and the speed of sound is about 10 times that of the wire's wave, the sound's wavelength must also be 10 time bigger.
Now to work out the wavelength of the wave on the wire: in the fundamental mode, the simplest vibration, there is a node at each of the fixed ends so half a wavelength fits on one length, 60 cm, of the wire. So the wavelength of wave on the wire is 120 cm. Multiply that by 10 to get the wavelength of the sound.
Question 6: Sound from a vibrating string
The vibration of the guitar string gradually dies away. As it does so
A: its frequency decreases
B*: its amplitude decreases
C: its phase decreases
D: all of the above.
?: Don't know.
Feedback:
The vibrating string loses energy. (Some of the lost energy goes into the sound waves that travel out in all directions, some is lost in other ways.) Less energy in the vibration implies that the amplitude must be smaller. Frequency is not affected by the amplitude and it does not make sense to talk about a decreasing phase.
Question 7: Blowing bubbles in the air
Morris Hults, a physics teacher in Fort Wayne, Indiana in the USA, does a demonstration like this. A man plays a short tune on a trumpet which has a big soap bubble over the end where the sound normally comes out. The class is asked to predict what they will see and hear.
As the player blows his tune into the trumpet and before the bubble bursts
A the bubble grows but no sound comes out.
B* the bubble grows and sound comes out
C the bubble stays the same size and sound comes out.
D the bubble stays the same size and no sound comes out.
?: Don't know.
Feedback:
The bubble grows because the player is blowing into the instrument and the sound generated in the trumpet can easily get though the bubble. Some people think, mistakenly, that because sound can be described as movements in the air that air has to move along with the sound. In fact the sound moves through the air and requires no large scale movement of air, only very small local vibrations of the air at each place where the sound passes through.
Question 8: More bubbles in the air
Continue to think about playing the trumpet with the soap bubble over its end. Suppose that we could have a very close and very fast look at the soap bubble by making a "slow-motion" movie of the surface of the bubble as the trumpeter plays his tune. What would you expect to see on the expanding surface of the bubble?
A*: Small complex vibrations whose complex pattern keeps changing.
B: Small simple harmonic vibrations whose frequency changes with each new note.
C: A small simple harmonic vibration with constant frequency and changing amplitude.
D: A small simple harmonic vibration with constant amplitude and changing frequency.
E: A perfectly smooth-looking expanding bubble.
?: Don't know.
Feedback:
The bubble will vibrate as it transmits the sound. Although the pitch of each note can be modelled using a fundamental simple harmonic oscillation, with real musical instruments each note of a tune consists of a different complex oscillation (which usually includes the fundamental).
Question 9: Two musicians
A microphone is placed in a hall at the same distance from a trumpeter and an clarinettist who are standing well apart and playing a tune in unison (the same notes together). The trumpeter is playing louder than the clarinettist.
Which one of the following statements is true?
A: A loud sound from the trumpet reaches the microphone before the same softer note from the clarinet.
B*: The microphone receives sound energy from the trumpet at a greater average rate than it does from the clarinet.
C: Waves from the trumpet and the clarinet arrive at the microphone in phase.
?: Don't know.
Feedback:
Since the trumpet is playing louder you can deduce that it is producing waves of higher intensity and since the distances are the same the intensity of the trumpet sound at the mike will be greater. (To complete the formal argument we need to realise that intensity is power per area and say that since the sound-collecting area of the mike is the same for both sounds, the rate of receiving energy is greater for the trumpet sound.)
Extra:
Alternative A contains the misconception that loud sounds travel faster than soft sounds - not true!
Alternative C is plausible for two point sources emitting identical sounds which are exactly in phase. That implies a control over the production of sound on a time scale which is much shorter than the oscillation period of the sound. In this case, not only are the sounds quite different, but such fine control over the timing is beyond the capabilities of even the best musicians. It does not make sense to compare phases from independent sources of sound.
Question 10: Speeding ambulance
Imagine that you are standing in the path of an ambulance that is coming straight towards you at a constant speed, while sounding a fixed note on its horn. (Remember to jump out of the way.)
As the ambulance approaches, the loudness of the note that you hear
A: decreases.
B: remains constant.
C*: increases.
?: Don't know.
Feedback:
Perceived loudness is determined mainly by the intensity of sound. The intensity of a sound decreases with increasing distance from its source so as the distance from the ambulance decreases the intensity of sound from the ambulance increases.
Question 11: Ambulance still coming
We are still considering the same example. Imagine that you are standing in the path of an ambulance that is coming straight towards you at a constant speed, while sounding a fixed note on its horn.
As the ambulance approaches, the fundamental frequency of the note that you hear
A: decreases.
B*: remains constant.
C: increases.
?: Don't know.
Feedback:
Although the sound gets louder its frequency remains constant. The effect of the source's motion on the sound wave depends on how the source is moving in the direction of travel of the sound. In this case the direction of the source's motion and the direction in which the sound (that interests us) travels are the same and the source's velocity is also constant. The things that affect frequency are constant so the frequency is constant.
Question 12: More ambulance
We are still considering the same example of the ambulance that is coming straight towards you at a constant speed, while sounding a fixed note on its horn. Suppose that the fundamental frequency of the note generated by the horn is 920 Hz. We have already seen that the note that you hear gets louder but has constant frequency. What about the value of that frequency compared with that of the horn?
The fundamental frequency of the note that you hear
A: is less than 920 Hz.
B: is 920 Hz.
C*: is more than 920 Hz.
D: could have any value near 920 Hz.
?: Don't know.
Feedback:
When the source is approaching the listener the frequency heard by the listener is higher than that of the source. Experience is probably the best reason to quote here but the next question will explore the theoretical reasons.
Question 13: Explaining the sound
We are still considering the same example of the ambulance that is coming straight towards you at a constant speed, while sounding a fixed note on its horn. The answer to the last question was that the frequency heard by the listener is higher than that of the moving source. What is the reason for that?
A: The motion of the horn actually changes the horn's frequency.
B: Some of the kinetic energy of the horn is transferred to the air which makes it vibrate faster.
C*: The horn catches up a bit with wavefronts that it has already emitted, thus compressing the sound's wavelength.
?: Don't know.
Feedback:
Alternatives A and B are fanciful (see the extra notes). That leaves C, which makes sense. The distance between successive wavefronts depends on how the source has moved in the time between emitting them (which is one period of the source). Since the speed of the sound does not depend on the speed of the source and since wave speed is equal to the product of wave frequency and wavelength (v = fl), shortening the wavelength requires that the wave frequency must be increased.
Extra:
Comments on the wrong alternatives.
A: There is no known physical mechanism whereby the mere constant motion of an object can affect any of its mechanical properties or behaviour - that is implied by Newton's laws of motion.
B: Bulk energy given to the air, say by pushing it out of the way, has no direct effect on the propagation of sound. It is true that sound waves travel with a fixed speed relative to the air, so a wind in the direction of travel would affect the wavelength but it's hard to see how the motion of the horn alone would produce a significant wind. Although motion of the air near the horn may affect the frequency of the sound in that region, the effect is unlikely to extend as far as the listener. In any case, transfer of kinetic energy makes no sense because the ambulance is moving with constant velocity and the kinetic energy of the horn does not change.
Question 14: Ambulance on a different course
Now it's time to consider a slightly more realistic example. Instead of being directly in the path of the ambulance, suppose that you are standing well off to one side of a straight road so that the ambulance is no longer heading straight at you. The ambulance is coming along the road at constant speed. We will explore what differences that may make to our previous answers? We can now also consider what happens as the ambulance goes past you.
To answer this question, let us assume that the horn emits sound equally in all directions (isotropically as they say in the text books).
When will the sound that you hear from the ambulance's horn will be loudest? To answer this, let us assume that the horn emits sound equally in all directions (isotropically as they say in the text books).
A: A little before the ambulance gets closest to you.
B: When the ambulance is closest to you.
C*: A little after the ambulance has come closest to you.
D: The loudness does not change.
?: Don't know.
Feedback:
The loudest sound to reach you will be the one that has the shortest distance to travel. That is emitted when the ambulance is closest, but by the time the sound gets to you the ambulance will have moved on a little.
Question 15:
We are still considering you standing well off to one side of a straight road and the ambulance coming along the road at constant speed.
How does the frequency of the sound that you hear change as the ambulance approaches and goes past you?
A: The frequency remains constant (as before).
B*: The frequency drops continuously throughout the motion.
C: The frequency drops as the ambulance gets closer and then rises again as the ambulance goes away.
?: Don't know.
Feedback:
This is the situation that you have probably experienced. What matters theoretically is the component of the ambulance's velocity in the direction of the sound that reaches you. Although the ambulance's velocity is constant, its component in the direction of a line from the ambulance to you keeps changing because that direction keeps changing. That component will be zero when the line from the horn to you is perpendicular to the motion (the point of closest approach) and it will be negative when the ambulance is retreating from you.
Question 16: Ambulance on a different course
We are still considering you standing well off to one side of a straight road and the ambulance coming along the road at constant speed.
In the answer to the previous question you saw that the frequency that you hear decreases all the time as the ambulance approaches and goes past you. Now we want to know how rapidly it changes. At which stage of the motion is the rate of decrease of the frequency greatest?
A: A little before the ambulance gets closest to you.
B: When the ambulance is closest to you.
C*: A little after the ambulance has come closest to you.
D: The rate of decrease in frequency is constant.
?: Don't know.
Feedback:
The rate of decrease in frequency will be greatest when the component of the ambulance's velocity in the direction towards you changes most rapidly. That occurs when the line from the horn to you is perpendicular to the motion (the point of closest approach). The most rapidly changing sound is emitted when the ambulance is closest, but by the time the sound gets to you the ambulance will have moved on a little, so C is a better answer than B.
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