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Unformatted text preview: 0001-1 Physics 1240 Oct 27, 2005 Version 0001 Midterm #2 After the exam, tear off the LAST PAGE (with your long answer question) PLEASE, CAREFULLY FOLLOW THESE DIRECTIONS. If you don't, we may lose your grade! Using a number 2 pencil: Write in and bubble in your name and CU ID number on the bubble sheet. Write and bubble the version number (0001) in the upper left corner of the bubble sheet. After you start, don't forget to neatly ALSO put your name and CU ID # on the last (long answer) page, which you'll be handing in. Be sure you have found all the problems: This exam has 14 multiple choice questions, followed by 1 page (double sided!) of "long answer" questions. The 14 multiple choice problems are 7 pts each. (Remember to bubble them in the bubble sheet!) The long answer problem is worth 12 pts total. Please budget your time appropriately. EXAM IS DOUBLE SIDED - DON'T MISS ANY PAGES! By handing in this exam, you agree to the following statement: "On my honor, as a University of Colorado Student, I have neither given nor received unauthorized assistance on this work" Please do not open the exam until instructed to do so. Good luck! SIL difference (in decibels) 1 dB 2 dB 3 dB 4 dB 5 dB 6 dB 7 dB 8 dB 9 dB Intensity ratio 1.3 1.6 2.0 2.5 3.2 4.0 5.0 6.3 7.9 0001-2 (This side intentionally left blank) 0001-3 Bubble in questions 1-14 ON YOUR BUBBLE SHEET! 1. 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 sec. 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 0.5 Hz lower D) The second tone must be 2 Hz higher E) The second tone must be 0.5 Hz different, but could be higher or lower, we can't easily tell which. 2. 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 sound 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 3. What is the frequency of the note which is three octaves above 100 Hz? A) 103 Hz B) 200 Hz C) 400 Hz D) 600 Hz E) 800 Hz 4. How will the sound of a wind instrument change if you alter your fingering on the holes in such a way that the tube gets shorter by a factor of 2? A) The pitch goes up by 1 octave B) The pitch goes up by 2 octave C) The pitch goes down by 1 octave D) The pitch goes down by 2 octaves E) The pitch stays the same, the amplitude (and therefore loudness) is primarily what changes. 5. My dog Sasha is barking, producing a sound of 75 dB. Her evil twin Rasha is barking louder still, producing a bark of 100 dB. What is the ratio of the intensity (in W/m2) produced by Rasha, to the intensity produced by Sasha? (To 2 digits of accuracy) A) 1.3 B) 25 C) 320 D) 14 E) None of the above is correct! 0001-4 6. You have two identical firecrackers. #1 explodes, making a noise lasting 0.01 seconds. #2 "fizzles": all the fuel is consumed, it just takes longer to release it, lasting 10 seconds. (For simplicity, assume all the energy of each firecracker goes into sound energy, and both firecrackers release the exact same total amount of energy.) Consider the following two statements (which refer to a moment when each firecracker is burning) and decide if each is true or false. i) The power of firecracker #1 equals the power of firecracker #2 ii) The intensity of sound produced by firecracker #1 equals the intensity of sound produced by #2 A) Both i and ii are true B) Both i and ii are false C) i is true, but ii is false D) i is false, but ii is true 7. 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 mike #1. (They both 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) i is true, but ii is false D) i is false, but ii is true 8. I am 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, i.e. 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, they will be heard in all parts of the room, they will go 'every which way' out of the door) A) Predominantly any pitches greater than about 3000 Hz B) Predominantly any pitches less than about 3000 Hz C) Predominantly any pitches greater than about 300 Hz D) Predominantly any pitches less than about 300 Hz E) All audible pitches will go 'every which way', the frequency makes no difference 0001-5 9. A child sits on a swing, swinging back and forth in front of you. She is blowing steadily on a whistle (playing a steady frequency "f") Consider half a swing, where she starts (at rest) at the highest point farthest away from you, and swings down and back up to the highest point closest to you. During this time, what do you hear (in terms of pitch)? A) Starts at f, gets higher through the whole swing, reaching its highest value (above f) when she reaches the peak of her swing closest to you. B) Starts at f, gets higher, reaching its highest value at the BOTTOM of the swing, then decreases, reaching f again at the peak of her swing closest to you. C) Starts below f, gets steadily higher, reaching exactly f at the BOTTOM of the swing, then continues getting higher, ending up above f when she's at the peak closest to you D) Starts below f, gets steadily higher, reaching exactly f at the BOTTOM of the swing, then gets lower and lower again, ending where it started when she reaches the peak closest to you E) None of the above is right, the pattern is something quite different! 10. You're playing a guitar string, tuned to "concert A" (440 Hz). Your friend plays the same note on their guitar, sitting right next to you, with the same loudness. You are almost but not perfectly in tune, your friend is about 1 Hz off. What do you hear? A) Close to concert A, louder than if you were playing alone. B) Close to concert A, basically the same loudness as if you played alone. C) Close to concert A, getting louder and softer, louder and softer. D) Close to concert A, but if you tilt your head ever so slightly to one side, the sound will go away. Tilting your head to a slightly different spot makes the sound louder (but steady at that place). E) A different pitch, which stays at constant loudness no matter where you tilt your head. 11. The walls and ceilings in a particular concert hall are built with lots of bumps, holes, and odd shapes, with the characteristic size of the "roughness" about 1 or 2 meters. (This is also the size of typical chairs, chandeliers, light fixtures, audience members, etc.!) Generally speaking, how will sound waves reflect off of the walls? A) frequencies above a couple Hz or so reflect specularly, lower frequencies reflect diffusely B) frequencies below a couple Hz or so reflect specularly, higher frequencies reflect diffusely C) frequencies above a couple hundred Hz or so reflect specularly, lower frequencies reflect diffusely D) frequencies below a couple hundred Hz or so reflect specularly, higher frequencies reflect diffusely E) With this level of roughness, no sound will reflect, it will all be absorbed. 0001-6 12. A small choir starts a song, later joined by the Mormon Tabernacle Choir. The music thus goes from a total of 10 singers to 400 singers. How many dB higher do you expect the new sound level to be (when all 400 are singing), compared to when only 10 were singing? A) 16 dB higher B) 40 dB higher C) 390 dB higher D) 104 dB higher E) 25 dB higher 13. 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. Which of the figures below shows the shape of the string at t=2 seconds? A) B) C) E) None of these is correct! D) 14. The atmosphere on Mars is mainly carbon dioxide, and the weather there is a bit cold. Both items 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) Visiting Martians 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) The two instruments will sound an octave lower than they do on earth (but will still be in unison) B) The two instruments will sound an octave higher than they do on earth (but will still be in unison) C) The two instruments will sound just the same as they do on earth. D) The flute will sound the same as it does on earth, but the guitar will sound an octave lower E) The guitar will sound the same as it does on earth, but the flute will sound an octave lower. 0001-7 IMPORTANT !! Your Name _________________________ YOUR ID: ______________________ Be sure your name+ID are on the bubble sheet and here! (Write them NEATLY!! Thanks! ) PLEASE READ!!: CHOOSE either the questions on THIS side of the page, or the ones on the BACK side. Do NOT answer both sides of the page - we will only grade one side! Please "X out" the side you do NOT want graded. We are looking here for BRIEF but clear reasons. (We don't give credit if an answer is correct but unexplained, or explained incorrectly!) (12 points total.) a) (4 pts) Describe, in your own words, briefly but concisely, what an "octave" means. Don't just give a purely formal or mathematical definition - what's special to humans (and music) about an octave? b) (8 pts) I would have loved to have everyone in this class build their own homebrew musical instrument. Suppose you chose to make a string-instrument, and it produced a nice clear pitch when you plucked it. In the space below, give two fundamentally different kinds of changes you could make (in the design or materials) which would INCREASE the pitch by one octave. Try to be as specific and quantitative as you can. Briefly explain why/how the physical changes impact the pitch of sound that you hear. As part of this explanation, tell us if the wavelength of the vibrating string is the same as the wavelength of the vibrating air? (If not, why not, and what is the same?) Hint: We discussed in class the formula for the speed of waves along a string: speed = Tension mass per meter ! Over for alternative choice... 0001-8 IMPORTANT !! Your Name _________________________ YOUR ID: ______________________ You should CHOOSE to answer either the questions on THIS side of the page, or the ones on the BACK side. Do NOT answer both sides of this page - we will only grade one side! Please "X out" the side you do NOT want graded. We are looking here for BRIEF but clear reasons. (We don't give credit if an answer is correct but unexplained, or explained incorrectly!) (12 points total.) OPTION 2) a) 4 pts) In your own words, briefly but concisely, what are the main DIFFERENCES between "intensity", "sound intensity level (SIL)", and "loudness"? b) (8 pts) In the space below, give two distinct examples of evidence that sound is a wave. An "example" should involve an experiment or demonstration that we have done (or could have done) including: a brief description of the example, and an explanation of how/why this provides evidence FOR the wave nature (and/or AGAINST some other model) of sound. ...
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This note was uploaded on 03/31/2008 for the course PHYS 1240 taught by Professor Holland,murray during the Spring '08 term at Colorado.
- Spring '08