SM_chapter17 - 17 Sound Waves CHAPTER OUTLINE 17.1 17.2...

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17 Sound Waves CHAPTER OUTLINE 17.1 Speed of Sound Waves 17.2 Periodic Sound Waves 17.3 Intensity of Periodic Sound Waves 17.4 The Doppler Effect 17.5 Digital Sound Recording 17.6 Motion Picture Sound ANSWERS TO QUESTIONS *Q17.1 Answer (b). The typically higher density would by itself make the speed of sound lower in a solid compared to a gas. Q17.2 We assume that a perfect vacuum surrounds the clock. The sound waves require a medium for them to travel to your ear. The hammer on the alarm will strike the bell, and the vibration will spread as sound waves through the body of the clock. If a bone of your skull were in contact with the clock, you would hear the bell. However, in the absence of a surrounding medium like air or water, no sound can be radiated away. A larger-scale example of the same effect: Colossal storms raging on the Sun are deathly still for us. What happens to the sound energy within the clock? Here is the answer: As the sound wave travels through the steel and plastic, traversing joints and going around corners, its energy is con- verted into additional internal energy, raising the temperature of the materials. After the sound has died away, the clock will glow very slightly brighter in the infrared portion of the electromagnetic spectrum. *Q17.3 (i) Answer (b). The frequency increases by a factor of 2 because the wave speed, which is depen- dent only on the medium through which the wave travels, remains constant. (ii) Answer (c). Q17.4 The speed of sound to two signiF cant F gures is 340 m s. Let’s assume that you can measure time to 1 10 second by using a stopwatch. To get a speed to two signiF cant F gures, you need to measure a time of at least 1.0 seconds. Since dt = v , the minimum distance is 340 meters. Q17.5 If an object is 1 2 meter from the sonic ranger, then the sensor would have to measure how long it would take for a sound pulse to travel one meter. Since sound of any frequency moves at about 343 m s, then the sonic ranger would have to be able to measure a time difference of under 0.003 seconds. This small time measurement is possible with modern electronics. But it would be more expensive to outF t sonic rangers with the more sensitive equipment than it is to print “do not use to measure distances less than 1 2 meter” in the users’ manual. 403
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404 Chapter 17 Q17.6 Our brave Siberian saw the f rst wave he encountered, light traveling at 300 10 8 . × ms. At the same moment, inFrared as well as visible light began warming his skin, but some time was required to raise the temperature oF the outer skin layers beFore he noticed it. The meteor produced compres- sional waves in the air and in the ground. The wave in the ground, which can be called either sound or a seismic wave, traveled much Faster than the wave in air, since the ground is much stiFFer against compression. Our witness received it next and noticed it as a little earthquake. He was no doubt unable to distinguish the P and S waves From each other. The f rst air-compression wave he received was a shock wave with an amplitude on the order oF meters. It transported him oFF his doorstep. Then he could hear some additional direct sound, refl
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This note was uploaded on 01/28/2011 for the course PHYS 011 taught by Professor Nianlin during the Fall '08 term at HKUST.

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SM_chapter17 - 17 Sound Waves CHAPTER OUTLINE 17.1 17.2...

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