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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 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 outfit 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. Q17.4 The speed of sound to two significant figures 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 significant figures, you need to measure a time of at least 1.0 seconds. Since d t = v , the minimum distance is 340 meters. *Q17.5 (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). *Q17.6 (i) Answer (c). Every crest in air produces one crest in water immediately as it reaches the interface, so there must be 500 in every second. (ii) Answer (a). The speed increases greatly so the wavelength must increase. 449 13794_17_ch17_p449-472.indd 449 13794_17_ch17_p449-472.indd 449 1/3/07 8:08:12 PM 1/3/07 8:08:12 PM
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450 Chapter 17 Q17.7 When listening, you are approximately the same distance from all of the members of the group. If different frequencies traveled at different speeds, then you might hear the higher pitched frequen- cies before you heard the lower ones produced at the same time. Although it might be interesting to think that each listener heard his or her own personal performance depending on where they were
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