Chapter 14 Prob Solutions

# Chapter 14 Prob Solutions - Chapter 14 Sound Quick Quizzes...

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Chapter 14 Sound Quick Quizzes 1. (c). The speed of sound in air is given by ( ) 331 m s 273 K vT = . Thus, increasing the absolute temperature, T , will increase the speed of sound. Changes in frequency, amplitude or air pressure have no affect on the speed of sound. 2. (c). The distance between you and the buzzer is increasing. Therefore, the intensity at your location is decreasing. As the buzzer falls, it moves away from you with increasing speed. This causes the detected frequency to decrease. 3. (b). The speed of sound increases in the warmer air, while the speed of the sound source (the plane) remains constant. Therefore, the ratio of the speed of the source to that of sound (that is, the Mach number) decreases. 4. (b) and (e). A string fastened at both ends can resonate at any integer multiple of the fundamental frequency. Of the choices listed, only 300 Hz and 600 Hz are integer multiples of the 150 Hz fundamental frequency. 5. (d). In the fundamental mode, an open pipe has a node at the center and antinodes at each end. The fundamental wavelength of the open pipe is then twice the length of the pipe and the fundamental frequency is open 2 f vL = . When one end of the pipe is closed, the fundamental mode has a node at the closed end and an antinode at the open end. In this case, the fundamental wavelength is four times the length of the pipe and the fundamental frequency is close 4 fv L = . 6. (a). The change in the length of the pipe, and hence the fundamental wavelength, is negligible. As the temperature increases, the speed of sound in air increases and this causes an increase in the fundamental frequency, 00 λ = . 7. (b). Since the beat frequency is steadily increasing, you are increasing the difference between the frequency of the string and the frequency of the tuning fork. Thus, your action is counterproductive and you should reverse course by loosening the string. 501

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502 CHAPTER 14 Answers to Even Numbered Conceptual Questions 2. The resonant frequency depends on the length of the pipe. Thus, changing the length of the pipe will cause different frequencies to be emphasized in the resulting sound. 4. Air flowing fast by a rim of the pipe creates a "shshshsh" sound called edgetone noise, a mixture of all frequencies, as the air turbulently switches between flowing on one side of the edge and the other. The air column inside the pipe finds one or more of its resonance frequencies in the noise. The air column starts vibrating with large amplitude in a standing vibration mode and radiates this sound into the surrounding air. 6. The distance around the opening of the bell must be an integer multiple of the wavelength. Actually, the circumference being equal to the wavelength would describe the bell moving from side to side without bending, which it can do without producing any sound. A tuned bell is cast and shaped so that some of these vibrations will have their frequencies constitute higher harmonics of a musical note, the strike tone. This tuning is lost if a crack develops in the bell. The sides of the crack vibrate as antinodes. Energy of
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Chapter 14 Prob Solutions - Chapter 14 Sound Quick Quizzes...

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