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Review:  Chapter 18 Waves II
Q1. In the sonar method of measuring distances under water, a sound pulse travels from the generator to a distant object and is reflected back to the location of the generator. If the bulk modulus of sea water is 2.16 * 10**9 N/(m**2) and the density of sea water is 1. 022 * 10 ** 3 kg/(m**3), what is the time interval between sending out and receiving the reflected pulse from an object 1200 m away? (ANS: 1. 65 s)
Q2. By what factor must the pressure amplitude of a harmonic sound wave be increased in order to increase the intensity by a factor 16? (ANS: 4 )
Q3. A sound wave travels in a medium of density 1.20 kg/ m**3. The displacement, s , of the molecules of medium from their equilibrium position is given by
s = 0. 00060 cos [ PI ((5. 83 x ) (2000 t )] m, where x is in m and t is in s. What is the pressure amplitude of the wave? (ANS: 1. 55* 10 **4 N/m**2 )
Q4. A sound wave with an intensity level of 65 dB strikes an eardrum of area 7. 85 * 10**( 5) m**2 ). How much energy is absorbed by the eardrum per second? Assume that all the energy is absorbed by the eardrum. (ANS: 2. 48*10**(10) J )
Q5. The sound level of a jet plane is measured as 150 dB and that of a vacuum cleaner as 70 dB. If the intensity of the sound from the jet plane is I 1 and the intensity of the sound from the vacuum cleaner is I2, what is the ratio I1/I2 ? (ANS: 10**8)
Q6. Two cars are traveling, one behind the other, in the same direction along a straight road. The car in front has a speed of 20 m/s and the car behind has a speed of 23 m/s. The horn of the car behind is sounded at a frequency of 1280 Hz. What is the apparent frequency of the horn as observed by the occupant of the leading car? Take the speed of sound as 343 m/s. (ANS: 1292 Hz)
Q7. An approaching train sounds its whistle and appears to have a frequency of 280 Hz, according to passengers waiting at the station. The frequency experience by the passengers aboard the train is 262 Hz. The velocity of sound in air is 1080 ft/s. Find the speed of the train? (ANS: 69. 4 ft/s ).
Q8. Two speakers S1 and S2 are driven by the same oscillator at a frequency of 390 Hz. A listener is originally located at P. The listener walks in a direction perpendicular to OP until he reaches point Q where the intensity of sound is minimum. If the difference between QS2 and QS1 is 44. 5 cm , what is the speed of sound in the air at the time of the experiment? (ANS: 347 m/s )
Q9. We want to measure how deep a water well is from the resonance condition of the well. A big speaker driven by a frequency generator is placed near the mouth of the well. Two consecutive resonances are observed, one at 155 Hz and the other at 165 Hz. Assuming the velocity of sound is 340 m/s, find how deep the well is. (ANS: 17 m )
Q10. The minimum length of a pipe, closed at one end, that resonates with a given source of sound is 7. 0 cm . Taking the speed of sound in air as 350 m/s, calculate the frequency of the source of sound. (ANS: 1250 Hz)
Q11. When a sound generator sweeps through all frequencies between 1000 Hz and 2000 Hz, resonance is excited in a hollow tube at just two frequencies, at 1400 Hz and 1800 Hz. The tube is open to air at one end and closed at the other. Find the fundamental resonance frequency for the tube. (ANS: 200 Hz )
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