Homework #10 2

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University of Southern California *

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135A

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Physics

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Dec 6, 2023

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pdf

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Homework #10 1. [ 2 points ] What is the sound level of a sound whose intensity is 1.5 x 10 -6 W/m 2 ? 2. [ 3 points ] A person standing a certain distance from an airplane with four equally noisy jet engines is experiencing a sound level of 140 dB. What sound level would this person experience if the captain shut down all but one engine? [Hint: Add intensities, not sound level levels] 3. [ 3 points ] Loudspeakers can produce intense sounds with surprisingly small energy input despite their low efficiencies. Calculate the power input needed to produce a 90.0-dB sound intensity level for a 12.0-cm-diameter speaker that has an efficiency of 1.00%. (This value is the sound intensity level right at the speaker.) 4. [ 3 points ] A certain dog whistle operates at 23.5 kHz, while another (brand X) operates at an unknown frequency. If humans can hear neither whistle when played separately, but a shrill whine of frequency 5000 Hz occurs when they are played simultaneously, estimate the operating frequency of brand X. 5. [ 2 points ] An unfingered guitar string is 0.68 m long and is tuned to play E above middle C (330 Hz). (a) How far from the end of this string must a fret (and your finger) be placed to play A above middle C (440 Hz)? (b) What is the wavelength on the string of this 440 Hz wave? 6. [ 3 points ] As a bat flies toward a wall at a speed of 6.0 m/s, the bat emits an ultrasonic sound wave with frequency 30.0 kHz. What frequency does the bat hear in the reflected wave? 7. [ 4 points ] A police car sounding a siren with a frequency of 1580 Hz is traveling at 120.0 km/h. (a) What frequencies does an observer standing next to the road hear as the car approaches and as it recedes? (b) What frequencies are heard in a car traveling at 90.0 km/h in the opposite direction before and after passing the police car? (c) The police car passes a car traveling in the same direction at 80.0 km/h. What two frequencies are heard in this car

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