Principle and Applications (Sound) Answer Key Part 2

24) A person stands between two speakers driven by the same source. Each speaker produces a tone with a frequency of 200 Hz on a day when the speed of sound is 330 m/s. The person is 1.65 m from one speaker and 4.95 m from the other. What type of interference does the person perceive? A) constructive B) destructive C) both constructive and destructive D) neither constructive nor destructive Answer: A 25) A sonic boom occurs only as an airplane just reaches the speed of sound in air, but not after it increases its speed further. A) True B) False Answer: B 26) As a supersonic airplane increases its speed more and more, the half-angle of the conical shock wave A) increases. B) decreases. C) is not affected. Answer: B 12.2 Problems 1) An elephant can hear sound with a frequency of 15 Hz. What is the wavelength of this wave if the speed of sound in air is 343 m/s? Answer: 23 m 2) You drop a stone down a well that is 9.5 m deep. How long is it before you hear the splash? The speed of sound in air is 343 m/s and air resistance is negligible. A) 1.2 s B) 1.3 s C) 1.4 s D) 1.5 s E) 1.6 s Answer: C 3) The speed of sound through the ground is about 6.0 km/s while the speed of sound in air is 343 m/s. A very powerful explosion occurs some distance away and you feel the ground vibrate 60 seconds before you hear the sound of the explosion. How far away is the explosion? A) 20 km B) 22 km C) 25 km D) 27 km E) 30 km Answer: B 4) Two motors in a factory are running at slightly different rates. One runs at 825.0 rpm and the other at You hear the sound intensity increase and then decrease periodically due to wave interference. How long does it take between successive instances of the sound intensity increasing? A) 1.54 s B) 1.43 s C) 1.66 s D) 1.79 s Answer: A 5) A plucked guitar string produces a sound wave of frequency 0.44 kHz on a day when the speed of sound is 340 m/s. (a) What is the wavelength of the sound wave? (b) How much time elapses between the impacts of two adjacent compressions of the sound wave on your eardrum? Answer: (a) 0.77 m (b) 2.3 ms 6) What is the intensity level in decibels of a sound with intensity 10-3 W/m2? The threshold of human hearing is 1.0 × 10-12 W/m2. A) 30 dB B) 60 dB C) 90 dB D) 96 dB Answer: C 7) What is the intensity of a 70-dB sound? The threshold of human hearing is 1.0 × 10-12 W/m2. A) 10-4 W/m2 B) 10-5 W/m2 C) 10-6 W/m2 D) 10-7 W/m2 Answer: B 8) What is the ratio of the intensities of two sounds with intensity levels of 70 dB and 40 dB? A) 10:1 B) 100:1 C) 1000:1 D) 10,000:1 Answer: C 9) If the intensity level by 15 identical engines in a garage is 100 dB, what is the intensity level generated by each one of these engines? A) 67 dB B) 13 dB C) 44 dB D) 88 dB Answer: D 10) The intensity at a distance of 6.0 m from a source that is radiating equally in all directions is 6.0 × 10-10 W/m2. What is the power emitted by the source? A) 2.1 × 10-8 W B) 2.7 × 10-7 W C) 2.1 × 10-6 W D) 2.7 × 10-5 W Answer: B 11) The intensity at a distance of 6.0 m from a source that is radiating equally in all directions is 6.0 × 10-10 W/m2. What is the intensity level in dB? The threshold of human hearing is 1.0 × 10-12 W/m2. A) 18 dB B) 23 dB C) 28 dB D) 32 dB Answer: C 12) A barking dog delivers about 1.0 mW of power, which is assumed to be uniformly distributed in all directions. What is the intensity level in decibels at a distance 5.00 m from the dog? The threshold of human hearing is 1.0 × 10-12 W/m2. A) 61 dB B) 63 dB C) 65 dB D) 68 dB Answer: C 13) The intensity level is 65 dB at a distance 5.00 m from a barking dog. What would be a reasonable estimate for the intensity level if two identical dogs very close to each other were barking? You can ignore any interference effects. A) 65 dB B) 68 dB C) 130 dB D) 136 dB Answer: B 14) At a distance of 15.0 m from a sound source, the intensity level is 60.0 dB. What is the intensity level (in decibels) at a point 2.00 m from the source if the source radiates equally in all directions? A) 55.7 dB B) 57.5 dB C) 75.5 dB D) 77.5 dB Answer: D 15) The sound intensity level 5.0 m from a point source is 95 dB. At what distance will it be 75 dB? A) 25 m B) 50 m C) 75 m D) 225 m Answer: B 16) A certain siren radiates sound uniformly in all directions. At a distance of 17 m from the siren, the intensity level is 49 db. How many watts of power does this siren put out? The threshold of human hearing is 1.0 × 10-12 W/m2. Answer: 2.9 × 10-4 W 17) You double your distance from a sound source that is radiating equally in all directions. What happens to the intensity level of the sound? It drops by A) 2 dB. B) 3 dB. C) 6 dB. D) 8 dB. Answer: C 18) By how many decibels does the intensity level of a sound decrease when you triple your distance from a source of sound? A) 9.5 dB B) 4.8 dB C) 6.0 dB D) 12 dB E) 3.0 dB Answer: A 19) A certain factory whistle can be heard up to a distance of 2.5 km. Assuming that the acoustic output of the whistle is uniform in all directions, how much acoustic power is emitted by the whistle? The threshold of human hearing is 1.0 × 10-12 W/m2. A) 0.079 mW B) 0.026 mW C) 0.26 mW D) 0.79 mW E) 2.6 mW Answer: A 20) A certain factory whistle can be heard up to a distance of 9.8 km. Assuming that the acoustic output of the whistle is uniform in all directions, at what distance from the factory is the intensity level of the whistle sound equal to 25 dB? The threshold of human hearing is 1.0 × 10-12 W/m2. A) 550 m B) 660 m C) 830 m D) 940 m E) 1100 m Answer: A 21) By how many decibels does the intensity level of a sound increase when you triple the intensity of a source of sound? A) 9.5 dB B) 4.8 dB C) 6.0 dB D) 3.0 dB E) 12 dB Answer: B