Principle and Applications (Sound) Answer Key Part 4

44) The wavelengths of the sounds produced by two horns are 6.0 m and 7.0 m, respectively. What beat frequency is heard when the horns are sounded on a day when the speed of sound is 340 m/s? A) 5 Hz B) 6 Hz C) 7 Hz D) 8 Hz Answer: D 45) Two taut strings of identical mass per unit length are stretched with the same tension with their ends fixed, but one string is 0.330 cm longer than the other. Waves on these strings propagate at 34.0 m/s. The fundamental frequency of the shorter string is 258 Hz. What is the beat frequency when each string is vibrating at its fundamental frequency? A) 12.3 Hz B) 9.00 Hz C) 12.0 Hz D) 12.7 Hz E) 11.3 Hz Answer: A 46) Two taut strings of identical mass and length are stretched with their ends fixed, but the tension in one string is 1.10 times greater than in the other. Waves on the string with the lower tension propagate at 35.2 m/s. The fundamental frequency of that string is 258 Hz. What is the beat frequency when each string is vibrating at its fundamental frequency? A) 11.0 Hz B) 9.00 Hz C) 13.7 Hz D) 12.6 Hz E) 17.1 Hz Answer: D 47) A policeman in a stationary car measures the speed of approaching cars by means of an ultrasonic device that emits a sound with a frequency of 39.6 kHz. A car is approaching him at a speed of 35.0 m/s. The wave is reflected by the car and interferes with the emitted sound producing beats. What is the frequency of the beats when the speed of sound in air is 343 m/s? A) 5000 Hz B) 4500 Hz C) 8490 Hz D) 9000 Hz E) 4250 Hz Answer: D 48) As shown in the figure, a man is traveling on a bicycle along a straight road that runs parallel to and right next to some railroad tracks. While moving at 10 m/s, he hears the whistle of a train that is behind him, as shown in the figure. The frequency emitted by the train's whistle is but the frequency the man hears is The speed of sound is 340 m/s. (a) What frequency is heard by a stationary observer located between the train and the bicycle? (b) What is the speed of the train, and is the train traveling away from or toward the bicycle? Answer: a) 797 Hz b) 9.8 m/s, away from the bicycle 49) A factory siren indicating the end of a shift has a frequency of 80 Hz where the speed of sound in air is 343 m/s. What frequency is perceived by the occupant of a car that is traveling at 30 m/s (a) towards the factory and (b) away from the factory? Answer: (a) 87 Hz (b) 73 Hz 50) A car approaches you at a constant speed, sounding its horn, and you hear a frequency of 76 Hz. After the car goes by, you hear a frequency of 65 Hz. What is the frequency of the sound emitted by the horn? The speed of sound in the air is 343 m/s. A) 68 Hz B) 69 Hz C) 70 Hz D) 71 Hz E) 72 Hz Answer: C 51) As you stand by the side of the road, a car approaches you at a constant speed, sounding its horn, and you hear a frequency of 76 Hz. After the car goes by, you hear a frequency of 65 Hz. What is the speed of the car? The speed of sound in the air is 343 m/s. A) 26 m/s B) 27 m/s C) 28 m/s D) 29 m/s E) 30 m/s Answer: B 52) You are driving along a highway at 35.0 m/s when you hear the siren of a police car approaching you from behind at constant speed and you perceive the frequency as . You are relieved that he is in pursuit of a different driver when he continues past you, but now you perceive the frequency as What is the speed of the police car? The speed of sound in air is A) 38.4 m/s B) 30 m/s C) 39.2 m/s D) 40.1 m/s E) 41.7 m/s Answer: D 53) You are driving along a highway at 35.0 m/s when you hear the siren of a police car approaching you from behind at constant speed and you perceive the frequency as 1310 Hz. You are relieved that he is in pursuit of a different driver when he continues past you, but now you perceive the frequency as 1240 Hz. What is the frequency of the siren in the police car? The speed of sound in air is 343 m/s. A) 1300 Hz B) 1320 Hz C) 1270 Hz D) 1360 Hz E) 1370 Hz Answer: C 54) You are driving along a highway at 25 m/s when you hear the siren of an emergency vehicle traveling at constant speed in the opposite direction on the other side of the highway. When the vehicle is approaching you, you hear the frequency of the siren as 2380 Hz, but when it is past you the frequency becomes 1680 Hz. What is the speed of the vehicle? The speed of sound in air is 343 m/s. A) 35 m/s B) 32 m/s C) 30 m/s D) 41 m/s E) 50 m/s Answer: A 55) You are driving along a highway at 25.0 m/s when you hear the siren of an emergency vehicle traveling at constant speed in the opposite direction on the other side of the highway. When the vehicle is approaching you, you hear the frequency of the siren as 2380 Hz, but when it is past you the frequency becomes 1680 Hz. What is the frequency of the siren? The speed of sound in air is 343 m/s. A) 1850 Hz B) 2000 Hz C) 2050 Hz D) 2010 Hz E) 1980 Hz Answer: B 56) A whistle produces sound of frequency of 1.00 kHz. If a listener moves with a speed of 30 m/s away from the whistle, what frequency does this person hear if the sound speed is 340 m/s? A) 912 Hz B) 919 Hz C) 1000 Hz D) 1090 Hz Answer: A 57) A police car has an 800-Hz siren. It is traveling at 35.0 m/s on a day when the speed of sound through air is 340 m/s. The car approaches and passes an observer who is standing along the roadside. What change of frequency does the observer hear? A) 0 Hz B) 82 HZ C) 166 Hz D) 249 Hz Answer: C 58) What is the frequency does a stationary observer hear when a train approaches her with a speed of 30 m/s. The frequency of the train horn is 0.600 kHz and the speed of sound is 340 m/s. A) 551 Hz B) 600 Hz C) 653 Hz D) 658 Hz Answer: D 59) You are moving at 120 km/h toward a stationary train. The train blows its 0.400-kHz whistle. Take the speed of sound to be 340 m/s. What frequency do you hear? A) 444 Hz B) 439 Hz C) 364 Hz D) 361 Hz Answer: B 60) A train is traveling away from you at 120 km/h. It blows its whistle, and you hear a tone of 0.400 kHz. Take the speed of sound to be 340 m/s. What frequency does the whistle actually produce? A) 444 Hz B) 439 Hz C) 364 Hz D) 361 Hz Answer: B 61) Sonar is used to determine the speed of an object. A 40.0-kHz signal is sent out, and a 42.0kHz signal is returned. If the speed of sound is 345 m/s, how fast is the object moving? A) 6.9 m/s B) 8.4 m/s C) 331 m/s D) 347 m/s Answer: B 62) A bat emits a sound at a frequency of 39.0 kHz as it approaches a wall. The bat detects beats with a frequency of 827 Hz between the sound it emits and the echo bouncing from the wall. What is the speed of the bat if the speed of sound in air is 343 m/s? A) 2.7 m/s B) 3.0 m/s C) 3.6 m/s D) 5.4 m/s E) 9.0 m/s Answer: C 63) Two in-phase loudspeakers are some distance apart. They emit sound with a frequency of 536 Hz. A microphone is moved between the speakers along the line joining the two speakers with a constant speed of 1.60 m/s. What beat frequency is observed? The speed of sound in air is 343 m/s. A) 5.00 Hz B) 6.00 Hz C) 7.00 Hz D) 8.00 Hz E) 9.00 Hz Answer: A 64) A siren emitting sound of frequency 1000 Hz approaches a stationary observer at one-half the speed of sound. The observer hears a frequency of A) 1000 Hz B) 2000 Hz. C) 500 Hz. D) 1500 Hz. E) none of the above. Answer: B 65) The sound from the whistle of a truck moving at 17.0 m/s toward a cliff reflects from the cliff and is heard by the driver of the truck. When it is parked at the side of the road, the whistle from the truck produces sound of frequency 2.267 kHz. As observed by the driver of the truck, what is the change in frequency the sound from the whistle due to the reflection? The speed of sound is 343 m/s. A) 236 Hz B) 228 Hz C) 207 Hz D) 118 Hz E) 472 Hz Answer: A 66) Two in-phase loudspeakers are 3.0 m apart. They emit sound with a frequency of 490 Hz. A microphone is placed half-way between the speakers and then moved along the line joining the two speakers until the first point of constructive interference is found. At what distance from that midpoint is that first point? The speed of sound in air is 343 m/s. A) 0.18 m B) 0.35 m C) 0.50 m D) 0.70 m E) There is no point in that line where constructive interference occurs. Answer: B 67) Two in-phase loudspeakers are placed along a wall and are separated by a distance of 4.00 m. They emit sound with a frequency of 514 Hz. A person is standing away from the wall, in front of one of the loudspeakers. What is the closest nonzero distance from the wall the person can stand and hear constructive interference? The speed of sound in air is 343 m/s. A) 0.34 m B) 0.73 m C) 1.2 m D) 1.6 m Answer: B 68) Two in-phase loudspeakers are placed along a wall and are separated by a distance of 4.00 m. They emit sound with a frequency of 514 Hz. A person is standing away from the wall, in front of one of the loudspeakers. What is the closest distance from the wall the person can stand and hear destructive interference? The speed of sound in air is 343 m/s. A) 0.34 m B) 0.37 m C) 0.73 m D) 1.2 m E) 1.6 m Answer: A 69) Two in-phase loudspeakers that emit sound with the same frequency are placed along a wall and are separated by a distance of 5.00 m. A person is standing 12.0 m away from the wall, equidistant from the loudspeakers. When the person moves 1.00 m parallel to the wall, she experiences destructive interference for the first time. What is the frequency of the sound? The speed of sound in air is 343 m/s. A) 211 Hz B) 256 Hz C) 422 Hz D) 512 Hz E) 674 Hz Answer: C 70) Two in-phase loudspeakers that emit sound with the same frequency are placed along a wall and are separated by a distance of 8.00 m. A person is standing 12.0 m away from the wall, equidistant from the loudspeakers. When the person moves 3.00 m parallel to the wall, she experiences destructive interference for the second time. What is the frequency of the sound? The speed of sound in air is 343 m/s. A) 278 Hz B) 422 Hz C) 452 Hz D) 562 Hz E) 694 Hz Answer: A 71) Two loudspeakers placed 6.00 m apart are driven in phase by an audio oscillator having a frequency range from 1908 Hz to 2471 Hz. A point P is located 4.70 m from one loudspeaker and 3.60 m from the other speaker. At what frequency of the oscillator does the sound reaching point P interfere constructively? The speed of sound is 344 m/s. A) 2190 Hz B) 2420 Hz C) 2110 Hz D) 2340 Hz E) 2270 Hz Answer: A 72) Two loudspeakers placed 6.00 m apart are driven in phase by an audio oscillator having a frequency range from 1595 Hz to 2158 Hz. A point P is located 4.70 m from one loudspeaker and 3.60 m from the other speaker. The speed of sound in the room is 344 m/s. At what frequency (or frequencies) of the oscillator does the sound reaching point P interfere destructively? (There could be more than one correct choice.) A) 2030 Hz B) 2130 Hz C) 2100 Hz D) 1720 Hz E) 1630 Hz Answer: A, D 73) Two stereo speakers mounted 4.52 m apart on a wall emit identical sound waves. You are standing at the opposite wall of the room at a point directly between the two speakers. You walk 2.11 m parallel to the wall, to a location where you first notice that the sound intensity is essentially zero. If the wall along which you are walking is from the wall with the speakers, what is the wavelength of the sound waves? The walls are cushioned to absorb sound reflections. A) 1.71 m B) 2.05 m C) 2.57 m D) 2.91 m Answer: A 74) Two speakers are placed side by side and driven by the same frequency of 0.50 kHz. If the distance from a person to one speaker is 5.0 m and the person detects little or no sound, which of the following is a possible the distance from the person to the other speaker? The sound speed in the room is 340 m/s. A) 7.7 m B) 8.1 m C) 8.4 m D) 9.1 m Answer: B 75) The half-angle of the conical shock wave produced by a supersonic aircraft is 60¡. What is the ratio of the speed of the aircraft to the speed of sound? A) 0.87 B) 1.2 C) 1.7 D) 2.0 Answer: B 76) A jet flies at 1.4 times the speed of sound. What is the half-angle of the conical shock wave formed? A) 30¡ B) 36¡ C) 44¡ D) 46¡ Answer: D