A siren emitting a sound of frequency 1080 Hz moves away from you toward the fac
ID: 1325039 • Letter: A
Question
A siren emitting a sound of frequency 1080 Hz moves away from you toward the face of a cliff at a speed of 8 m/s. Take the speed of sound in air as 330 m/s. (a) What is the frequency of the sound you hear coming directly from the siren? (Enter your answer to one decimal place.) (b) What is the frequency of the sound you hear reflected off the cliff? (Enter your answer to one decimal place.) (c) what is the beat frequency between the two sounds? (Enter your answer to one decimal place.) Did you use the general Doppler equation? The source is moving away from you? Does that motion shift the frequency up or down? What sign goes with the speed in the denominator? To get the frequency from the cliff, you need to calculate the frequency detected by the cliff. The sound source is moving toward the cliff. Does that shift the frequency up or down? What sign do you use in the denominator?Explanation / Answer
For 1080 cycles to reach your ear, the last bit has to travel an extra = 8 / 330 = 0.0242 s.
So the frequency = 1080 cycles / (1 + 0.0242s) = 1054 Hz
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The last bit has 8 less to travel, same time difference but subtracted from the time.
1080 cycles / (1 - 0.0242 s) = 1106.78 Hz
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Beat frequency f = 1106.78 Hz - 1054 Hz
= 52.78 Hz.