2. The Na vy Blue Angels fly F/A Hornet aircraft that are capable of flying clos

Question

2. The Na vy Blue Angels fly F/A Hornet aircraft that are capable of flying close to Mach 2. In their public air shows, however, they typically fly at up to Mach 0.912. The jets are loud with a variety of frequencies of noise. In a crowd pleasing flyover of the spectators a plane flies low over the audience approaching them at Mach 0.750 then receding at the same speed. a) For engine noise at a frequency of 630 Hz what frequency does the crowd hear for the noise as the aircraft approaches them? b) After the jet passes overhead and recedes from the crowd, what is the frequency of the noise that the crowd hears? c) When the jet is directly overhead, what frequency does the crowd hear for the noise?

Explanation / Answer

2. given spee dof aircraft , u = 0.75c ( where c = 343 m/s speed of sound)

a. given fo = 630 Hz

frequency heard by the crowd when the plane is approaching = f

from doppler's effect

f = fo*c/(c - u) = fo*c/(c - 0.75c) = fo/0.25 = 4fo = 2520 Hz

b. again from doppler's effect , when the plane receeds

f = fo*c/(c(1 + 0.75)) = fo/1.75 = 360 Hz

c. when the jet is directly overhead, there is no apparent shift in the frequenc of the plane and hence the from doppler's effect we can say that the frequency heard by the crowd is same as the original frequemcy of the engine

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