A person in a parked car sounds the horn. The frequency of the horn\'s sound is

Question

A person in a parked car sounds the horn. The frequency of the horn's sound is 460 Hz. A driver in an approaching car measures the frequency of the horn's sound as 505 Hz. What is the speed of the approaching car? (Use 343 m/s for the speed of sound.) 69.69 m/s 31.7 m/s 25.3 m/s 33.55 m/s 23.276 m/s A very loud train whistle has an acoustic power output of 90 watts. If the sound energy spreads out spherically, what is the intensity level in dB at a distance of 115 meters from the train? (I_0 = 10^-12 Watts/m^2) 108 dB 51.84 dB 87.34 dB 194.58 dB 86.4 dB

Explanation / Answer

Doppler's effect,

f = f0 (v + v_r) / (v + v_s)

v = velocity of wave = 343 m/s

17. v_r = velocity of receiver = v1

v_s = velocity of source = 0

  
505 = 460 (343 + v1) / (343 + 0)

v1 = 33.55 m/s

Ans(D)

18. I = P / (4 pi r^2 )

I = 90 / (4 pi 115^2) = 5.415 x 10^-4 W/m^2

Sound level = 10 log(I / 10^-12)

= 87.34 dB

Abs(C)

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