42. If the distance between a point sound source and a dB detector is increased

Question

42. If the distance between a point sound source and a dB detector is increased by a factor of 4, what will be the reduction in intensity level? a. 16 dB b. 12 dB c. 4 dEB d. 0.5 dB 43. The intensity level of sound 20 m from a jet airliner is 120 dB. At what distance from the airplane will the sound intensity level be a tolerable 100 dB? (Assume spherical spreading of sound.) a. 90 m b. 120 m c. 150 m d. 200 m 44. A very loud train whistle has an acoustic power output of 100 W. If the sound energy spreads out spherically, what is the intensity level in dB at a distance of 100 meters from the train? o 10-12 W/m2) a. 78.3 dB b. 81.6 dB c. 89.0 dB d. 95.0 dB

Explanation / Answer

42. sound Level = 10 log (I / I0)

and I = P / (4 pi r^2)

  
SL1 - SL2 = 10 log(I1 / I0) - 10 log (I2/I0)

= 10 log(I1/I2)

= 10 log(4^2)

= 12 dB
Ans(b)

43. 120 - 100 = 10 log (r/20)^2

20 log(r/20) = 20

log(r / 20) = 1

r/20 = 10

r = 200 m

Ans(d)

44. I = P/ (4 pi r^2) = 100 / (4 x pi x 100^2)

I = 7.958 x 10^-4 W/m^2

SL = 10 log(I / I0) =89.0  

Ans(c)

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