Physics-
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Easy

Question

Intensity level 200 cm from a source of sound is 80 blank d B. If there is no loss of acoustic power in air and intensity of threshold hearing is 10 to the power of negative 12 end exponent W m to the power of negative 2 end exponent then, what is the intensity level at a distance of 4000 cm from source

  1. Aero    
  2. 54 dB    
  3. 64 dB    
  4. 44 dB    

The correct answer is: 54 dB


    I proportional to fraction numerator 1 over denominator r to the power of 2 end exponent end fraction rightwards double arrow fraction numerator I subscript 2 end subscript over denominator I subscript 1 end subscript end fraction equals fraction numerator r subscript 1 end subscript superscript 2 end superscript over denominator r subscript 2 end subscript superscript 2 end superscript end fraction equals fraction numerator 2 to the power of 2 end exponent over denominator open parentheses 40 close parentheses to the power of 2 end exponent end fraction equals fraction numerator 1 over denominator 400 end fraction rightwards double arrow I subscript 1 end subscript equals 400 I subscript 2 end subscript
    Intensity level at point 1 comma blank L subscript 1 end subscript equals 10 log subscript 10 end subscript invisible function application open parentheses fraction numerator I subscript 1 end subscript over denominator I subscript 0 end subscript end fraction close parentheses
    and intensity at point 2 comma blank L subscript 2 end subscript equals 10 log subscript 10 end subscript invisible function application open parentheses fraction numerator I subscript 2 end subscript over denominator I subscript 0 end subscript end fraction close parentheses
    therefore L subscript 1 end subscript minus L subscript 2 end subscript equals 10 log invisible function application fraction numerator I subscript 1 end subscript over denominator I subscript 2 end subscript end fraction equals 10 log subscript 10 end subscript invisible function application left parenthesis 400 right parenthesis
    rightwards double arrow L subscript 1 end subscript minus L subscript 2 end subscript equals 10 cross times 2.602 equals 26
    L subscript 2 end subscript equals L subscript 1 end subscript minus 26 equals 80 minus 26 equals 54 blank d B

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