a) A siren produces a sound that has an intensity level of 120dB at a distance o

Question

a) A siren produces a sound that has an intensity level of 120dB at a distance of 2.0 m. Treat the siren as a point source ofspherical waves. Find the intensity in decibels heard by anobserver 12 m away from the source.
b) You are parked at the side of the road. As a fire truckapproaches you hear a tone of 460 Hz. As the fire truckrecedes, you hear a tone of 410 Hz. (vsound=340 m/s) Find the constant velocity of the fire truck and the frequencyof the siren. a) A siren produces a sound that has an intensity level of 120dB at a distance of 2.0 m. Treat the siren as a point source ofspherical waves. Find the intensity in decibels heard by anobserver 12 m away from the source.
b) You are parked at the side of the road. As a fire truckapproaches you hear a tone of 460 Hz. As the fire truckrecedes, you hear a tone of 410 Hz. (vsound=340 m/s) Find the constant velocity of the fire truck and the frequencyof the siren. Find the constant velocity of the fire truck and the frequencyof the siren.

Explanation / Answer

Sound intensity level L = 120 dB

We know L = 10 log ( I / Io)

          120 = 10log ( I / Io)

Log ( I / Io) = 12

          I / I0 =10 ^ 12

              I = Io * 10^ 12

                = 10 ^ -12 * 10 ^ 12

                = 1 W / m^ 2

i.e., intensity at a distance 2 m from the source I = 1 W /m^ 2

intensity at a distance 12 m is I ‘ =?

We know I ‘ / I = ( r/ r’ ) ^ 2

                         = ( 2/ 12 ) ^ 2 = 1/ 36

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