Problem 12.45: Eavesdropping. You are trying to overhear a juicy conversation, b

Question

Problem 12.45: Eavesdropping.

You are trying to overhear a juicy conversation, but from your distance of 24.0 m , it sounds like only an average whisper of 20.0 dB . So you decide to move closer to give the conversation a sound level of 65.0 dB instead.

Part A

How close should you come?

1331

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Incorrect; Try Again; 3 attempts remaining

Problem 12.45: Eavesdropping.

You are trying to overhear a juicy conversation, but from your distance of 24.0 m , it sounds like only an average whisper of 20.0 dB . So you decide to move closer to give the conversation a sound level of 65.0 dB instead.

Part A

How close should you come?

1331

SubmitMy AnswersGive Up

Incorrect; Try Again; 3 attempts remaining

Explanation / Answer

Intensity level B = 10 dB log(I/Io)

so Io = 1 e -12 W/m^2

so log(I/Io) = 20/10 = 2

I/Io = 10^2 = 100

I = 100 *10^-12 = 1 e -10 W/m^2

I = power/area

Power = IA

P = 1 e-10 * 4* 3.14 * 24*24

P = 7.23 e-7 Watts

now fron this power we need B = 65 dB

log(I/Io) = 65/10 = 6.5

I/Io 10^6.5

I = 3.16 e 6 * 1 e-12

I = power/area = 3.16e-6

Area = 7.23 e -7/(3.16 e -6)

A = 4 pi r^2 = 0.228

r^2 = 0.0182

r = 0.1349 m or 13.5 cm

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