- #1

gmmstr827

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## Homework Statement

At a rock concert, a dB meter registered 130 dB when placed 2.2 m in front of a loudspeaker on the stage. (a) What was the power output of the speaker, assuming uniform spherical spreading of the sound and neglecting absorption in the air? (b) How far away would the sound level be a somewhat reasonable 85 dB?

## Homework Equations

I = 10^(ß/10)*I_0 where I_0 = 1*10^-12 W/m^2

I_1 = I_2/(4πr^2)

## The Attempt at a Solution

a)

I = 10^(ß/10)*I_0

I = 10^(130 dB/10)*10^-12

I_2 = 10 W/m^2

b)

Convert dB to W/m^2:

I = 10^(ß/10)*I_0

I = 10^(85 dB/10)*10^-12

I_1 = 3.16 * 10^-4 W/m^2

Solve for r in formula: I_1 = I_2/(4πr^2) >>> r = √(I_2/(4πI_1))

r = √(10 W/m^2/(4π(3.16 * 10^-4 W/m^2))

r = 501824 m

Part b) is obviously way too large to be right. I need a formula that incorporates the original distance. Any ideas?