# Physics 101

A 90-dB sound is absorbed by an eardrum 0.75 cm in diameter for 2 hours. How much energy in joules does the eardrum absorb at that time?

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asked by DJ
1. (Power/area)*(area)*(time)= Energy

Convert 90 dB to power/area. Use SI units. See
http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm
if you need help with that.

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posted by drwls
2. I figured it out already, thanks. I was just having trouble with converting dB to W/m^2.

Anyway, here's my answer. Feel free to correct it.

I - Intensity
P - Power
E - Energy
t - time
A - Area

I = P/A ; I = E/t / A ; E = IAt

Io = 1x10^-12 W/m^2 (treshold of hearing)

Conversion of dB to W/m^2 :

B = 10 log I/Io

90 = 10log I/Io
9 = log I/Io
I = 10^9 Io

I = 10^9 (10^-12)
I = 1x10^-3 W/m^2

E = IAt
= 1x10^-3 W/m^2 [1/4 pi (0.0075m)^2] (7200 sec)
= 3.18086 x 10^-4 Joules

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posted by DJ

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