Homework Help: Precalculus

Posted by jh on Tuesday, March 9, 2010 at 8:33am.

I have posted this question earlier as well but was unable to receive an answer. I don't know how to do this question at all, so PLEASE help me with this question and also show me the steps, so that I can learn. I would appreciate it a lot.

A high sound level of 120 dB is regarded as the threshold of pain for the average listener. If the threshold of hearing is 10^−12 watts/m^2, then the intensity of the sound corresponding to 120 dB is
_________ watts/m^2.

Precalculus - Reiny, Tuesday, March 9, 2010 at 8:45am
You must be referring to
http://www.jiskha.com/display.cgi?id=1268004723

Using bobpursley's formul, he is very seldom wrong,
and my 10 digit display calculator, I got

10log(120/10^-12) as

130.79 or 131 decibels.
Precalculus - jh, Tuesday, March 9, 2010 at 8:59am
No unfortunately this is not the correct answer :(, is there any other way of doing this question?
Precalculus - Reiny, Tuesday, March 9, 2010 at 9:13am
Does the formula use natural logs perhaps (ln) ?

Was there a formula or relation given?
By how much is our answer off ?
Precalculus - jh, Tuesday, March 9, 2010 at 9:24am
This is an online assignment, we write the answer and the computer system recognizes whether the answer is right or wrong, but doesn't give any hint.
See if this helps:
The instructor gave us the formula 10log E/E0 and this
question told us what EO is so we just plugged it in then
we got rid of the 10 in front of the log by making it equal
to 12 instead of 120.....Someone already gave the equation
to solve earlier in the discussion Consider log(E/10^-12)=12

just solve for E we know that 10^12 = (E/10^-12) as this is
a log of common base ten then from here you solve for E

we also know that 10^-12 is also 1/10^12

hope these hints help
Precalculus - Reiny, Tuesday, March 9, 2010 at 9:39am
Ok then, if we are just solving

120 = 10log(E/10^-12) then it is easy

divide both sides by 10
12 = log(E(10^12))
which means e(10^12) = 10^12
E = 1

try that
Precalculus - jh, Tuesday, March 9, 2010 at 9:42am
Yes, 1 is the answer. Thanks a lot for your help Reiny :)

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