Solving Sound Intensity with 2 Shut-Off Engines

[physicsnewby]

Homework Statement

A person is a certain distance away from a plane with three equally noisy engines at 120 dB. What is the sound intensity if two of the engines are shut off?

Homework Equations

B (in dB) = log I/Io

The Attempt at a Solution

using this equation:
120 dB = 10 log I/Io
10^12 = 10 log (I/10^-12 W/m2)
I = (10^12)(10^-12)
I = 1 W/m2

So, intensity for three engines is 1 W/m2. If two engines turned off, then I'm pretty sure you don't divide the answer by three to get 0.33 W/m2 per engine because the answer is 116. I can't figure out what I'm doing wrong.

 

[cepheid]

On the second line in your solution, where did 10¹² come from?

 

[physicsnewby]

Sorry, this should read:

120 dB = 10 log (I/10^-12 W/m2)
10^12 = I \(10^-12)
I = (10^12)(10^-12)
I = 1 W/m2

 

[physicsnewby]

any ideas?

 

[Doc Al]

I think you need to better understand the meaning of decibel. This might help: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/db.html"