# Homework Help: Sound intensity

1. Jun 24, 2007

### physicsnewby

1. The problem statement, all variables and given/known data
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?

2. Relevant equations
B (in dB) = log I/Io

3. 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.

2. Jun 24, 2007

### cepheid

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

3. Jun 24, 2007

### physicsnewby

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

4. Jun 25, 2007

### physicsnewby

any ideas?

5. Jun 25, 2007

### Staff: Mentor

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

Last edited by a moderator: May 2, 2017