Calculating Sound Level Intensity with Multiple Jet Engines

[mortho]

Homework Statement

A person standing a certain distance from an airplane with four equally noisy jet engines is feeling a sound level intensity bordering on pain, 114 dB. What sound level intensity would this person experience if the captain shut down all but one engine?



I have no idea how to do this problem. I thought this was relative intensity so i plugged it into the formula and found intensity. Now I'm not sure what to do. Thanks!

 

[quantumdude]

I thought this was relative intensity so i plugged it into the formula and found intensity.

It is a relative intensity problem. Which formula did you use? You were supposed to enter that into Part 2 of the posting template.

Now I'm not sure what to do. Thanks!

Show us what you did. That was supposed to go into Part 3 of the posting template.

 

[mortho]

Yeah, i was in a rush when i posted that because my computer's being repaired and I'm at the library and time was running out. Here's my work

I used relative intensity=10log (I/Io)

114=10log(I/1e-12) and got 0.251 w/m2 for I. I do not know what to do after this.

 

[lavalamp]

OK, you have the value for I, divide by four and calculate the new sound level intensity.

 

[mortho]

Is it 108 Hz?

 

[mortho]

i just submitted and it was correct. THanks for your help :)

 

[lavalamp]

No problem.

The thing to realize is, you had the value for the power / m^2 at whatever arbitrary distance from the plane, then 3/4 of the engines were shut down, so you automatically know the new power / m^2 at this arbitrary distance.

 

[zrome]

I have the same problem. Except instead of 114 dB it's 121 dB. I do not understand where the 1e-12 came from. Can someone tell me please?

Also, here is what I have done so far:

121=10log(I/Io)
121/10=log(I/Io)
10^12.1=I/Io

Now I do not know what to do... I am stuck there since I have no clue to what Io is and we're trying to solve for I- two variables one equation...

Please Help.

 

[LowlyPion]

Io = 10^-12 w/m²

This is the lowest threshold of human hearing.

So the ratio of the sound intensity to that is 121/10 or

I = 121dB can also be written as 10^12.1*10^-12 w/m²

 

[zrome]

I did that and I got an answer of .315 dB
I need the answer to be in dB to submit. Can you tell me what I am doing wrong here?

 

[LowlyPion]

I did that and I got an answer of .315 dB
I need the answer to be in dB to submit. Can you tell me what I am doing wrong here?

Perhaps you should show your work. And maybe even what the problem is, because, if it is the same as the OP here where 3 jet engines cut out, you won't be getting .315 dB, if you calculated it correctly.