Sound Intensity Equation Help

[ilovelucy]

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I am desperate need for someone to explain an easy way to use the sound intensity equation:


B = 10 + log (I/Io)

I understand how to use logs, but I get so confused in calculating the decibles when I get a problem where intensity increases or decreases by a certain amount.

For example: If a person whispers and teh intensity decreases by a factor of 1,000. If the original I was 34 decibels, what is the new Intensity (B)?

The answer is suppose to be 4 decibles, but I can't seem to get to that! Ugh.
Thanks in advance.

 

[jochi1543]

1000 is 10 to the power of 3.....each unit of power is equal to 10 decibels, hence a 30 decibel decrease here.

 

[ilovelucy]

Ok, that makes sense.

But then my TPR teacher gave us this problem:

What if I increases by a factor or 6 and Bo=22 db. What is the new level?

He said that the answer is approx. 29 db?

Can you please explain. I am forever in your debt.

 

[jochi1543]

What's Bo?

 

[BrokenGlass]

Ok, that makes sense.

But then my TPR teacher gave us this problem:

What if I increases by a factor or 6 and Bo=22 db. What is the new level?

He said that the answer is approx. 29 db?

Can you please explain. I am forever in your debt.

Basically, "times y" in intensity means "+10*logy" in intensity level, where intensity level is Beta.

"Divided by y" in intensity means "-10 * log y" in intensity level.

So "times 6" in intensity means "+ 10* log 6" in intensity level

10* log 6 = 10 * log (3*2) = 10 * (log 3 + log 2) = 10 * (0.5 + 0.3) = 10 * 0.8 = 8.

22 + 8 = 30.

You should know the values of log 2 and log 3 by heart.


HTH

 

[ilovelucy]

Bo is the original Intensity (B) in the equation B = 10 + log (I/Io)

Sorry for the confusion

 

[ilovelucy]

thanks Brokenglass.