Homework Help: Sound problem

1. Apr 17, 2005

Jayhawk1

Here is the trouble problem...

11) A tornado warning siren on top of a tall pole radiates sound waves uniformly in all directions. At a distance of 15 m the sound intensity of the siren is 0.39 . Neglect any effects from reflection of the sound waves from the ground. a) At what distance from the siren is the intensity 0.19 ? b) What is the total acoustical power output of the siren? c) At what distance is the sound intensity reduced by 15 dB from its level at 15 m?

a) 21.5 m
b) 1100 Watts

Ok... so I figured out parts a and c, but I have no clue for part c- and the same seems to go for most of my class. I think the change might be very small, but I don't know how to get it. Any help would be appreciated. Thanks!

2. Apr 17, 2005

OlderDan

What are the dimensions of .39 and .19? Knowing that should lead you to an answer for b) I believe the customary dimensions would give you intensity in watts/meter^2. Intensity is the power per unit area, and the area in your case is the total area of the sphere centered at the source.

3. Apr 18, 2005

Jayhawk1

Sorry... I meant I didn't understand part C... any help?

4. Apr 18, 2005

OlderDan

The decibel scale is based on logarithms. In terms of intensity

$$L = 10log({\frac{I}{I_0}})$$

You need to find the intensity ratio that gives you L = -15 and from that determine the distance ratio and then the final distance