# Homework Help: Sound level of explosion

1. Oct 18, 2012

### songoku

1. The problem statement, all variables and given/known data
A firework charge is detonated many metres above the ground. At a distance d1=550 m from the explosion, the acoustic pressure reaches a maximum of ΔPmax = 10 Pa. Assume the speed is constant at 343 m/s throughout the atmosphere over the region considered and the ground absorbs all the sound falling on it. Assume that the density of air 1.2 kgm-3. What is the sound level at a distance of d2 = 4.30 x 103 from the explosion?

2. Relevant equations
Don't know

3. The attempt at a solution
I don't even know what the relevant equations that should be used to solve this question. Please help me to start, I don't have clue

2. Oct 18, 2012

### tiny-tim

hi songoku!
i'll guess that you're supposed to assume that the height is negligible, and that there's no reflection (ie, the ground isn't there)

3. Oct 18, 2012

### songoku

hi tiny-tim

Sorry still not know how to proceed. Acoustic pressure is the difference between the total pressure and atmospheric pressure, so the total pressure at that point is almost the same as atmospheric pressure.

I don't understand how to combine all the information given; I have pressure, density, speed. What is the relation between them?

Should I use TI2=TI1 + 10.log (r1/r2) to find the sound level?
TI1 = 10 log (I/I0) and I = power / area and I don't have the information to find all the variables needed. I even don't know whether I am on the right track or not

4. Oct 18, 2012

### tiny-tim

why so complicated?

5. Oct 18, 2012

### songoku

hi tiny-tim

Oh I never know that formula...

P1 = 10 + 1 x 105 = 100010 Pa
r1 = 550 m
r2 = 4.3 x 103 m

So P2 = 127.9 Pa

sound level = 10 log (P / Pref)2

Do I use P1 as the Pref?

6. Oct 19, 2012

### tiny-tim

hi songoku!

(just got up :zzz:)

from that wikipedia: The distance law for the sound pressure p in 3D is inverse-proportional to the distance r of a punctual sound source

so you apply it directly to the 10 Pa
sorry, no idea

7. Oct 19, 2012

### songoku

hi tiny-tim
What does it mean by "punctual sound source"? Does it mean the pressure at that point or just the difference of pressure from atmospheric pressure?

Do you have idea of alternative equation that can be used?

8. Oct 19, 2012

### tiny-tim

it means a point source (as opposed to a spread-out source)

"punctual" is a word meaning "at the correct time" which some idiots are trying to re-define as meaning "related to a point"

9. Oct 19, 2012

### songoku

Ok. Thanks for the help