How Is Sound Intensity Calculated at Different Distances from a Trumpet?

[dimpledur]

Homework Statement

When playing fortissimo (very loudly), a trumpet emits sound energy
at a rate of 0.800 W out of a bell (opening) of diameter 12.7 cm. (a) What is the sound
intensity level right in front of the trumpet? (b) If the trumpet radiates sound waves
uniformly in all directions, what is the sound intensity level at a distance of 10.0 m?

The Attempt at a Solution

My only question for the following question has to do with area. I know that I = P / A.. However, for a), since it is at the very end of the trumpet, would the area be pi*r^2, rather than the area for a sphere?

I know that for b) it is the area of a sphere, but I was thinking for a) it would be a circle. Apparently this question is supposed to be hard, so that's why I was thinking that.

Thanks!

 

[Delphi51]

You are quite right on all counts.

 

[dimpledur]

Thank you. I was a bit unsure about the area for a), but it's nice to have some clarification that it is a circle.