Sound intensity does not rely on distance?

A forum set up for physics questions from students in the courses PHYS1121, 1131, 1221 and 1331 at the University of New South Wales. It is intended for questions that cannot readily be answered in class,
either because they fall outside the main syllabus and therefore would be distraction (however interesting) or for other reasons.

Moderator: msmod

Post Reply
z5017731
Posts: 1
Joined: Sat Jun 07, 2014 8:19 am

Sound intensity does not rely on distance?

Post by z5017731 »

Probably something obvious I have misinterpereted, but I don't understand how for the equation I=(1/2)*Ro*v*omega^2*(Smax)^2, not rely on distance from the source (no distance terms), as I=Power/Area suggests. This confuses me for questions such as calculate intensity at a distance.
User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Re: Sound intensity does not rely on distance?

Post by joe »

Whether or not the intensity depends on distance depends on the geometry. For sound propagating down a pipe, the intensity is approximately constant. For uniform radiation in 3D space, I goes as 1/r^2.

In both cases, the formula you quote applies.
In the (lossless) pipe, the maximum displacement (Smax) does not depend on distance.
In the 3D radiation, the maximum displacement (Smax) is proportional to 1/r, so I is proportional to 1/r^2.

Joe
Post Reply