Understanding Phase Difference in 3D Waves

[Kites]

Homework Statement

A wave at the origin expands outwards passing the points (.01, .03, .02) and (-.01, .015, .025). This wave has a wave speed, V_w = 346 m/s and frequency = 13100 HZ.

What is the phase difference between these two points?

Homework Equations

v = (wavelength)(frequency)

Phase Difference = 2pi (delta x) / Wave length

The Attempt at a Solution

I attempted to use the following:


[(distance between points using 3D formula)/(wave length)] * 2pi = phase difference.


This was in correct. I am going to guess that it's because "Phase Difference = 2pi (delta x) / Wave length" is for a wave in one dimension. My question is how does the phase difference change in 3 dimensions? I attempted to use the surface area of a sphere but it also failed. Something is quite wrong.

 

[learningphysics]

Use the distance from the origin... the delta you need is the difference of the second point from the origin - the distance of the first point from the origin.

 

[Kites]

Wonderful thanks.