# Homework Help: Find the point of destructive interference of two waves.

1. Aug 3, 2012

### HunterDX77M

1. The problem statement, all variables and given/known data

Two coherent in-phase point sources of sound are located at the points (-2 m, 0 m) and (2 m, 0 m). If the wavelength of the sound is 0.9 m, at which of the following x values on the x-axis does destructive interference occur?

2. Relevant equations

Wave Equation:
y(x, t) = Acos(kx + ωt)

3. The attempt at a solution

I have no idea how to even start this one. Could someone just nudge me in the right direction? This is the last practice problem I have to do, but I am really stuck.

2. Aug 3, 2012

### cepheid

Staff Emeritus
Well, maybe start with this: what is the condition for destructive interference to occur?

3. Aug 3, 2012

### HunterDX77M

Well, let's see. They have to be out of phase by 180 degrees. But the part that confuses me is that the problem says they have the same phase.

4. Aug 4, 2012

### SammyS

Staff Emeritus
The sources are in phase.

However, the waves move and that's at a finite speed.