Find the point of destructive interference of two waves.

[HunterDX77M]

Homework Statement

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?

Homework Equations

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

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.

 

[cepheid]

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

 

[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.

 

[SammyS]

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.

The sources are in phase.

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