Two sound sources 4 m apart have the same frequency of 880 Hz. The sources emit energy in the form of spherical waves. An observer initially hears a minimum sound, since the sound sources are out of phase, at a distance of 15 m along the perpendicular bisectorof the line joining the two sources. What distance x must the observer move along a line parallel to the line joining the two sources before hearing another sound of minimum intensity? THe speed of sound is 336 m/s. I'm not even sure where to start for this problem. Do I need to find the equations for both the sound waves and find where they interfere deconstructively? I would really appreciate any hints to get me started.