Two speakers in the drawing vibrating in phase $($meaning they create sound waves in the exactly same way $--$ peaks are created at the same moments, and troughs are created at the same moments$)$ with the same frequency ff $=1000.$ HzHz $,$ the speed of sound in air is vv $=343.$ m$/$sm$/$s $.$

Initially the two speakere A and B are at the same location. A listener is a distance LL $=4\mathrm{.}6$ mm from both speakers.