Hi, Question: X and Y are two identical sources of sound which emit in phase. Calculate the lowest possible value of frequency of the sources for there to be (a) constructive (b) destructive interference at Q. So, X is at 1.8 m from Q and Y is 1.2 m away from Q. a) If they leave in phase and are to arrive in phase at Q, the path difference must equal the 1, 2, 3, 4, etc. wavelengths. Since we want the lowest possible frequency we want the largest wavelength possible. So my answer for the lowest frequency would be 340/(1.8-1.2) b) For this part, since we want again the lowest frequency possible we want the largest wavelength possible again. In this case, the path difference would have to be equal to half a wavelength: 0.6/0.5 = 1.2. The frequency this time round would be 340/1.2? Thanks, Peter G.