Two Speaker Problem

Two speakers are arranged as shown. Assume that point O is 12m along the center line and the speakers are separated by a distance of 1.5m. As the listener moves toward point P from point O, a series of alternating minima and maxima are encountered. The distance between the first minimum and the next maximum is 0.4m. Using 340 m/s as speed of sound in air, determine the frequency of the speakers.

Three metal rods are located relative to each other as shown in the figure where L1 + L2 = L3. Values of density and Young’s modulus for the three materials are:
  d1 = 2.7 ´ 103 kgm-3, Y1 = 7 ´ 1010 Nm-2
  d2 = 11.3 ´ 103 kgm-3, Y2 = 1.6 ´ 1010 Nm-2
  d3 = 8.8 ´ 103 kgm-3, Y3 = 11 ´ 1010 Nm-2
If L3 = 1.5 m, what must be the ratio L1/L2 be if a sound wave is to travel the length of the rods 1 and 2 in the same time required to travel the length of rod 3? If the frequency of the source is 4000hz, determine the phase difference between the wave traveling along rods 1 and 2 and the one traveling along rod 3.