The two sound sources in Fig. 23-1 vibrate in-phase. A loud sound is heard at P when L1 = L2. As L1 is slowly increased, the weakest sound is heard when L1 - L2 has the values 20.0 cm, 60.0 cm, and 100 cm. What is the frequency of the sound source if the speed of sound is 340 m/s?