Randall D. Knight — Traveling Waves: Problem 73

A $20.0\text{ m} \times 20.0\text{ m}$ square is marked off around a loudspeaker that is in the center of the square. Carla stands at the center of one side of the square to listen to the sound, and Diego stands at one of the corners. Measurements find that the sound waves they hear have a phase difference of $180^\circ$ when the temperature is $20^\circ\text{C}$. What is the sound frequency?