Loudspeakers A and B are vibrating in phase and are playing the same tone, which has a frequency of 430 Hz. They are set up as in the figure, and point C is located as shown there. However, the distance between the speakers and the distance between speaker B and point C have the same value d. The speed of sound is 343 m/s. What is the smallest value of d, such that constructive interference occurs at point C?