Two loudspeakers, 2.0 m apart, emit sound waves with the same frequency along the positive x-axis. Victor, standing on the axis to the right of the speakers, hears no sound. As the frequency is slowly tripled, Victor hears the sound go through the sequence loud-soft-loud-soft-loud before becoming quiet again.

What was the original sound frequency? Assume room temperature of 20C.

Express your answer with the appropriate units.