Two speakers emitting identical 210 Hz tones are placed on a line a distance Delta x away from each other and pointed in

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Two speakers emitting identical 210 Hz tones are placed on a
line a distance Delta x away from each other and pointed in the
same direction. As you walk along this line away from the pair of
speakers (both are behind you and pointed in your direction), you
don't hear anything even though the speakers are both producing
sound. This is true no matter how far away you are from the nearest
speaker. After some experimentation, you find that there are
multiple separations between the pair of speakers, LaTeX: \Delta x,
that give rise to this same phenomenon where no sound is heard
along the line despite the fact that both speakers are working.
What is the smallest value of Delta x? Assume that the speed of
sound for the temperature of the air surrounding the speakers is
349 m/s.