Physics/EE students please correct me, I've mostly forgotten this stuff.

Assuming the speaker is many wavelengths away (in the "far field"), the distance between individual speakers needs to be larger (comparable with the wavelength) to make a difference in the radiation pattern in the far field. Speakers which are closer together only make a difference in the near field (meaning the listener is within a few wavelengths of the array).

If you want to understand this, look into https://en.wikipedia.org/wiki/Phased_array