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).
While there is certainly a difference between near- and far-field approximations, the short answer is that no, the speakers can be closer than one wavelength together in order to have a steerable far field. In fact, the link you provided has a 1/4 wavelength spaced array as one of its first examples.
You may be confused here because typically it is harder to make lower frequency waves from a single emitter more directional -- but that has more to do with waveguide and aperture geometry.
Looking at the equation for the radiation pattern of the phased array, the angle dependence goes like sin(pi * (N * d/lambda) * sin theta). If N * d (i.e. the size of the array) is much smaller than lambda, there's no interference pattern.
To be more precise, for the radiation pattern to have a null, N * d must be larger than the wavelength.
Yes, but I think that's why you're confused. The distance between the individual speakers would be d, not N*d. Because your original comment was about this spacing, that's what I addressed in my answer.
Obviously a larger effective aperture (either physical or synthetic) would be more effective at beam steering.
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