Hi Joe, sorry for the deley in reply, I do not check this site that often.
I think instead of point-source you mean line source. When thinking about line sources, wave length and array length are related, but it is not a relation ship when a 3.4m length array will only beam down to 100 Hz (wavelength of 100 Hz is 3.4m), but it will continue doing so bellow that as well, but loosing the efficiency as going away from that frequency.
In the example in the question, a 19 feet tall array (around 5.8m, that is the wavelength of around 59 Hz)  will beam energy down to 59 Hz, but it does not stop there, it will beam energy bellow that as well  but less efficiently. At 59 Hz we have a strong directivity, at 45 Hz, it is less, but still effective and at 29.5 Hz, it will still have it, but a lot less effective. Still, there is reduction of energy bellow and above the array.
The actual result also depends on the physical position of the array and the acoustical properties of the venue. This is why checking the energy at the audience and the stage using the TM array is always important during system tuning.