The biggest advantage of a (well designed) line-array is the efficiency and relative simplicity of the sound system, in providing SPL and coverage control over a defined area.
For a line-array (basically “an array of line sources”), a true cylindrical wave front is the (only) key to a good working line-array system and many attempts are being made to create this type of wave front with traditional compression drivers.

The main criteria for getting a well designed array, is to get all drivers working together, instead of working against each other. Since each frequency has a different wave-length, the secret to “summing” is that the sources/speakers must be closely coupled, with a distance less than half a wavelength of the highest frequency they have to reproduce.

For low frequencies this is no problem, where the size of the speaker is well within “half of the wavelength”. The real challenge is in mid and high frequencies; Here the physical size of the speaker is too big for its wavelength, resulting in individual point sources instead of a seamless cylindrical wavefront.

If we calculate half of the wavelength with the formula “Wavelength = Speed of sound divided by Frequency”, we can see that for correct summing, a frequency of 20.000 Hz, should be reproduced by a transducer not bigger than 8,6mm.. Knowing that (one of) the smallest HF speakers measures 24mm (so effective up to 4,1kHz), it is clear that this is not possible. Taking half the frequency (10kHz.), the required speaker size is still only 17,2mm. And we didn’t even mention usable SPL with a speaker this size…!