Line arrays have some very attractive properties, but the there is one major problem. The physical size of the driver causes time smearing in the high frequencies. Let's take a typical example. The length
L of the array is 1 meter and the listening distance
dmin is 3 meters. So,
dmin is the minimum distance from the listening position to a point on the line array - that being the center point. The maximum distance
dmax occurs at the ends of the array. Using a little geometry we find:
dmax = sqrt(dmin^2 + (L/2)^2) = 3.0414 m
dmax - dmin = 0.0414m
Now we can calculate the delay corresponding to dmax - dmin.
delay = 0.0414 m ÷ 344 m/s = 0.12 msec
This difference in distance also corresponded to one wavelength at 8308Hz. So, the sound radiating from the ends of the array will be shifted by more than a wavelength relative to the sound radiating from the center of the array for all frequncies above 8308Hz.
This would present no problem if we were just listening to continuous sine waves. The resultant waveform simply sums to a flat response. Music, however, isn't just sine waves. Music contains a lot of short duration pulses. And a pulse emitted from any other point on the array will arrive slightly later than that emitted from the center. We can perceive these differences in arrival times. This is why line arrays tend to have a strange ethereal, almost reverberant quality to them.
The difference in arrival times can be reduced my moving far away from the array. To achieve the same SPL at the listening position, however, you need more power in the array. This defeats the whole point of having a large surface area moving very little.
From a purely performance perspective an outstanding solution is to build a concave array where all points on the array are equidistant from the listening position. This give all the benefits without any time smearing. Practically, however, it creates a very small focal sweet spot.
Hope this helps.
Thomas
