Why so few high end line arrays?

09-04-09: Shadorne
You are getting serious comb filtering from listening close up to multiple drivers with the same bandwidth.

Not entirely true. Comb filtering is NOT an issue in nearfield listening if the center-to-center distance of the drivers used does not exceed a distance equal to the speed of sound (at ocean level) divided by the highest frequency handled by that particular line of drivers (bass, mid-bass, treble).

Dr. James R. Griffin, Ph.D, has a fairly definitive white paper on the subject titled "Design Guidelines for Practical Near Field Line Arrays"(.pdf) that discusses the subject in-depth. In the paper, he provides all the basic criterion for creating a successful line array:

Center-to-center Driver Separation (Circular drivers). We want our discrete driver array to approximate a continuous line source. This spacing is the separation between the centers of the adjacent drivers in the line and includes any mounting allowances and the flanges surrounding the drivers. In the limit the closest spacing would be dictated by the flange diameters of the drivers although some drivers have truncated flanges that would allow closer spacing. Two different solutions (Table I) for the driver separation guidelines are presented in the literature for circular drivers. These cases are:

1. Far Field. Ureda [3] uses driver directivity to determine that circular drivers need to be positioned within one wavelength center-to-center at their highest operating frequency. Wavelength is equal to the velocity of sound (344 m/s or 1130 feet/s) divided by the frequency. Directivity of the multiple drivers in the line increases until one wavelength spacing is reached and starts to decrease beyond this spacing. Figure 7 illustrates how the sound wavefront is created by a line array. Spacing less than one wavelength creates a constant phase front but comb lines start to form beyond one wavelength separation. At two wavelengths separation the first cancellation occurs. Directivity continues to decrease with more severe comb line effects as the spacing increases beyond two wavelengths.

2. Near field. Urban, et al [1] derives a more restrictive criterion of no more than a half wavelength separation between drivers at their highest operating frequency. Fresnel analysis is used and a disruption grid is used to shutter a continuous line source in their work. This analysis is based upon their desire to place any far field dips (nulls) in the angle off axis response of the array beyond p/2 (90 degrees). This assures that secondary (off-axis) lobes in the sound field are greater than 12 dB down from the on-axis response (main lobe)...

For the tweeter line very close center-to-center spacing is difficult to attain as very small circular drivers would be necessitated for either the one wavelength or especially the half wavelength criteria. Consider operation to 20 kHz where one wavelength is 17.2 mm (0.68”) and a half wavelength is only 8.6 mm (0.34”). Without regard to their surrounding flanges, dome tweeters are available in 25 mm (1”), 19 mm (0.75”) and 13 mm (0.5”) diameters. Hence, with any mounting flange allowance at all, the one or half wavelength c-t-c criteria are very difficult—if not impossible--to satisfy at 20 kHz. But, if we relax the c-t-c criterion, more secondary lobes would appear in the 10 to 20 kHz frequency range. Fortunately, in this octave the ear is less sensitive (per Fletcher-Munson curves) so any secondary lobes likely would be less audible to the listener. Thus, if one wavelength spacing at 10 kHz is adopted as a compromise, then tweeter spacing would need to be 34.4 mm (1.35”) c-t-c apart. While more off axis secondary lobes would be generated in the far field, small flange tweeters are available to meet this dimension. The tradeoff is possible sound degradation from comb lines near 20 kHz.

09-11-09: Tbg
If you ever look at the frequency response of line arrays, you will see what looks like a comb. Each driver interferes with the others at a regular interval throughout the frequency range.

Selah Audio Alexandrite - on-access Freq Response(200hz-20K), on-access Freq Response(10-200hz)

Selah Audio Symmetrica - on-access Freq Response(200hz-20K), on-access Freq Response(10-200hz)

Tbg,

I've been researching line arrays for the last couple of weeks, but that only assures I have way more questions than answers or experience...

But, the website which you refer seems to be only concerned with is only concerned with farfield sound reproduction(stadium, club). Not that nearfield acoustic completely diverge from farfield, but the goals are significantly different.

The "lobing" and "Nulls" which you(and your link) refer to primarily become a problem for listening at the point/frequency where c-to-c spacing of the source(drivers) exceeds the distance of one-half to one full wavelength of the highest frequency produced by that particular array(low, mid, or high).

The "trick" is in designing a widerange(mid/bass) array which crosses over to the next higher frequency array below the threshold where lobing and comb filter effects become troublesome. Within the home audio industry there are a variety of drivers small enough in diameter to achieve those goals.

These issues really seem(to my inexperienced eye) to be no more difficult than the many design limitations and compromises which plague all speaker designs(including traditional 2 or 3-way "box").

Here's a far better, and more technical, analysis(besides Dr Griffin's link above) of the design parameters and limitations of array design in this ElectroVoice paper.