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.

Darkmoebius,

For the sake of brevity I did not go into as much detail. Yes it is true that a line array of woofers operating below 300 Hz is generally no problem due to the wavelengths of low frequencies. As you go up in frequency the problems start, unfortunately right in the mid range is where it can begin to be a problem.

It is much the same reason that some people argue that center channels are more trouble than they are worth....

After hearing some outstanding line arrays, I totally understand why people like them. I am about to get a just released line array from a manufacturer for an audition. I was worried about combing as well, but the drivers are small and quite close together, so I suspect at normal (not nearfield) listening distances this should not be a problem.

I'll post back in a few weeks with some initial thoughts, but at 16 ohms, (perfect for my OTL's) and no crossover, I'm thinking this might be an ideal speaker, (for me anyway).

Emailists,

I use Atma MA-1s with mine and the arrays are custom wired to yield 13 ohms nominal, 10 ohms minimum. I wasn't prepared for the difference that proper impedance matching makes with OTLs. You are likely in for a treat.

As for comb filtering, diffraction effects manifest in all manner of speakers - even single driver/baffle interactions occur. Not unlike our listening rooms themselves.

It is what we perceive that matters. I hear the bass nodes/nulls in my room quite well, ameliorated as best as possible with traps and PARC. Comb filtering in the "line array physics" sense is imperceptible in my setup. FWIW, I sit 10-11 feet back.

There is another approach that is worth considering - The Genesis Horn from Danley Sound Labs. In essence Danley's interpretation of a line array http://www.danleysoundlabs.com/pdf/GenesisHorn-anewbeginning.pdf
As for all line arrays, you are going to need a large room. It is pro audio so it needs a face lift if in the home environment and on display.