John: It was reprinted in April of ‘74 by the AES. Basically up to a certain frequency, limited by the driver's diameter, you put the mic right next to the dust cap of the driver and in the port for the lowest 1/2 octave. He proved mathematically that this response measurement is equivalent to a far-field measurement.

Moderator: You recently posted an interesting concept on the SAC list about the aiming of a horn's axis for vertical coverage. Can you go over your approach in this regard?

John: Is this about using the top half of a horn's coverage, or is this about frequency shading long-throw/short-throw pairs?

keith: Actually, both are of interest.

John: First item - someone thought you could not use the top half of coverage of any horn in a cluster. This is generally true for the top far-throw horn, and you aim its axis at the last row. But for short- or mid-throw devices, their 6-dB down point at the top of their coverage, when aligned with the upper device, will sum to unity, thus being useful.

John (continued): The second item is a trick that we developed in the (Syn-Aud-Con) Horns I Workshop. It makes use of the fact that all but the largest horns lose vertical control well before they are crossed over at the low end of their passband. With a far-throw (60 X 40) and a near-throw (120 X 40) you can high-pass the lower horn at, say 2 kHz, and let the upper 60 X 40 go on down to, say, 800 Hz. This eliminates the severe lobing that occurs due to the excessive overlap in vertical coverage. After that you delay one horn enough to get a dense comb filtering between them so that we perceive a seamless transition in coverage.

KD: I would think the concept would be somewhat valid for clusters of full-range cabinets as well. We just don't have much opportunity to install horn clusters.

John: Yes, now that full-range multi-cells are popular ;>), it requires a little more thinking, but the horns in cabinets are almost always too small to pattern control their entire bandwidth, and horns are horns inside or outside of boxes. This is what makes this technique work. As Don Davis said, ‘if you can't be perfectly aligned, its better to be way off’.

John (continued): That is the seamless transition part, the dense comb-filtering- just be sure not to go too far or a rain barrel effect results. By the way, Dave Gunness was one of the instructors that had the most to do with this idea.

KD: What do you mean, "rain barrel"?