My simple minded way of understanding it is that the end of a pipe/horn has a very thin 'film-like diaphragm' attached to it that loosely holds in the air mass 'slug' that vibrates within it and since it weighs much more, it bulges out the end at ~ its radius*0.613 [end correction] assuming it's not laying on a surface.
With a horn it gets a little more complicated since one has to factor in the flare frequency to find its effective perimeter, i.e. can't just use the horn mouth's perimeter.
When the horn sets on a surface, then there's a different end correction and if in a baffle, yet another, but don't recall these. Mostly it affects its polar response.
In general though, it's normally measured in a polar response plot or just use Altec's spacing. The sectoral horns OTOH terminate such that it can only be measured AFAIK, though again AFAIK, the wood 'L' mounting board plus spacer is the correct gap: http://www.lansingheritage.org/images/altec/plans/1960's-lf-design/page14.jpg
Altec A7 with cinema horn spacing.jpg
GM
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Historically, vertically aligning the VCs was sufficient: http://www.lansingheritage.org/image...ign/page16.jpg
Better though to use a test signal to do it. There's an instruction doc, but was surprised to not find it on any of the Forum's various 'libraries'.
GM
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