What is the science behind the gap?
A number of ribbon and ESL dipole systems I've seen include a small air gap between the woofer panel and the HF drivers. This gap, I believe, is intended to minimize woofer modulation of the low mass HF drivers.
Does it work and, if so, what determines the proper dimensions of the gap?
the gap is to minimize acoustic coupling. Coupling can lead to intermodulation and in worst case might "blow" the mebrane of anESL into the stators.
The gap should be as small as possible, since it increases the baffle step effect, means the benefitial effect of the baffle width is reduced. This is trial and error, since the gap need to be selected regarding driver compliance and moved air mass of the entire system. Especially ESL cant be really calculated for those parameters
The Martin Logan Statement 2 is one example of complete physical isolation between midwoofer line array and ESL linesource. Very important when you have a loose suspension on a 2-3 gram MM film/foil next to 200 grams MM of midrange and 2000 grams MM of subwoofer. The ML Statement 2 also includes a large bass dipole array that is placed several feet away and on a diagonal to the ESL
It is not uncommon to put the ESL/ribbon on the inside of the sound stage and the mid-woofer line array on the outside. This seems to create the best 3D soundstage and also allows the mid-woofer line array to be angled slightly outward toward the walls. I think this breaks up some of the bass room modes and also keeps some for the bass air pressure from moving the ESL/ribbon.
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