May I be pertinent ask what is in those slots?Did you not see the pictures I posted before? It's exactly the same principle as used by D&D.
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In bad weather I measure in an old church, using gated measurements. In good weather I'd like to move outside from some groundplane measurements. The gated measurements are certainly not perfect in terms of resolution, although relative levels still come out well.
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Since then I've got a better mic stand ..
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Also, at 20Hz wavelength is 17 metres, as large as a church will be measurements of such low frequencies is not reliable. There are very few anechoic chambers in the world that can measure such low frequencies.
Obviously ground plane measurement will not show any polar pattern beyond +/- 90 degrees.
If the box is filled with compressed rockwool we can expect the speed of sound to be as fast as 200m/s. That reduces the wavelength of 20Hz to 10 metres. In other words the sound coming out of the slots are 70cm in front. This difference may well give you the cancellation you need but that is at one frequency. How are you going to spread that linearly to the operating range of the speaker?