apologies for the later response, i may have gotten a little distracted.
as to links to the research for this, the theory was taken from “home audio” research and then applied to what we use. (in the hope it would work)
unfortunately i have since deleted the recordings and my notes recycled so i have nothing to back what i have found with this particular area. (passive radiators)
from what i can remember, the sound produced heightened the midrange rather well.
this in turn though had a negative effect on the lower end frequencies and i think just drowned them out.
the goal was to get a nice rounded / balanced and more defined sound from a smaller set up.
but what i ended up creating was quite the opposite.
i tried many different types of set up but i think that our small speakers just didn’t have the necessary energy involved to make it work the way i hoped it would.
needless to say i did not pursue this any further.
i’m sure that we could still get some gains with getting better sound from our speakers but i don’t think that Passive radiators will work with what we use currently.
i could be wrong as i most likely missed something while testing this theory.
i think if a real sound engineer (as I’m definitely not) had a play with this area, we might see some additional improvement.