I have been thinking about this design.
There has been minimal work on using different drivers is isobarik configuration and practically no good data, but it is not recommended.
I think the attraction here is two fold. The rear driver is much cheaper than the front driver. The other is that there is some data to suggest you can average the Vas. So if the rear driver has a lower Vas than the front, then cabinet volume is reduced further.
The best suggestion I found was to mount the two drivers front to front, and measure the Thiele/Small parameters.
The drivers must have similar sensitivity. The xmax of the rear driver must not be greater than the xmax of the front, or damage is likely.
One rule they have broken is to use an isobarik configuration above 300 Hz. This is because of rear speaker radiation emanating through the front driver with irregular response as it transits through the front driver.
The designers have tried to get round this by using a 130 Hz low pass filter to the rear driver, but then got a huge null at crossover due to the phase shift of the crossover. This is not solvable with a passive crossover. So they accepted it.
So, this is the front driver
. US price is $585.00 each, but with volume discounts.
The rear driver is from Eton. I have looked at the drivers, and this is the only one
that looks a possible contender. The Vas is 28 lt. versus 44 lt. for the front driver, and it meats xmax criteria, for he potential to lower total volume compared to using two front drivers.
This is the tweeter
. Nothing to complain about. Though obviously "exotic" has a dollar sign.
I would point out that unless the Vas is high, then an isobarik design just adds complication for no discernable gain. My view here is that the designers have added no benefit just problems. This is not a good speaker in terns of value for money. On a value for money rating it is abysmal.
To illustrate the point, it so happens that just before Shady's excellent review we had a member want to build a pair of bookshelves for no more than $500.00. It so happens that I designed with Morel drivers and a Scanspeak tweeters in our in wall system. As you know my policy generally is only to design subs for members except under unique circumstances. In the event I thought I could use a pair of the same drivers and tweeters and give him a reliable design. I think he will build it. He has engaged me to build the crossovers, as he feel confident only with the carpentry. I am attaching the design.
This $500.00 dollar design will be a significant improvement over that 25K effort. True the midbass driver is less sensitive, but it is 8 ohms, so the power drawn by the two speakers from the amp will be virtually identical.
If the owner is not happy, then he can ship them to me for measurement and if necessary tweaking of the crossover. However I suspect he will beat the 25K effort at one fiftieth of the cost.
Now I realize you could not market that design for $500.00, but it would not be near 25K. I think it will also be smaller.
I have told you guys that building speakers gets you way ahead financially!
I enjoy Shady's reviews. I understand that Audioholics needs advertisers and he has a tactful line to walk. Keep up the good work Shady.