System that sounds so real it is easy to mistaken it is not live

@rauliruegas just because a sub is a balanced force design does not necessarily mean it is going to be good. It depends on the quality of the drivers and the construction of the enclosure. All internal bracing does is change the frequency of the resonance. Play a 20 Hertz test tone at 90 dB and not only will your sub be shaking but so will the whole house. There is no coating you can put on a sub that will keep this from happening. There is no subwoofer enclosure made of MDF that can perform at the state of the art. 

Next using a subwoofer with a two way crossover under a 3 way dynamic system will relieve the system's woofer from taking long excursions which will keep it in a more linear zone of operation lowering distortion and Doppler effect of the woofer. If any of the woofer's "upper harmonics" are getting through to the rest of the loudspeaker someone really f-ed up on the design of the woofer to midrange crossover. In which case I would toss that loudspeaker and buy another one. 

The reason that using a subwoofer under an ESL is because there is only one driver thus, keeping it from taking long excursions cleans up everything and allows it to go VERY LOUD. And because it is a full range line source it sounds VERY BIG just like a real rock and roll concert. 

Raul, I have been building and designing subwoofers for 40 years. I think it is also pretty obvious that I am very talented cabinetmaker with enough equipment to open a commercial shop. I am building what I think will be the worlds finest subwoofer. You could at least wish me good luck.

Dear @mijostyn  : "  90 dB and not only will your sub be shaking but so will the whole house.  " , not in my place.

" If any of the woofer's "upper harmonics" are getting through to the rest of the loudspeaker someone really f-ed up on the design of the woofer to midrange crossover.  ", that's not what I said.

" dynamic system will relieve the system's woofer from taking long excursions which will keep it in a more linear zone of operation lowering distortion and Doppler effect of the woofer. "  this is what I'm saying and this effect means the  IMD goes lower and that permits the mdYhi frequencies been listened with new clarity/transparency and the like.

Good luck. When will be in the market?

R.

Mijo, In a distant way, there is a relationship between a transmission line and a dipole design, or whatever you call the type where two woofers are working in or out of phase, back to front or front to front. The transmission line is akin to an open baffle where the rear radiation is used to augment the very low frequency bass output by undergoing a phase change in the course of passing through the transmission line and out through the port at the base. But the cabinet does not damp the motion of the woofer, as in open baffle.

Raul, I was a lowly intern when I first heard the IMF Monitor at Lyric Hi-Fi in NYC. I wanted that speaker very badly but could not afford what seemed to be the stupendous price at the time, $1000 I think. I had a patient who offered me the use of his bench saw, so I bought the HDF, clamps, glue, drill, etc, and built the transmission line exactly according to the IMF Monitor, which was based on a published paper in a British journal called "Wireless World". A guy named Bailey described the TL in one issue of the journal and gave all information needed to make the cabinet using a KEF B139 woofer. Then my home-made version used a KEF B100 midrange, as in the Monitor, and RTR Electrostatic tweeters, 4 per side, that I bought from a guy in CA who was associated with Infinity, which was then making the Servo-Statik 1. The problem then was that I knew nothing about crossover design. I got some help with that from an MIT-trained engineer who worked for NSA here in the DC area. I eventually sold the speakers to my cousin, and then bought them back from him about 20 years later. I cut off the midrange and tweeters and saved the TL woofer cabinet, which I now use along with the Beveridge 2SW, as the outboard woofers.

     People have been fooled for for quite a few decades.

                                  Even by Edison!

ie:  https://phonographia.com/Factola/Edison%20Tone%20Tests.htm

@lewm , transmission lines are arguably the hardest type of enclosure to design. Yes, you can effectively double the size/efficiency of the driver at some frequencies. The problem is that the front and rear waves are only exactly in phase at certain frequencies. Then there is the problem of the construct. The enclosure is composed of a number of dividers and pathways. It becomes much harder to control all the panel resonances that develop and to make sure every panel is locked down solid. Subwoofers, by nature shake the hell out of everything. It is what they do for a living. I lean towards small enclosures because it is much easier to make them stiff and solid (the soap bubble rule). The lower efficiency is now easily covered by the powerful amps we have and amplitude errors can be corrected in room digitally. Setting them up as a line array and placing them directly against a wall then minimizes room interaction.

@rauliruegas , Is less distortion in the woofer's frequency range going to make the rest of the loudspeaker sound better? Since the loudspeaker as a whole will sound better I suppose you could say that is true. It will in no any way effect the actual performance of the higher frequency drivers. 

Raul, do you live in a bomb shelter? Put on a 20 Hz test tone, crank it to 90 dB and I absolutely guarantee you that your house will become a symphony of rattles. 

When are the final versions of the subwoofers going to be ready? About two months after my wife stops handing me stuff to do. The prototype has been made and it works as advertised but it is nothing special to look at. It is used more or less to develop construction methods and procedures that will work and minimize waste.