A full range speaker?

@cleeds , that is exactly what I wrote. That thump is not the note it is an associated percussive sound and I think that is pretty obvious. 

Oldhvymec has it right. There is a visceral aspect to live music that is missing in most HiFi systems. People will try to get it by turning up the volume way too high. This does not work and blows out your ears. This aspect is coming from bass below 40 Hz. Reproducing these frequencies in residential settings is difficult and it is easy to "damage" the the rest of the music trying. I believe this is why there are such polar opinions on the subject. 

@ivan_nosnibor , interesting project! You need drivers with a free air resonance below 20 Hz. Infinite baffle speakers are much more efficient but the enclosures are large and larger enclosures tend to have more resonance issues. I know one person who mounted the drivers in his floor so that the basement was the enclosure. I did not like it because there was a floor resonance that ruined bass detail. He needed a stiffer floor. I toyed with the idea of mounting drivers in the front wall which in my situation is an outside wall. Glad I did not. If you go outside when the system is running that wall buzzes and rattles like crazy. The Hardy Plank siding is creating most if not all of the racket. Fortunately, I used  staggered stud sound proof walls so you can't hear it inside. I used the same construction in the master bedroom so the kids could not hear mom and dad having fun:-)
I am not sure that 18" drivers are the way to go. I would use multiple smaller drivers. I would use two of these instead of one 18" driver.
https://www.parts-express.com/Dayton-Audio-UM15-22-15-Ultimax-DVC-Subwoofer-2-ohms-Per-Co-295-514  
It is easier to control a smaller cone. A cone's motion has to be pistonic. Larger cones have a tendency to move asymmetrically. 15" drivers are plenty big enough. I use 12" drivers to keep the individual enclosures small enough to fit in my situation. The new system will use 8 drivers. Dayton subwoofers are excellent and a great value. I have been very pleased with them. I got two Morel drivers and was not impressed. I sent them back.

mijostyn

@cleeds , that is exactly what I wrote. That thump is not the note it is an associated percussive sound and I think that is pretty obvious.

You also think it "obvious" that the percussive strike is lower in frequency than the fundamental:

You have to forget about fundamentals. Things like the thumb striking low E produce a thump that is below the fundamental. Percussion will do this also.

That is completely mistaken, as can easily be shown on even a simple modern software program.

In other threads, you've reported on your need to use rumble filters in your system to prevent your woofers from "flapping." It's increasingly apparent that something is wrong with your system; you're inclined to accept it because you believe those low frequencies are inherent and filtering is the only solution. (While that approach works for you, I've always preferred to tackle LF disturbances at the source.)

FYI.

All sounds made by musicians are notes in this post. Rimshots to tapping on a sound board to clapping hands. If it's part of the artists repertoire, it should be reproduced as a coherent whole.

Oh yeah, even saws and wash tubs…

@mijosytn,

Yes, the fs is 18.4 Hz. Acoustic Elegance is something of a ’Rolls Royce" brand of woofers for many different apps actually (sealed, IB, OB, etc) b/c John, up there in Michigan, takes the time to personally tune the design of each woofer for that particular app and that app only. He doesn’t go in for designing ’dual or multi purpose’ woofers...or off the shelf - he makes everything in-house to his custom spec. Full specs are here:http://http//www.loudspeakerdatabase.com/AE/IB18HT#8%CE%A9
I have the "Apollo" version here:http://aespeakers.com/shop/ibht-woofers/ib18ht/

Gotta laugh sometimes when I visit IB sites and start perusing through the mass graves of the unfortunate souls who have gone before and had fearlessly waded into the swamp waters of IB territory, having heard past tales of glory to be had on the cheap and armed with little more than a tape measure and a skill saw. The sites (like cult of the infinitely baffled and others) are strewn with examples of people who’ve made all kinds of ill-advised major surgery to their homes in an effort to DIY and ’save some money’. In the extreme some of them all but destroyed the resale value of their homes(!) And most severely underestimated the vibration problem. But, I salute those who posted their horror stories and have contributed to my sobriety of mind before taking on this project of mine.

So, going in, I’ve held few if any illusions about how easy an undertaking this all is. I’ve had the last 2 yrs to think things all the way through as much as I could even before I bought anything, let alone broke ground for construction. Many advantages in going for a standalone structure, don’t have to worry about sympathetic transmission to the rest of my house. It will be built on a concrete slab with the system front wall made of concrete block, all interior walls, floor, ceiling will be made of 2x12’s with walls and ceiling having at least a 3" space between them and the damped metal shell (room within a room construction). There will be the unexpected challenges I’m sure, but I will have to allow for that and come up with solutions as I go.

The 18" woofers should be ok. I’ve been using them in my living room in an OB config and their behavior so far has been exemplary. I know you’re referring to the sag as they age, but I’ve intended all along to rotate them once a year to keep the spider material from wearing asymmetrically over the life of the woofer. A calculated risk, as it were, as to whether or not that works, but I’m thrilled with their sound right now, we’ll see.

@mozartfan - I'm glad you found the chart useful.

There's also a comment somewhere above that sub bass isn't audible... sub bass is generally (perhaps not universally?) understood to reference the lowest two octaves which occupy 16-62Hz, most people can hear 20Hz and up.