Acoustic treatment

I must have gone farther in math than you did. No offense,

The reason I ask about adding smaller subs to a big one is that I, like a lot of people I'm sure, already have one big expensive sub.  I would never buy three more like it.  I measured the response with the Behringer ultracurve and measurement mic, and equalized out the worst of the problems.  I'm not unhappy with it how it is but if I could add a few smaller, cheaper subs and get a dramatic improvement I might consider it.  

Deeply discounted subs are common.  For example, I could get three KEF Q400s for about a grand.  It's a sealed box like my current sub so I'd expect them to blend fairly easily.  Adding three more drivers would allow me to let the big sub take it easier.  The question is whether adding fairly cheap subs would solve more problems than it causes.  Would they start audibly distorting if I turn the volume up a bit or would the fact that there are three of them plus the big one allow for more clean dynamic range than I have now?  Maybe I'd leave them off if I want to listen loud and only use them to create really high quality bass when I'm listening at moderate volumes.  

Here is a good site with various materials and their effectiveness
https://www.bobgolds.com/AbsorptionCoefficients.htm

I have had good results with some bass absorbing materials, multiple subwoofers and using the free software REW and MSO (Multi-sub Optimiser). 

Most anything big and absorbing can work. I have a wall to wall closet stuffed with pillows, sleeping bags, boxes of clothes etc.

Working with fiberglass is not bad unless you cut it. You can wrap 703 in muslin (cheap cotton fabric) before working with it or just leave in the box and throw the whole sealed box in the corner. Wrap it if you like. Aesthetics is another issue. Thinner panels only do so much   

mijostyn:
" Tim, I think you should find a job as a used car salesman, excuse me salesperson.
Actually summed bass below 100 Hz is an advantage in most acoustic situations you would have in the typical home."

Hello mijostyn,

     No thanks, I enjoy being semi-retired and performing only the occasional brain surgery.
     All 4 of the subs in my system are run as mono and I have no current issues with summed bass below 100 Hz, although those wanting or expecting stereo deep bass from their systems will likely be disappointed.

  mijostyn:
" Back to Tim, The term "room control" is a misnomer. You are not controlling the room. You are controlling the speaker to correct response abnormalities of the speaker/room system, to make both sides have exactly the same response and correct timing so that the sound from each driver hits the listening position at exactly the same time."

     I don't recall ever using the term "room control".  However, I do understand the 4-sub DBA concept works so well due to creating numerous well dispersed additional bass room modes (bass peaks and dips) at various frequencies in the room.  And I understand the psycho acoustics identified principle of our brains processing these numerous varied bass room modes by averaging them out which results in the perception of the bass as accurate, detailed, fast, smooth and natural. But I agree with you that this is properly described as controlling the speakers to correct abnormalities of the speaker/room system, not as "room control".
     The time domain aspect, or the timing of bass sound waves reaching the listening position, is of much less importance for deep bass frequencies than it is for mid-range and treble frequencies.   We're actually not very sensitive to arrival time in the bass region.  It's been proven that we don't even hear a deep bass frequency until the full duty cycle of the frequency sound wave has been released into the room.  In other words, we cannot detect the presence of bass from less than one full wavelength.  We also need to hear multiple full wavelengths before we can detect pitch.   A 56 foot 20 Hz sound wave released into a normal sized room means it is going to bounce or reflect at least once before the entire duty cycle is detected and be able to be heard at the listening position.  We also need to detect multiple cycles at the listening position to determine pitch. 
      This implies that relatively small timing differences - 'small' relative to the wavelengths in the bass region - are not going to make an audible difference in and of themselves.
     I think you're stating there's a requirement for the "correct timing so that the sound from each driver hits the listening position at exactly the same time"  seems unreasonable and unlikely given the reality of our bass sound wave perception capabilities.  

Tim

I'm using Owen corning 703 rigid 48"x24"x2".

Thanks