Two medium size woofers Vs One big size woofer

All other things being equal, all things are never equal.

Well, the first thing you need to do is make sure the smaller drivers move the same amount of air.  You can do a coarse guess-estimate by computing the area of the cone using the diameter divided by 2, squaring it, then multiplying by two.  A more accurate formula is using the volume of the cone - google it to see how to measure and compute it.   However, once you have the two areas the same, you will find the smaller drivers are much more expensive than one larger sub.  The advantage is smaller can mean faster and better resolution.  The disadvantage is they have to be matched otherwise it might blur the wavefront.   If the drives are in one speaker, then one could assume the manufacturer has matched the drivers.  For your example here, one would need about four  5 inch drivers to about equal one 10 in terms of moving the same amount of air.

IMHO I think the 2 smaller woofers would present a better mid bass quality with a little more dynamics. While the one larger woofer would provide a lower frequency sound but perhaps not as dynamic.
That being said, 5" woofers are pretty small.

ozzy

Thanks you all for your comments.
I currently have a pair of 3-way Sonus Fabers (one tweeter, one midrange and two 7 inches woofers). So I've looking into a new pair of speakers that could keep up with the space (1000 Sqft former factory loft, 13ft height ceilings, open space, no walls). *Room has some minimal treatment.
Aesthetics are somehow important, so that's why I've considering thinner towers over boxy (JBL type speakers).

the key specification is a thing called acceleration factor.

the smaller driver has a larger acceleration factor.

two smaller drivers with the same swept area as a single larger driver..the two smaller ones (~7") will create a notably more articulate bass and have better integration with say, a electrostatic panel- vs that of lets say..a single 10".

eg:

www.audiotechnology.dk

C-Quenze 15 H 52 06 13 SD (5")
Acceleration factor: 674
C-Quenze 18 H 52 17 06 SD (7")

Acceleration factor: 499

C-Quenze 23 I 52 20 06 SD (9")

Acceleration factor: 423

So you get more cheap thump with the larger single driver, but you don’t get articulation. Articulation requires a high acceleration factor. Which is generally with smaller drivers, due to lower mass smaller cones.

IIRC, a tweeter runs near 1000, or just above it, maybe. Thus the 674 of the audio technology driver is crazy fast. This is a difficult spec to find as many driver manufacturers might be embarrassed to print it.

Great, if that's the case, I get good sound along with aesthetics.

There's no substitute for surface area and maximum linear displacement in absolute terms. The idea of small drivers being faster is rarely proven.  Two 8" drivers have about the surface area of a single 10" driver but often with lower impedance.  It's not like you get a 16" driver by buying 2x 8". Also, the 8" drivers tend to have shorter displacements and power handling, so assuming they had the equivalent surface area of a 16" (and they don't) they'd never have the same linear movement capability.

Having said this, I will say I'm a big fan of 2.5 way systems for their efficiency and foot print, but they do tend to have lower impedance with the parallel drivers which can make them underperform with less capable amps.

The big issue in terms of having "fast articular bass" tends to be that the lower you go, the more issues you run into in a room. A speaker that is anechoically flat to 16 Hz may sound really crappy vs. a smaller speaker which is flat to 40 Hz as the big speaker will aggravate the room modes more.  

There's a big difference between the anechoic bass response of a speaker, or the -3 dB point, and how it will sound in a room. Smaller is often better. 

two 5" woofers is equivalent one 7" woofer , not 10"
this is simple , check any speaker design book

approximately

I have two Vandersteen 2wq subwoofers. Each has three 8 inch drivers driven by a 300 watt internal amp. They have a "Q" control on the cabinet back to increase or decrease the tautness of the sub response. The three 8" drivers with the "Q" set low provide amazingly tight bass. If you want it to boom, for home theater movie reproduction you just turn that dial up.
I just bought a new disc player and power cord for my system amplifier. The disc player is rendering a tighter bass profile and the power cord (7 gauge) has really given heft and gestalt to the bass and system sound overall.
On the other hand, one large driver can do a nice job especially if it is used in a servo controlled system.

How many subwoofers have you seen using ten 6" drivers?

I wonder how the larger surface area of a large woofer factors into the acceleration factor.  A large driver with twice the surface area of a smaller one only needs half the movement of the smaller one to move the same amount of air.  That has to factor into the equation.  I think you'd need to understand the relationships between size, extension, distortion, etc. to be able to have a meaningful opinion on this.  Big drivers are definitely better for deep bass.   You'd need 9 6" drivers to be the equivalent of my 18" sub and in order to have a comparable excursion the surround would need to have as much surface area as the cone.  That would have to contribute to distortion.

"Acceleration factor" is the ratio of force or motor strength (Bl) divided by moving mass (Mms). It ignores cone area, and is imo therefore misleading.

Let me illustrate why: Suppose we are comparing a single 10-inch woofer against four 10-inch woofers, and they are all identical. So the acceleration factor (the ratio of motor strength to moving mass) is the same for the single 10-inch woofer as for the four 10-inch woofers. But it would be a mistake to claim that the ability of the single 10-inch woofer to accelerate air is the same as the four 10-inch woofers. The four woofers can accelerate four times as much air as the single woofer.

And ultimately it is the ability to accelerate air that we are most interested in. 

As for whether a single big cone is better for bass than multiple smaller cones, assuming the capability of the multiple small cones adds up to the equivalent of the single big cone, I believe that I could get better performance out of the multiple small cones by distributing them to different locations on the loudspeaker enclosure to improve their interaction with the room.  There would be tradeoffs for doing so, and the cost would almost certainly be much higher for the multiple small cones.  

Duke

No speaker has ever given me the attack, and thump of my old D-9 wardogs!

  They were , and still are amazing monkey coffin speakers!

Yes, there are tradeoffs  either way. Can't have it both ways. 

Another factor to consider is a large, 15" or so, woofer properly loaded into a well designed bass front horn can make any combinations or 7" of 5" drivers ashamed of themselves.  The transients, impact, tunefulness, definition and clarity is unequaled by other systems for bass reproduction.

@kingharold --

Another factor to consider is a large, 15" or so, woofer properly loaded into a well designed bass front horn can make any combinations or 7" of 5" drivers ashamed of themselves. The transients, impact, tunefulness, definition and clarity is unequaled by other systems for bass reproduction.

I wholeheartedly agree. Myself I’ve chosen the tapped horn principle - do you know of it, and have you heard any tapped horn subs? A traditional front loaded horn would have suited my requirements very well also, but that’s another discussion. It’s great to see the sound of horn bass/subs acknowledged - you put it just right.

What’s interesting is how the horn acts as a force multiplier in being an acoustic transformer, and thus a horn-loaded 15" like you mention acts and feels quite differently compared to a 15" (or bigger) direct radiator. Even compared to ported subs with 2 x 18" drivers per cab, a horn-loaded 15" to my ears feels more effortless, present and "malleable" somehow, as if the bass flows in more organically and naturally with the rest of the frequency spectrum. Ported 2 x 18" is typically more of a "pounding" experience that potentially stumbles over itself with very demanding bass lines; 15" horn-loaded bass is more a floating/flowing experience that vibrates, or even shakes the whole air volume in the room, making for a very present, enveloping and tactile feel. It’s a shame so few have heard horn (sub) bass..