Bass Driver Size - how much better is an extra inch?

Have older b&w speakers and bass drivers are 7 inches vs 804 d3 speakers that are 6 1/2.

the larger drivers seem to really open the sound stage, more open and less constrained.  Imaging of older speakers not as good but a very enjoyable listen.
Not sure why driver sizes tend to be smaller these days unless you pay a lot more.  Hear that most consumers want more compact mains so drivers are smaller.  Kinda sad.
Bigger definitely will open it up more. Although there is much to be said for stroke, frequency, and duration. Especially duration. The longer the session the more it opens things up and the less constrained they get.
There is no replacement for displacement. Axiomatic in the car world, and as well should be for producing bass.

Of course, on both fronts there ARE replacements for displacement. But slapping on a turbo or just increasing xmax isn't a true replacement Hence the non-linear price increases for well executed power at smaller displacements.
Much value being offered herein albeit cryptic. 

I believe it's largely fashion.

Not everyone wants to, or is even able to accommodate speakers with huge 15 inch bass units like me. 

Drive unit size is certainly a contentious subject with various arguments for and against large and small drivers.

It's certainly not a question of bass content either as some small units can dig lower than some larger ones. Depending upon factors such design (greater number of long throw units) and cabinet loading.

However there do seem to be differences, size does matter.

Permit me a gross generalisation if you will, but I'd say that smaller units sound noticeably quicker with transients.

On the other hand, larger units tend to have a far greater sense of ease as they go about their work.

The 15 inch Dual Concentrics on my Tannoys are easier on the ear than the slimline Revolution 3s that they replaced, if perhaps surprisingly a little lighter in the bass.

If I had to chose, and if I wanted a more relaxing sound, I'd take the second option. 
To answer your question " how much better is an extra inch? " the answer is, not much -- unless you have a very large space that your present speakers fail to fill.If that is the case, a 8' woofer will give you a slightly bigger image and will be somewhat louder in the bass (i.e. they do move more air!).

Of course, at equal SQ (speed, accuracy, etc) large drivers are much more expensive... there are other considerations as well
Right, most people (women) do not want big speakers. 

Assuming proper driver design larger woofers can go lower and have less distortion because they do not have to move as far to create the equivalent output of the smaller driver. There is a limit. As the woofers start getting over 18 inches (some say 15") it gets harder to control the motion of the cone creating more distortion not to mention the size of the enclosure gets prohibitive for home use. I think 12 and 15 inch woofers do fine in the home environment. There are 15" drivers now that will work in a 1.5 cubic foot enclosure which is darn small.

Many people notice that for some reason the midrange opens up when subwoofers are added. The low bass gives you a different midrange perspective. If a two way crossover is used distortion in the main speaker is reduced. If a lower order crossover is use enough midrange may come from the sub to change the perspective but I notice this even with steep slopes and we are talking about 10th order. The problem with steeps slopes is the crossover becomes more obvious. The compromise is 4th and 6th order which works great at least in my case. 

Most reference speakers, in the bass/mid-bass area are mid 6-10 mm at the most on Xmax, and higher sensitivity.  If they even address sub at all, other than to roll it off sooner..

Better idea..

Monitors can't do it all well..

Full range speakers are full range speakers, doesn't mean it's a good idea..or that they do it very well.. I cut anything to do with the monitor at  40, no matter.. the set up.. Small monitors (bookshelf type stuff) and no sub.. Just easy on the ol ears that way.. Higher when we add separate subs. maybe 60-80 on the roll off.

Depends on HOW they have it set up.. Biamp able, I disconnect the bass. My bass/sub system is ALWAYS better. 300 hz and down..

Can't put everything in the same box, it can't work.. :-)

BTW every inch counts.. that's X 2 there is a back side of the bass drivers too. Can't just dismiss that... Passive radiators, and ports, make NOISE.. all part of the measurement process.

IB (no port) maybe not, at first, anyways.. The enclosure might.. Depends who made it and how well it's dampened. Panels breaks, are number one in IB structures...

B&w 802 speakers, $22k, have two base drivers at 8 inches.

to me that still seems restrained. 
Better to have 2 smaller base drivers vs 1 bigger size?

At $16K new, the Revel 328be uses three 8 inch woofers. In room response should be in the 20's and the measurements I've seen show low distortion. 
Honestly, bass drivers should be 8” or bigger in my opinion.  10” or 12” is even better.  Thats why you should just augment the low end with a sub but many 2 way speakers today with 6” drivers produce sufficient bass.

If you sit there though and go up a line of speakers, the drivers get bigger and the sound fuller.  I’ve found that no 5” drivers can cope with the abuse I want to throw at them.  It’s got to be at least 6” if not 7”.  I learned this early on in the Paradigm line.  The difference between dynamics between studio 10’s and studio 20’s was pretty substantial.  The bigger driver on the 20’s vs the 60’s would even play louder.

So yes, with regards to bass drivers, bigger is better!
If you think you can't put everything in the same box you haven't been paying attention to newer designs. The Devaliet Phantom Premiere will go down to the teens with two 6.5 inch drivers. Genelec 8361A in room down in the 20's same with Dutch and Dutch. 
Cabinet design and driver excursion  more important than diameter.
I've never owned speakers with less than 10" bass drivers but I suspect a dual 8" driver arrangement would be fine for even larger rooms.  (3)8" drivers, probably better though.  Obviously, implementation is key.  When I went from dual 10" drivers to dual 12" drivers, the difference was subtle in my 16x28 listening room.  A little deeper, yes.  But the 10" were certainly plentiful.  I love being able to "feel" the music.  I have a hard time believing 6.5" "bass" drivers would give me anywhere near the same sensation.
The half inch, as a single metric, is irrelevant.

Much more important is the motor quality, room acoustics, and integration.

I know some 6 1/2" drivers I like a lot more than some 8"s. :0

Room fullness makes all the difference in the world, and with no distortion these simple things will rank a speaker very highly.  

High end brand names take notice.
Im not sure that Im tracking that last post...??

Anyhow, yes, 6” drivers, especially dual, can produce a good amount of bass if high quality.  But a high quality 6” driver vs the same high quality 10” driver, and the larger driver will put out more bass.  This is just physics, right?  More cone surface area...
I went from two way monitors with a 6" woofer to two way monitors with a 8" woofer. Not only is there more bass but the quality of the bass changed. It seems more tuneful and resonant for lack of a better description.
I use a pair of 18" drivers in a 14 x 27 x 8 1/2 ft room.  No complaints.  An extra inch would basically take the drivers up to 20" or 21".  Not sure that would make much difference.
It is an engineering compromise, just like almost everything is in life. It is basic physics.

The larger cones move more air and hence provide a much more realistic and voluminous bass. However, the larger the cone gets, the less rigid it becomes, affecting the sound negatively.

If you could find an infinitely rigid material, which would never lose its shape regardless of how much air pressure it was exposed to, then you could build infinitely large speakers. But real life has limits :-)

There is also one more very important factor as to why most drivers (and hence the speakers) are smaller these days.  It is the WAF :-)

Makes sense very large drivers loss some ability to handle detail.  So there is a sweet spot between, say, 7 to 12 inches. And beyond 12, you would be in a subwoofer zone where detail is less important and less feasible.  
Personally, 10 inches on the b&w 800 seems ideal.  
There is only so much tech research can do while trying to cruelly shrink a bass driver and maintain sq.  Physics has its limits
Manufacturers and designers need to wake up and not artificially  suppress quality bass sound to accommodate a smaller cabinet.  How cheap are things getting.  Remember how they trimmed the size of a carton of oj a few yrs ago.

The wharfedale 225's as reviewed by three different reviewers in stereophile, who all agreed that the speaker has phenomenal bass, uses only a 6.5 inch bass driver, ported at the bottom surface of cabinet. One reviewer, I think the late Art Dudley, was taken back by how much useable response the speaker had at 31.5 hz! It was confirmed with measurements. He said he heard what he heard....just one example. I still own a pair and still love them, but My tannoys with 10 inch driver somehow seem effortless, maybe more relaxed. 
MC, I got it!!!! Clever and fun.

More seriously,  size not critical - but only nowadays. Design and new tech are changing the old parameters.

A big sound depends on how much air is moved forwards.  So this meant earlier large cone size vs breakup was the issue. Larger and stiffer=more bass directed forwards.

Nowadays with high excursions being permitted by the new materiels, and some creative thinking, it's possible to get smaller cones (almost zero breakup) to do similar stuff.  

A recent development has been to design the speakers port so the back wave is phased to reinforce the front. Eg D&D, some open baffle designs.

Kii use a slightly diff approach.  They have a back speaker which slightly out of phase so that it cancels the front wave behind the speaker and reinforces it in front of the speaker.   Along with dsp and long throw cones their speakers measure down to 19 -20 in room +- 0.5 dB!

KEF's new approach is to simply cancel the back wave internally - with no consequent penalties.  That is unbelievable in my book but they claim to have done it.

So the answer to your question is that the new slightly larger bass drivers may improve bass response, but you should look at these new technologies first, if possible.

Best wishes

Quite agree AubreyBob.

Back in the day most woofers were 12 inches and some were 15.  Who remembers the 15 inch Fane that could be bought very reasonablly and put in home-built cabinets?  It was designed for use in cheap PA and guitar amps.

The issue with such big units was big lack of cone stiffness.  Yes, they moved a lot of air and delivered a lot of bass but flexure of the cone meant it was very flabby, with poor cut-off of notes and inclined to tunelessness.

So from around the 80s, designs featured multiple smaller units.  Note as a good example the B&W 801, a 'budget' high-end speaker introduced around 1997 with a single 12 inch woofer.  After some years the 801 was withdrawn and replaced by the 802 that has two smaller units.  Reviewers reported cleaner bass with no loss of sound pressure.

More recently the application of new exotic hi-tech materials has enabled the revival of 12 inch cones that have sufficient stiffness to allow low distortion AND big air movement.

So OP, the choice is yours....
@emergingsoul --

Not sure why driver sizes tend to be smaller these days unless you pay a lot more.

Indeed, a lot more; size is progressively expensive in hi-fi, not to mention high-end, and for physics to be more closely approximated - taking a small, already expensive package and making it into a bigger one - it’s an expenditure eventually only the very few can accommodate. A shame really obsessing about painting that little corner of the canvas in the most minute of detail, when the rest of it, its sheer scale and totality, is neglected. That is, at least when one knows what’s missing.

Smaller size is by and large about convenience/practical considerations/interior decoration and spousal acceptance, from where effort is often invested to make it appear as if the smaller package is actually a desirable trait, or one overall sufficient sonically (some justification is obviously sought into buying small-ish products that expensive, or even less expensive). And who wants to pay $1,000 for a measly sound bar? Many, it seems; they’re selling like warm bread - go figure.

I’m not trying to negate the potential prowess of small(er) speakers in smaller spaces, but it’s when smaller sized speakers become excessively expensive that bothers me, because vital aspects of the sound only achieved thorough an adherence to physics (i.e.: sheer displacement area, high sensitivity, etc.), important to me, are still sorely missing. Why then bother, and at that price? 

Simplicity topologically (and what it in-effect leads to) is something to aspire to, a trait with smaller speakers in particular, but it’s likely a side effect of a purchase incentive here that’s not always consciously sought to begin with, at least not as much as seeking smaller size per se. Holding on to that trait, however - and for good reason, I’d claim - the question could be how to scale up in size without throwing out the baby with the bathwater, and I’m thinking mostly complexity and price here. It’s certainly possible to achieve, I might add, but not through the package one would usually expect, while also needing a healthy dose of change in mentality towards sound reproduction in a home environment.
the cone area will increase 29% per driver or 58% per speaker pair.  all else being equal the larger drivers will be capable of more impactful and deeper bass.   this is a significant plus.  
however a larger cone will lower the frequency at which the speaker starts beaming, or become directional.  
this will drive compromises in the design to maintain driver to driver coherence and dispersion, usually a lower crossover frequency and a more expensive, capable tweeter.  
the best sounding most coherent speakers i have heard use smaller woofers, e.g. a ls3/ 5a type monitor that can sound spooky good.  

Bass driver size depends on the design of the cabinet and the driver design to utilize that drivers best potential.  In other words the components and cabinet they are mounted need to  be designed together with an intended sound.
The difference between a 6" woofer and a 6.5" or 8"  woofer wont matter.  The design specification of the woofer matters.  You can have a 6" woofer that has a usable frequency range of 30hz to 5000hz  and a woofer of 6.5" with a usable frequency range of 80hz to 5000hz.  Just because the woofer is larger does not mean it will have as good of dynamics as a better designed woofer.  Either woofer in the wrong cabinet will sound crummy.  A cheap $20 Woofer matched in the right cabinet  will sound better that an $80 woofer in the wrong cabinet.  All depends on the design and quality of materials. not size. Let your ears tell you what is best.
This is like saying a built 2 liter, 4 cylinder turbocharged engine can outperform a V-8.  Well sure, but can it outperform a built, turbocharged V-8?  No.

I’ll buy that a 10” driver isn’t as fast as an 8” driver, but all other things being equal, a 10” driver will put out more bass, lower bass etc.
Just look at speaker manufacturers.  As you move up the line they get bigger... Most flagship models within a speaker line are there largest.

I do believe though that you can overpower a room with speakers to large for that room but the room needs to be small, like my 11x13 room, and the speakers have to be rather large.
Don't confuse frequency response with impulse response (impact) in regards to low frequency drivers. While a well designed 6.5" driver can cycle quite low in a well designed cabinet, it will only have a fraction of  the impulse response of a larger driver. Not all music styles require impulse response, like folk music or traditional jazz to name a couple, but impulse response is a major part of most modern music created since the early seventies and you will not get the full effect of that music out of a small speaker.
One way to get the transient response of a six or eight inch woofer out of a fifteen inch woofer is to put an appropriate fifteen in a well designed corner horn.  Admittedly this mandates a largish enclosure.  Mine are 18 cu.ft. each.
I have a 16' x 15' room with a pair of Klipsch K-horns in the corners of the long dimension (they're 1978 models that have an updated (per spec) crossover and modern wiring (lamp cord was original).
I've often wondered if I would significantly improve the sound by adding a subwoofer?

Emergingsoul wrote: 

"Have older b&w speakers and bass drivers are 7 inches vs 804 d3 speakers that are 6 1/2.  The larger drivers seem to really open the sound stage, more open and less constrained.  Imaging of older speakers not as good but a very enjoyable listen." 

Imo the difference between 6.5" diameter and 7" diameter drivers is inconsequential. 

I think you are SEEING a cone size difference and HEARING a sonic difference, assuming that the one is the cause of the other. 

My guess is that there are other factors involved which are not readily apparent, but which have a greater effect than the small difference in cone diameter. 
I have older 803's with dual 6" woofers. My even older B&W's with 6.5 inch woofers do add something that is pleasurable. but I liked my old 8" Peerless subs that I put in my Acoustats better than the identically designed 10"ers I now use, because the tens are not quite as tight on acoustic bass. Interestingly, the difference between successive model preamps from the same company is nearly as great.
You cannot predict what you will experience when sitting back in your chair from specs such as these.  You MUST listen to the speakers and preferably in your home.  Your example is 0.5 in difference from the same mfg, but from a different generation.  You wouldn't expect any difference here but the newer generation may be far superior to the older design.  The factors that contribute to a speaker's sound is extensive as others have mentioned.  I have Platinum Audio Quattros that have four 4.5 in drivers, two crossed for mids and two for bass in a nice slim cabinet.  No way this speaker can extend low.....right??  Well, they would shock you on how effectively they can play down in the 30s and loud too!  I tried out the Focal 1038Be2 with three 7 in all dedicated to the low end and in my room they were weaker in the bass compared to the Quattros.  Platinum Audio speakers were designed by Phil Jones, who is a bassist and has a great love for good bass from his speakers.  This goes back to all the other factors and how he designed his speakers.  Now I have Wilson Audio Sophias with a 10 inch driver in each.  They far surpassed my Quattros in their lower extension and have become my new speaker.  Let's do the math and measure the amount of air displaced from the cones surface area.

Platinums(two 4.5in) 32 sq inches
Focal(three 7in) 115 sq inches
Wilson(one 10in) 78 sq inches

This would imply a different scorecard for sure!
The "speed" a driver is capable of is indicted by its frequency response curve. A 15" driver pushing 100 Hz is every bit as fast as a 6" driver pushing 100 Hz and if the motor is designed correctly the transient response can be just as good or better. This concept of "speed" or fastness need to be thrown out.

6-8" woofers can perform admirably down to 50 Hz or so. You can force them to go lower but forget about energetic bass below 50 Hz. Small woofers are being used to support the tower concept that the ladies like. Using multiples helps but none of these speakers do well below 50 Hz. If you think they do it means to me that you have not heard a system that accurately produces low bass. I use a subwoofer array that uses 4 12" drivers with a total of 8000 watts in a 16 X 30 foot room. If I play a 10 Hz sine wave the entire house shakes and buzzes. You can actually see some items shaking. Everything upstairs and in the shop is singing. If I turn it up loud enough your vision blurs. Little woofers simply can not do this, physically impossible. What they really are is midbass drivers which they do wonderfully well. For the best performance you cross over to subwoofers at 80 Hz. If you use a symmetrical array around the main speakers you can cross higher. Good midbass drivers can run nicely up to 2500 Hz or so and there are many tweeters that will easily go down to 2500 Hz. So, you really only need a two way speaker and subwoofers. 
The minimum is four 10" subwoofer drivers or two 12" subwoofer drivers.
Subwoofer drivers have a hard time making it to 500 Hz for design reasons. 
Why not fill in the low end with a pair of REL S2 SHO subwoofers.  You can dial them so they don't over power the bass and with their high level connection they will act more like woofers.  Be curious to hear what this group thinks about this.  These really helped to broaden the sound stage.  Trick is to make sure you do not over power the bass.  I noticed my mid frequencies and higher frequencies seemed to clean up.
I've been using a pair of Velodyne HGS-15s crossed at 40 Hz with KEF Reference 1s.  Today I raised the crossover to 80 Hz, still using a 24 dB/octave slope, and ran acoustic room correction.  I was surprised how much that enlarged the sound stage.  My concern about the setup being suitable for large orchestrations vanished.

I don’t have a lot of experience with different speakers, but I can tell you there is a big difference in bass between my current Klipsch Cornwall IV’s and my Goldenear Triton 5’s. On paper the Tritons went deeper into the 20’s while the Cornwalls only go down to 38 however the sound is bigger. When I added two JL Audio Dominion D108 Subs with the Goldenears the Sound was great but still not as powerful as my Cornwalls. I’m actually thinking of adding my Subs back into my system to see how they’d sound with the Klipsch. But is it useless to use two 8 inch 500 watt subs with big 15 inch woofers? So in my experience albeit limited bigger woofers can provide bigger sound but not always deeper bass. 

"So in my experience albeit limited bigger woofers can provide bigger sound but not always deeper bass."

Mine too. My 15 inch Tannoy DC drivers certainly have a big sound but I don’t think it goes down as deep as the slimline floorstanding Tannoy R3s with their twin 6 inch drivers.

It’s not always easy to be sure as most of the music I listen to hardly features much, if anything below 40Hz. Besides it’s a common misconception to think that 60Hz is where real deep bass is. That's why a lot of small speakers seem to give perfectly adequate bass without disturbing the furniture.

Horn loading like your Klipsch Cornwall’s will result in a different bass sound than ported designs which will sound different to sealed boxes.

As @mijostyn said earlier, real deep bass (sub 40Hz) is where you start to feel it rather than hear it. A key difference I think between live and reproduced sound.

Live sound is often equally felt as it is heard.
@cd318 --

Horn loading like your Klipsch Cornwall’s will result in a different bass sound than ported designs which will sound different to sealed boxes.

The Cornwall’s 15" bass driver is ported. The sibling La Scala’s on the other hand are using a 15"-loaded folded bass horn, and yes that’s certainly a different bass presentation vs. a ported, or even a direct radiating sealed design.
that's a question you should be asking a female 
I was thinking it really mattershow many inches you’re already working with...
@phusis ,

I rather suspected that my front ported Tannoy Berkeley's didn't go down that far despite their 15 inch drivers.

According to one site their frequency response is 35Hz - 20kHz. I strongly suspect that it barely scrapes 35Hz let alone being flat.

Come to think of it, wouldn't FR stats be far more meaningful if they showed only the flat range, as opposed to -2db down or even worse?
@cd318 --

I rather suspected that my front ported Tannoy Berkeley’s didn’t go down that far despite their 15 inch drivers.

According to one site their frequency response is 35Hz - 20kHz. I strongly suspect that it barely scrapes 35Hz let alone being flat.

Come to think of it, wouldn’t FR stats be far more meaningful if they showed only the flat range, as opposed to -2db down or even worse?

Extension downwards is about the relation of bass driver size to enclosure volume (or horn path length in a bass horn) - unless we’re talking EQ’d sealed designs - and those Tannoy’s, from the looks of ’em, would seem to accommodate the response you point to - certainly in-room.

More important than extension per se is how they come about the bass that is, and I would expect it to be very satisfying here. Big paper coned woofers of at least 15" has a tuneful way with the lower frequencies that smaller units in multiples can’t replicate, and with the proper driver type (for this particular context at 15") they’re very good into the central midrange as well (though 900-1kHz is about as far as they get, I’d say), as you’ve no doubt come to experience with your Tannoy’s. Though, this would seem to go contrary to audiophile "wisdom" with the mention of beaming, cone break-ups, etc. My counter reply: try it out with actual listening, and if it isn’t one’s cup of tea - fair enough.

Regarding FR stats: Insofar the specs are trustworthy to begin with I suppose an -xdB number is meaningful in the sense of indicating the useful frequency extension, which goes beyond flat response and not least with the interaction of the listening room. I don’t really care about the bass stats of main speakers; listening will do the talking here, and in any case I’d by inclined to high-pass them in the bass, actively, for subs augmentation. Extension stats (or rather: tune/lower knee) for subs is another matter though, as this is specifically dealt with and important in the DIY designs I’m interesting in and use.
I have been using 2 dual 12 inches subs with my stereo system for 6 years and very happy with it until I bought 2 21 inches subs. The 2 21 inches subs just upgraded all my CD in my stereo system and I retired the 2 dual 12 inches subs for HT. If I have more space in my tiny music room, I would go for the 24 inches subs. Enjoy.
All things being equal there's no replacement for displacement...But use the sportbike analogy and the experienced will say smaller and lighter is better on a tight track. My old fischers with 15'' woofers would allow me to feel the air pressure in the radio studio move when the door opened and I've never experienced that from any other design, but I could also watch that paper driver flex and distort its shape when playing music. To make a 15'' woofer that's as rigid as a 6.5? is it even possible.
@steve59 --

All things being equal there’s no replacement for displacement...But use the sportbike analogy and the experienced will say smaller and lighter is better on a tight track.

The problem with analogies is finding the ones that apply properly and illuminates a matter. Given proportionately stronger motor force a larger diameter driver will see no lack of nimbleness compared to a smaller unit, within the constraints given re: upper frequency bandwidth dictated with the use of bigger cones. Moreover added surface area AND better efficiency to boot equals less inertia build-up in the moving system via less cone movement, not to mention the benefits of a surplus in headroom with all that entails in regards to lower distortion and minimizing thermal compression.

It’s also worth keeping in mind there are 5" woofers and 12" midranges; a smaller diameter low efficiency woofer in a 2-way system acting as well as a midrange isn’t necessarily the last word in performance prowess in its upper range (nor in the bass for that matter), whereas a suitable 12" or even a 15" high efficiency unit can do very well into the lower to central mids, not least when high-passed below 80-100Hz - something that would turn these drivers into "mitten raketen," as they say in German (i.e.: midrange rockets).

Even though bigger units are limited in regards to upper band frequency response they can be properly met within their safer operating range (typically between 500-1kHz) with a compression fitted to a horn/waveguide, something a dome tweeter would never be able to achieve unless mounted to a waveguide; the power response in the cross-over region between a dome tweeter and a woofer/midrange isn’t ideal, to say the least, something that can be attained quite successfully with the use of a waveguide/horn crossing over from a larger woofer/midrange - all in the name of a more (energy) coherent presentation.

My old fischers with 15’’ woofers would allow me to feel the air pressure in the radio studio move when the door opened and I’ve never experienced that from any other design, but I could also watch that paper driver flex and distort its shape when playing music. To make a 15’’ woofer that’s as rigid as a 6.5? is it even possible.

This should be relative to what I outlined above.