Why nominal diameter tells you nothing about tone
Long-held beliefs are the hardest to break!
When confronted with a statement that speaker diameter and tone / frequency response are independent the sceptic will ask, "Well why are PA subwoofers big speakers like 18" and why are hi-fi tweeters tiny 1" domes?" Good point, well presented!
The creation of loud fat bass
If you want to create high SPL low frequency output you have to move large quantities of air (this is termed Volume displacement or Vd). This requires a large cone area and high maximum cone excursion, which can be achieved in one of two ways - you can use a relatively small cone area but with greater cone excursion or vice versa. In the real world the best route is to have plenty of both. Let's assume that you can use 8", 10", 12", 15" or 18" woofers, that they all have 10mm Xmax and you need a total Vd of 4000cc. An 8" woofer is roughly 200 sq.cm, a 10" is 350 sq.cm, a 12" is 550 sq.cm, a 15" is 850 sq.cm, and an 18" is 1500 sq.cm. So you could use twenty 8"s, twelve 10"s, eight 12"s, five 15"s or three 18"s.
So the first thing we see is that unless each 18" costs more than six times as much as each 8" then it makes economic sense to go with the bigger driver. Furthermore, the larger the nominal diameter of the speaker then the easier it is to design a suspension system (that's the cone surround and the voice coil spiders) that can handle high excursion and remain linear, so in the real world within the proportional cost bracket the larger drivers are likely to have greater Xmax, which means that you need even more of the smaller drivers than their mere cone area suggests.
But what about transient response, I thought big drivers were slower to respond?
It is certainly understandable that most bassists think big driver equals slow respond - it's pretty intuitive that a large heavy cone should move more slowly, plus a certain groundbreaking cab with eight 10"s has led its manufacturer to keep pushing this inaccurate agenda for the last forty years. However, there's a formula that most will have heard of which states that F=ma (i.e Force equals mass times acceleration). So if we turn that around we can see that acceleration equals force divided by mass. Let's forget speakers for the moment and think about cars. What accelerates quicker from standstill, a two tonne Bugatti Veyron or a one tonne VW Golf? The Bugatti Veyron by far, despite weighing twice as much. Why is this? It's because it can supply more than enough extra force to counteract its greater mass. And it's the same with woofers - stick a powerful motor on a large heavy cone and it can exhibit better transient response than a lightweight cone with a less powerful motor. What's the downside? The cost (and until the advent of neodymium magnets, also the weight - a big motor requires a powerful magnet and powerful ceramic magnets are very heavy).
So why do smaller woofers exist then?
Three reasons - firstly sometimes you only need the output of a single small woofer, in which case the bigger woofer is overkill. Secondly as cones get larger it becomes more and more difficult to stop the cone flexing too much, which then means that the cone gets heavier which starts to affect the high frequency response. And thirdly, the bigger the size of the source, the less wide the dispersion of sound.
As we've said elsewhere, there has been a tendency in the past for bassists to write off 15" speakers as being muddy and boomy, simply because the vast majority of 15"s in bass cabs have been pretty cheap and poorly implemented. But once you start looking at high quality PA oriented 15"s you find that not only do you get more output for your weight than with typical bass guitar 10"s and 12"s, you also have the shocking discovery that the long held myth that 15"s don't go high enough is untrue. Yes, everyone has different tastes but the number of users we have who love the tone of their simple 1x15" Compact is ever increasing and they don't just play in quiet bands - some are even having to punch and cut through dense loud metal bands. The reason these nice 15"s manage to go high is that they have found the right balance of cone mass, cone stiffness, cone flexure patterns and motor strength. The downside is their cost. Once you start looking at these measured frequency response plots you see that the only advantage the 10"s and 12"s have over the 15"s is their broader dispersion.
But I'm sure I've read that dispersion is critical?
And you'd be absolute correct! A single 10" does have better dispersion than a single 12" which has better dispersion than a single 15". But what about two 10"s or two 12"s? If you stack them in a vertical line then they maintain the dispersion advantage but place them side by side and they act like a ~21" or ~25" wide speaker which means the 15" outperforms them by a significant margin. So the ever popular 2x2 arrangement of a 4x10" means it has worse dispersion than a 21" woofer.
It's due to this dispersion issue that midrange drivers and tweeters are smaller than woofers, and also because they don't need to move large quantities of air because they don't deal with lows then there is no benefit from making them big.
So if woofer size doesn't affect tone then why do you sell both 12" and 15" models?
There are quite a few reasons for this. Firstly although the Compact is incredibly light it isn't tiny, so we always wanted to offer a smaller cab but it was a question of waiting until the right driver was developed. If an equally high performance but fairly priced 10" driver comes onto the market then we see no reason not to offer 1x10" and 2x10" cabs as well. We'll never do a traditional 4x10" due to the dispersion problems explained above. Secondly we know that the Compact is not always loud enough but the Vintage can be too big, hence a 2x12" model makes sense due to the higher voltage sensitivity (4 ohm load plus greater cone area and cab size) and greater power handling yet relatively small size. And thirdly although the Compact has plenty of midrange and treble output for many bassists, our new 12" (as in the Midget and Super Twelve) has proportionally more midrange and treble (though it doesn't go any higher, it's just more aggressive sounding with your EQ flat) so bassists in particularly noisy bands or with relatively mellow playing styles/instruments/amplifiers (or any balance of band noise vs tone going into the cab) can benefit from the increased cut and punch. With some careful EQ tweaking it would be pretty hard to tell those 12" and 15" models apart but unEQ'd there is a clear difference. We also use another 12" which goes lower than both our 15"s or the other 12".
So if woofer size doesn't affect tone then why do your 12" and 15" models sound different?
They sound different because there are more differences than merely the nominal cone diameter: In fact the 12" in the Big Baby and Big Twin sounds much more like the 15" in the Dubster than it sounds like the 12" in the Midget and Super Twelve. And the 15" in the Compact and Super Fifteen sounds much more like the 12" in the Midget and Super Twelve than it sounds like the 15" in the Dubster!