Are big drivers evil?
First, a story. Once I walked into a local store looking for a demo. I heard a speaker with some 6.5" drivers and a number of subs. Not one of the subs sounded nearly as good as the bass from the smaller drivers. In fact, all of their subs were just plain awful. It's tempting at this point to conclude that big drivers are the problem. A small driver is tight, fast and accurate while a big heavy driver is muddly and sloppy. Kind of like comparing a truck to a luxury car. This particular myth is seductive because it's one that intuition and experience would often suggest.
Many audiophiles quickly form a belief about things like this. Human nature then steps in as once an opinion is formed, the brain tends to order any extra information according to the belief. Truth is often suppressed in order to propagate an established belief. This is often applied to religion, but in reality this works across the board. It's a way for our minds to establish stability and avoid changing opinions about everything all the time. We don't worry about floating off into space because we instinctively trust the law of gravity.
Why does it appear that smaller drivers are more accurate?
There are a few reasons for this. In particular, when the bigger driver is a subwoofer, and the smaller driver is in a fullrange speaker, we have an apples to oranges comparison. Firstly, with a subwoofer, there is more that can go wrong.
A fullrange speaker is plug and play and already has the levels balanced - it's passive. A sub on the other hand is just too easy to set the level wrong. If done by ear a person will probably set the level much higher than the mains. That tends to amplify any bass problems in the room. You will now become much more aware of room modes, since the bass is turned up too much, some of them will boom. A fullrange speaker draws less attention to this as you are hearing a flat response. The sub could easily end up 10 - 15 db louder, and then if you have a 15 db room mode peak which is very common, it really booms. Just by turning up my subs too loud, I can get them to sound like boom boxes. Yet done right I have not heard better bass at any price.
Another issue is cost. Subs use big expensive drivers. If you compare a good woofer to a cheapie sub, what do you expect? Good woofers cost ~$80 - 300 but decent sub drivers cost generally $300 - 1000. Comparing at the same price is like comparing a sedan to a bus at the same price point for ride comfort.
Also there is a difference between woofers and subwoofers. Woofers have lower excursion and have an easier task. They are designed generally for wider bandwidth and usually need less power handling. They don't do the same job and they have different compromises. Sub drivers need generally an extra octave extension, but less top end. Therefore they need to move 4x as much air as well as more power handling. This will mean a bigger VC diameter which in turn has higher inductance, but we can live with that in a sub. Good design also helps keep inductance reasonable. We can move that extra air by making the driver bigger, or increasing excursion.
Increasing SD (driver piston area), will increase efficiency. That means we need less power, less excursion and can live with a smaller voice coil. Those factors are the ones that we struggle with here, and which cause distortion to hike up. The mass goes up in proportion to the size, and the motor strength may also go up, although this will depend on if the driver is designed to have a bigger box or not. The alternative, if we buy into the driver size myth, is to increase excursion. That is a change that will cause problems. Let's suppose we believe a 6.5" driver is "faster" than an 18" driver. We want smaller drivers to do the same job as an 18" driver with 18mm p-p excursion.
The 6.5" driver has a piston area of 145 sq cm, but the 18" has 1220. That means one 6.5" driver needs 150mm excursion! It is compensating for 1/8 piston area. We could compensate by using 8x as many drivers but even there 9mm is quite high excursion for a 6.5" driver. It can be done, but we probably want to use more like 12 drivers with about 6mm xmax instead. Now that we can move as much air, we have a cost issue. A decent midbass costs around $100 - 300 for something high end. The 18" driver for similar quality would cost around $400 - 1000. So if we pick a good 18" driver even at the top end, we have to use lesser quality midbass drivers as there are so many of them. If we choose high end midbass drivers, the cost is nearly 4x as much! So if we consider this on the basis of price, we are at a big disadvantage with the smaller drivers.
What happens in reality is that the small midbass drivers are placed in a slim box that will give accurate bass at a moderate level. They are affordable and practical, but are limited in dynamics. This choice is based on domestic acceptance rather than performance, but audiophile myths and subs not properly set up have confused the issue.
If you want the ultimate in bass performance, you can't go past bigger drivers. Small drivers for bass is always a compromise. Achieving accurate bass is far more complex than most are ready to get their minds around, but in general it is true to say that size matters. Depending on how loud and how deep you want to go, bass drivers should idealy be 10 - 24" in size. You can get good quality bass from a 6.5" driver with enough extension and output for moderate music enjoyment, but you should realise that this is always a compromise and certainly not as good as it gets. All else being equal (that happens only in theory), bigger is better up to a point.
One 6.5" driver