24" drivers

MPASOUND: Maybe Eminence. But as I pointed out, that parameter you look for isn´t absolute measure of the performance. For example Xmax for underhung coil systems is usually fatal. For overhung systems with great pole piece thickness, it might be less of an issue, and it will yield different performance.

Edited by 70,s hero - 23 February 2017 at 6:29pm

If you want a bass driver for PA applications with a good balance of Xmax/efficiency, when you consider all the magnet/top plate/voice coil combinations you tend to end up with a voice coil height around 25%-30% of the vc diameter and a top plate thickness about 40%-50% this, which gives an Xmax around 10%-15% of the vc diameter -- if you look at the T-S parameters for a lot of drivers you'll see this. Of course you can have longer-coil drivers (more Xmax) at the cost or lower efficiency.

Similarly the optimum voice-coil diameter is around 25% of the nominal driver diameter to get the right balance between coil/cone mass and stiffness, which means you end up with Xmax typically around 3% of driver diameter.

So if you increase the driver diameter everything else goes up proportionally, assuming you've done the design properly and not just but the same motor/coil onto a bigger cone. Here are some typical example numbers which ought to look familiar to those in the know -- apologies for mixing the units...

Driver : 12"/15"/18"/21"/24"
Voice coil diameter: 3"/4"/4.5"/5"/6"
Xmax : 8mm/10mm/12mm/14mm/16mm

So if you match total cone area, fewer big drivers will move more air than more smaller drivers. But there's a much smaller choice above 18" so costs are a lot higher, and also power handling doesn't go up as fast as driver diameter (or power needed to reach Xmax) so you're more likely to cook them, and very expensively.

All this is why for many years 18" has been the best choice for bass drivers in most cases, if you need more output then use more of them. There are now some good 21" drivers around but the the extra air they'll move doesn't always justify the big price hike if properly designed, and the power handling isn't as high relative to the cone area -- even more so for 24".

M-force takes all this to the logical extreme, with a massive cone (30"-40") with enormous driving force (can't be done with a voice coil) and ridiculous power handing, to get stupidly high output from a given box size.But with an even stupider price...

does anyone know a rough cost of said 40" M-Force?

IanD. I would agree, but the catch is, that the larger driver is really cheaper solution with more displacement volume in B&C line. Also frying the driver by so much power is not absolute truth. This fact is weakened by efficiency factor. Those real, smaller diameter drivers have stiff suspension and high FS, so these are not very efficient down there. I had live comparison with 3x12PS100 box, and 18", and those three buggers didn't win. We desperately need 10" and 12" drivers with Fs of 30Hz and Xmax of 15mm...

I wasn't saying smaller drivers (12" or 15") were better, I was saying that in most cases 18" was the best choice for subs, or sometimes 21" nowadays (there's a lot more choice than there used to be and prices have come down e.g. B&C 21DS115), but not often bigger (24" and up) unless you can afford (and get your hands on) M-force.

10" and 12" drivers with big Xmax tend to be low efficiency even when paralleled up, to get big Xmax with decent efficiency you need a long big diameter coil with a tall top plate and this has too much mass for a small diameter driver -- and costs almost as much as a bigger driver with the same motor, because that's where most of the cost is.