[Duke]

Over in the prosound world, I run into the implications of motor strength (BL) fairly often.  All else being equal, a bass cab with higher BL will have more impact.   And as far as I can tell, that trend carries over to home audio as well; there's no reason why it shouldn't.

The woofers in the Swarm have unusually high BL for a 10" unit, at 17.5.  BL is additive.  Four subs... four times 17.5 = 70.  Big deal, or not?  How does that compare with the uberdrivers in ubersubs that are out there?

The 18" Aurasound has a BL of 17.7.  The mighty 21" Maelstrom had a BL of 21.5.  The mighty Acoupower 18 had a BL of 23.3.  The best 18" TC Sounds has a BL of 26.8.  JL Audio doesn't publish BL but their best is probably in the same ballpark as TC Sounds.  The highest I could find was a 21" RCF prosound subwoofer at 34.5.  The Swarm still beats that by a factor of TWO!!

[Edit: BL^2/Re is probably a better measurement of motor strength than BL, and by that yardstick, the Swarm beats the RCS woofer by a factor of 2.5 but only beats the TC Sounds by a factor of 1.5.]

So basically the Swarm gives you the combined radiating area of a single 20" woofer (but spread out so that it interacts with the room much better), with enough excursion to handle a full-power sine wave down to 18 Hz, and twice as much BL as the next most powerful woofer I could find. 

[Edit:  If we use BL^2/Re the Swarm still has more motor strength than any home audio uberwoofer I know of; there is one specialty prosound woofer with higher motor strength but its resonant frequency is 38 Hz and its Qes is .20 so massive EQ would be required.]

I don't know whether anyone has done a comparison of subwoofers that looks at raw motor strength per dollar, but my guess is the $2800/set Swarm would measure rather well by that yardstick. 

[Russell Dawkins]

Wow. Those look like numbers you could take to the bank! Clearly presented, too.
It would seem that the only other salient considerations if comparing bass systems could be how the distortion figures compare, and perhaps how settling times - resonant characteristics - compare. To my ears a big variable in how various systems handle bass is the nature of the resonances created as a byproduct in the production of bass—and it seems there always are.

[Duke]

Wow. Those look like numbers you could take to the bank! Clearly presented, too.
It would seem that the only other salient considerations if comparing bass systems could be how the distortion figures compare, and perhaps how settling times - resonant characteristics - compare. To my ears a big variable in how various systems handle bass is the nature of the resonances created as a byproduct in the production of bass—and it seems there always are.

The Swarm uses an undersized vented box that's tuned "too low"... this because our target curve isn't "flat", but rather "flat after the effects of boundary reinforcement".   So we end up getting considerably less contribution from the Helmholtz resonance than if we were using a box optimized for deepest loudest bass. 

That being said, in sealed mode we have a roughly Qtc = .48 box, which is pretty good from the standpoint of settling time.  But expect to crank in some bass boost via the amp, as in most rooms the sealed box's native rolloff is steeper than what room gain can offset. 

[mojave]

BL is referred as force factor or motor power. The discussion about motor strength is most often about BL²/Re. You can read the definition on TC Sounds glossary.

BL²/Re is a composite terms used to define the raw efficiency of the motor relative to a given power level. BL is the actual cross product of the magnet field “B” with the conductor length “L”. It is not a scalar, but rather a vector. Combine this vector with the current I and you’ll get exactly the force (up or down according to the DC or AC input). But with the way amplifiers are designed, the current I of the speaker is never constant, only the voltage is. The BL squared divided by the resistance of the voice coil Re yields a generic force factor in newtons squared per watt. This number is relative for all speakers and the higher the number the more force a motor can invoke on a cone with the same input power. It’s important to distinguish that BL and Re are related much like inductance and Re are. For example, 32Tm over 8 ohms is identical to 16Tm over 2 ohms, namely 128 N²/W. All other things equal (moving mass and cone size in particular), A higher BL²/Re will increase the sensitivity of the driver which is a generally a very good thing.

Data-bass.com provides a sort-able list of BL²/Re of all drivers tested.

If going by BL alone, my GR-Research line source speakers have a total BL of 179.2 for two channel music. My infinite baffle subwoofer system adds another 99.92. Now were are at 279.12.

Changing BL changes Qes. No matter how many drivers you use in a single subwoofer, the Qes stays the same.

What is additive with drivers is displacement. This is because it takes an exact amount of displacement to output the same SPL at the same frequency regardless of which size subwoofer you are using. Displacement is two times the area of the driver (Sd) times the X-Max. A 10" driver with an X-Max of 13.25 mm will usually have a displacement of around .86 L. An 18" driver with an X-Max of 22 mm will have a displacement of around 5.36 L. It takes over six ofe these example 10" drivers to have the same displacement as a single 18" driver.

My system's total displacement for two channel playback is 30.74 L. What that means in real life is that at almost any volume level, you will never see the drivers move because they are moving so little to produce the sound. 
 

[Duke]

BL is referred as force factor or motor power. The discussion about motor strength is most often about BL²/Re. You can read the definition on TC Sounds glossary.

Using BL^2/Re, the combined motor strength of the Swarm is 4 x 88.5 = 354 newtons squared per watt, which is a unit that I have a hard time visualizing.  Anyway thanks for pointing this out, I think BL^2/Re is indeed a better yardstick. 

Data-bass.com provides a sort-able list of BL²/Re of all drivers tested.

Thanks for the link.  Looking at total BL^2/Re, the Swarm still has more motor strength than all but one uberwoofer on the list, a 21", 50-pound, neo-magnet specialty prosound woofer made by B&C that has an Re of only .7 ohms.   In particular, by this metric the Swarm outscores the best TC Sounds 18" woofer, 354 to 229 (the TC Sounds is the highest-BL^2/Re home audio subwoofer driver on the list).  I'm NOT picking on TC Sounds - I think they make the best subwoofer drivers on the market; rather, I'm ganging up on their big boy with my four little guys.  When I did a custom 16-Hz Swarm for someone a few years ago, I used TC Sounds woofers.

What is additive with drivers is displacement...

If we increase displacement by doubling cone area (and moving mass) without also doubling motor strength, we end up with a boomy mess.  So in that sense BL (or BL^2/Re) is additive alongside displacement, if we want to retain the same basic transfer function. 

If we add up all the BL in the Swarm and divide it by all the Re, we see that Qes hasn't changed.  But we displace four times as much air for the same amplifier input, relative to a single woofer.  Where does the additional power needed to do that come from?  From the permanent magnets in the motors.