actually a very hard question but basically:
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subwoofer drivers largely disregard effects of cone breakup and inductance
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subwoofer drivers largely disregard effects of moving mass on efficiency
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subwoofer drivers are optimized for performance around the corner frequency of the enclosure
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typically subwoofer drivers are designed to be able to optimally energize a bass reflex vent
in essence it comes down to a few things
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having a very powerful motor that has a long linear throw
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having suspension capable of supporting that long throw
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having a strong cone capable of withstanding the high pressure in a box
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having enough motor force to be able to move the cone in the face of the pressure created by the bass reflex vent
now the problem here is that high motor force actually reduces bass output from the cone due to back EMF … so how do subwoofer drivers reconcile that ?
well …
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the extra mass of the long throw coil and the extra mass of the reinforced cone reduce midbass efficiency of subwoofers which makes their bass output appear higher by comparison to low midbass output
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subwoofers are usually powered with much bigger amps, which again masks their inefficiency ( this is why they need very high power handling )
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subwoofers typically rely on bass reflex vent for output … the vent pulls down their impedance allowing them to extract power from the amp and produce deep bass …
i know you probably didn’t understand any of this but the good news is i actually do understand all of this and more. if you are struggling just ask me - i will help you out.
what you basically need to know is that there is a legitimate reason to use subwoofers. they are optimized differently from woofers. if you don’t want to get heavily into physics a subwoofer driver will basically have …
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much higher power handling than a woofer
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significantly lower efficiency than a woofer of same size
the reason most people may not realize that subwoofers are so inefficient is because they aren’t comparing apples to apples. if you compare a 12" woofer to 18" subwoofer they may have the same efficiency. but efficiency is supposed to increase with driver size … yet it doesn’t increase from 12" woofer to 18" subwoofer … of course since there is no such thing as 18" midrange you can’t make that comparison and you end up not realizing that low efficiency is actually a property of subwoofer drivers, yes even in prosound.
the reason subwoofers make that compromise is because by operating in only a very narrow range of frequencies the need to have high efficiency is reduced. they are purpose built to focus their performance on the corner frequency of the box. efficiency is sacrificed because frequencies much above the corner frequency will be handled by another, more efficient woofer.
because subwoofers are typically driven by class D amps efficiency is less of a concern, instead power handling is. fortunately the long winding depth of subwoofer voice coils naturally increases the area for heat dissipation thus increasing power handling.
i would probably like to see 80-100 hz crossover from subwoofer to woofer and 250-400 hz crossover from woofer to cone midrange with something like 95 db efficient 21" subwoofer, 97 db efficient 15" woofer and 99 db efficient 8" midrange.
note in above example efficiency from subwoofer to woofer to midrange increases ever so slightly despite dramatic reduction in cone area … that’s because that sub is likely to have 350 gram moving mass while that midrange only perhaps 15 gram moving mass.
i will have separate entries on how midrange drivers and other types of drivers are engineered.