Sub output: Is it the woofer size or the rated RMS

The design is of the utmost importance for a sub it's true. It's just easier as the driver size increases. That being said a system with multiple small drivers would probably be the best but would require a lot a of equalization and power to have real low bass.

"Why do we use eight 10” speakers? Because we learned early on that 10” speakers work much more efficiently than fifteens or eighteens—and if you put eight 10” speakers together, you can move a much larger column of air."

Mmarvin19 - excellent point. Bass players know that 10" speakers provide the best compromise between power and bass definition and use arrays of many 10" speakers.

Speakers in array made in the same enclosure work better. (Acoustic impedance lowers, resonance frequency drops down) Making simple reasoning 18" speaker moves 3.24 times more air than 10" speaker. To avoid membrane bending (cone breakup), that Almarg mentioned, membrane has to be 3.24 time thicker and in order to keeps everything proportional coil and magnet have to be 3.24 times larger to move 3.24x heavier membrane with 3.24x more air pressure. Am I right? Speaker people - are you there?

One correction - membrane of 18" speaker should be 10.5 times heavier because it should be 3.24 times thicker and the area is 3.24 times larger. That is probably why definition is getting poor (too heavy).

>One correction - membrane of 18" speaker should be 10.5 times heavier because it should be 3.24 times thicker and the area is 3.24 times larger. That is probably why definition is getting poor (too heavy).

It's entirely about extended frequency response, which only matters when you're using the driver at high frequencies as in a musical instrument amp. It's not an issue for sub-bass drivers in multi-way audio playback systems.

In spite of the name, bass guitars generate harmonics out to 5 kHz which is well into tweeter territory. Punch can be an 800 Hz phenomenon.

No matter what you do with the motor, larger diameter speakers without a phase plug have reduced power response (total power output in a sphere at a given frequency) at high frequencies the problem being that their radiating diameter is large compared to the wave lengths being reproduced so the sound from two points can be out of phase and cancel or at least add incoherently for less total output. For instance, a hypothetical 18" diameter cone (maybe a 21" driver) would have the 90 degree off-axis output 180 degrees out of phase at 376 Hz (sound travels at 1130 feet/second in air; 1130/2/1.5 = 376)

The force generated by the motor is a product of the magnetic field strength
(B), length of wire in the magnetic gap (L), and current flowing through the wire. Current is voltage divided by impedance. Voltage is fixed - it's just the instantaneous musical signal.

There are limits to how strong you can make the magnetic field especially given money, space, and or weight budgets - 40 pounds of motor isn't cheap or small so to overcome more moving mass (Mms) means a longer wire (L). More wire means more resistance which reduces current at all frequencies. You can increase the voice coil wire diameter for less resistance to compensate at the expense of weight and a wider magnetic gap which in turn means less field strength. Wire coils form inductors, and more turns mean more inductance. Inductor impedance is proportional to frequency (2 pi f * L) so current is less at high frequencies with the net effect being a heavy speaker cone + strong motor has less high frequency output.

On-axis this is compensated some by the driver coupling more efficiently to the air as its diameter becomes large relative to the wave length produced.

Cross the sub-woofer over at 40-120Hz and it's not an issue. With a good motor design you can even mate a 15" mid-bass to a wave guide at 1Khz.

161 ten inch woofers have met their match!

http://www.ohgizmo.com/2006/03/06/the-biggest-subwoofer-ever-made/