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

>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/

Kijanki: 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).

Drew_eckhardt: 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.

Drew -- doesn't your response overlook bass damping, inertia of the cone and the other parts of the moving assembly, and the ability of the cone to stop quickly when the input signal stops? All of which I think support what Kijanki was saying.

Regards,
-- Al

Interesting debate. IMHO, there is no question that a larger well built subwoofer will sound better than than ten small cheap ones sharing a box. The issue is that the single large woofer may cost up to $2K! Most high quality 15" subs are expensive and most will use 3 or 4 inch voice coils - not your regular 1 inch VC found in most WAF friendly speakers. This all makes the amplifiers tough job as easy as possible which is what it is all about ....effortless clean dynamic sound with little or no compression is generally why a single large driver matched to an amplifier works well.

I do agree that 10 inch woofers with a whizzer cone tend to work well for a bass player (as in Zu) because there is a slap sound to the bass which is up around 3 to 5 K HZ. But this is not a valid reason to say that a 10 inch for a subwoofer is ideal. Certainly a 10" sub is likely far better than a 6 inch but likewise a 15" will be a lot better than a 10" (assuming good quality in both drivers).

IMHO, ideal is a super big sealed box with low Q (0.5 or so) and a large woofer. The very large woofer and super big box allow for the very poor efficiency of a low Q design.

the ability of the cone to stop quickly when the input signal stops?

That would be system Q or damping. An overdamped design would be extremely inefficient (low SPL output at 20 Hz) with a small woofer in a small box.

Compare a WAF friendly product with an ugly DIY brute. (Source HT Shack subwoofer tests)

Both probably sound equally as good but the brute is going to play a whole lot louder cleanly and effortlessly.