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

It stands to reason that for a given cost it is easier to design and manufacture a smaller driver that will be more accurate than a larger driver, with respect to the inevitable tradeoffs between cone resonances, stiffness, transient response, overhang (the ability to stop quickly when the music stops), various forms of distortion, etc.

It is also true, of course, that a larger driver in a larger enclosure will provide more bass for less power, everything else being equal. But so what? I don't think that Dave's insulting response to Stan's well intentioned comment was called for, and I don't think that "no other reason for doing it, purely marketing" tells the whole story.

Regards,
-- Al

I'm not an expert in this area, but I believe that what creates the subjective impression of "fast bass" is simply the ability to accurately follow the input waveform, without harmonic or intermodulation distortion, and perhaps most importantly the ability to stop moving quickly when the musical note stops.

Increased driver size obviously will produce more volume at lower frequencies than a smaller driver, everything else being equal. Or, as Rwwear points out, the smaller driver will have the disadvantage of having to move further to produce a similar output. On the other hand, as I indicated in my previous post, producing a larger cone that will be as accurate as a smaller cone within their respective ranges of motion is more expensive. Everything else being equal, a larger cone will have to be stiffer to avoid flexing (what I believe is called "cone breakup"), cone resonances, and other contributors to non-linearity and distortion. A stiffer cone, everything else being equal, will be heavier, have greater inertia, and be less able to stop abruptly. Implementing a better combination of stiffness and lightness will tend to require better and more expensive materials.

As Mapman said, "it's the quality of the driver and soundness of the overall design that matters most." And producing a larger driver of similar quality to a smaller driver simply costs more, particularly if the design doesn't require the smaller driver to provide the volume or bass depth of the larger driver. I would not say that "a fast sub is not reproducing deep bass," just that it can't reproduce bass quite AS deep and/or loud as a larger driver or a driver of the same size but greater excursion.

Regards,
-- Al

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

Shadorne: ... 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 .... 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 issue is that the single large woofer may cost up to $2K!

Drew_eckhardt: Bass damping is a separate issue which also isn't related to driver size. It's determined entirely by the transfer function. How you get to a given Q (the ratio of stored to dissipated energy) isn't relevant to decay.

I don't doubt that what you are both saying is correct, but it seems counter-intuitive to me. It seems to me that to provide accurate "piston-like" motion, with minimal flexing of the cone, a larger driver would have to be thicker and heavier (as Kijanki stated), especially if we want to limit the cost increase associated with it.

It seems to me that a bigger, thicker, heavier cone would have reduced "compliance," if that is the right term, and therefore require a greater degree of damping than a smaller, lighter cone. Why would bass damping be independent of this?

Putting it another way, couldn't the smaller, lighter, more compliant driver get away with a higher-Q enclosure, which would partially offset its limitations in low frequency extension and volume?

Dogmatix -- Although your kind acknowledgement is more relevant to some of the other contributors to this thread than to mine, let me say that it's nice to see such words here.

Regards,
-- Al

Isn't Q the dude from Star Trek w/ God-like powers?

Maybe, but he was also the guy who created James Bond's gadgets. :)

Shadorne & Drew -- Thanks! I guess we've established, at the very least, that there's a lot more to it all than just WAF and marketing.

Regards,
-- Al