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

This is the best lecture series I have ever had, and I have attended a few to get a Ph.D that made me prematurely bald in the process.
I am no student of wave physics. I am only interested in fiddling with mammalian genes. But I wanted to learn more about the components I listen to and what affects their output. I got a treat.

Thanks guys. And I mean that.

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

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?

Good point. It is actually quite complicated because designers can also make choices about voice coil diameter, magnet size, VC length and mag gap length etc - to compensate for added cone weight.

However, simply put - the big advantage that a large woofer has over a small one is simply that it moves much less for a given output. Smaller long throw cones with long coil in short mag gap (large Xmax) tend to be less well controlled than larger short throw cones (short coil large mag gap). Another issue is heat - small long voice coils have more trouble getting rid of heat and the amp loses control (as impedance rises). Wiring many small woofers together is also going to dilute the "control" that the amplifier exerts over each woofer.

Shadorne - I was thinking of three 10" woofers versus one 18" woofer (same area).

>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.

By the time you've built a speaker with a given pair of high-pass poles it doesn't matter whether the driver in question is a 26 gram 8" mid-bass, 166 gram 12" sub-woofer, something smaller, or something bigger.

Most of the damping is electrical. For instance, the Peerless 830452 bass drivers in my Orions have an electrical Q of .22, mechanical Qms of 3.90, and total Qts of .21 (those drop a bit in the H-frames due to the mass of the air).

When you make the motor stronger to maintain efficiency in spite of the heavy cone and stiff air spring, it's stronger when braking too.

>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?

No. Qs higher than .707 produce a pass-band peak which is audible.

While a reasonable psycho-acoustic trick to give the impression of missing octaves for tiny speakers you don't want it in sub-woofers where tightly spaced equal loudness curves make a small peak especially noticeable. High-Q resonances are what give box store sub-woofers that anoying one-note boom-boom quality.