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

>Shadorne - I wasn't thinking of slap and 5kHz when I mentioned 10" woofer's bass definition - for that bass enclosures have tweeters. I was thinking of low frequencies. 10" woofer arrays have better controlled/damped (shorter) bass while 18" woofers tend to produce "woolly" bass. The question is what is cheaper - 18" woofer or 3 x 10" woofers (to obtain the same surface area).

You can't extrapolate from instrument speakers driven with a wide bandwidth signal (even with a tweeter you may be running the main driver out to 2Khz) that are engineered to break up in a pleasing way to hi-fi drivers run in a narrow bandwidth (not even out to 200Hz) containing entirely wave lengths that are much larger than the speaker that are supposed to be true to whatever signal you're feeding them.

Just because a driver is large doesn't mean it will sound less good. Remember it is trying to reproduce such devices as ten foot bass drums and long stringed instruments. In order for ten inch drivers to play deeply enough they will require much more equalization and power than an eighteen inch model. doubling or tripling drivers does lower their f3 point. It just decreases their distortion and increases power handling and output.

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.