[Beardy]

Newbie question:

I have read the arguments and reviews for servo subs and from a conceptual standpoint they make sense; the servo control enables a large driver to follow the input waveform accurately. 
I have also seen designs using arrays of smaller drivers that provide good bass response.  The large number of small drivers provide sufficient cone area and they offer better control by using `more motor' for a given mass.
So from a practical standpoint how do they compare:
A folded dipole (a la Phoenix) could be made quite small using either a couple of 8 inch servo drivers, or via more smaller non-servoed drivers..?

cheers

[Angaria]

It depends what frequency you're talking about... distortion increases with excursion, and excursion quadruples every time you drop an octave.  So the choice of optimal configuration will be a complex function of frequency and SPL targets.  What you're not going to do is snag a bunch of $3 tv speakers from PE and compete with servo subs.

[Beardy]

thanks.

I was thinking more in terms of how a 12" servo driver/amp compares with something like 4 good quality 6" woofers and a more generic amp.  I am guessing that given the same cone area the excursion will be similar and the servo will be better.  But what about 6x 6" drivers with less extension? 

but given
"...distortion increases with excursion, and excursion quadruples every time you drop an octave."
it sounds like if you want to go as low as possible, the servo is the way to go...

Just trying to get some qualitative idea of the benefits of the servo technology.

[Bob in St. Louis]

Good question, good thread, and good conversation.

Bob...watching with interest.....

[Davey]

I don't think it's a given that smaller drivers provide "better control" because of a better motor-to-mass ratio.    Maybe, maybe not.

It's all about displacement when it comes to SPL capability....all other things being equal.  Take (Sd * Xmax) and multiply by the number of drivers used, and compare that to another prospective combination.
SPL capability difference (in db) is 20 * log(combo-A/combo-B).

Smaller drivers will do doubt have a higher roll-off point, but that can be lowered with EQ.

The servo advocates are struggling to provide objective data to support their subjective claims.  But subjective evaluation always trumps measured data so it's a moot point.

Cheers,

Dave.

[JohnR]

Generally, a larger number of smaller drivers will cost more than fewer large drivers, other things being equal. Other things usually not being equal, you need to look at the specifics of the drivers you are considering. But the parameters tend to not get better (for OB sub use) as drivers get smaller. I have some nice 10's in use, but with 6s I think I'd be thinking along the lines of a floor-to-ceiling array of this. Even then, some sims will be needed...

With smaller drivers, there's also a big temptation to make the (effective) baffle smaller, which works against them as well...