[tubamark]

Hello all.

I know that the figure Xmax is related to the linear control limits of cone travel.
While not all do, many cones have significant useful travel beyond that figure, spec'd as either "Xlim" or "Xmech".   In cases where the figure isn't given, it can be easily estimated by using the hold-bare-driver-in-your-hands test of the cone travel limits with sine wave < Fs.  Generally, the excursion point just before mechanical noise or (worse yet) physical damage to the polepiece, surround, or cone occurs is the Xlim.

My question is this:
While Linkwitz's SPLmax calculator has us sensibly input Xmax, can we "count" some of that additional cone travel in figuring actual SPL limits?
For example, I have an MCM woofer with an Xmax of 2mm, but actual quiet excursion out to perhaps 12mm (one way) given a 20 Hz sine wave (essentially inaudible when unbaffled).  It has a bumped backplate and very porus cloth dustcap that makes this possible.

Should I chalk-up all that nice travel as just "headroom", or allow it into my SPL calculations?
Forgive me if this is covered somewhere on SL's site - It's a whole universe unto itself there!

[JohninCR]

Xmax is the limit where the coil is fully in the magnetic gap.  Visaton actually claims linear operation past what is technically the Xmax.  Pro type drivers are designed to be driven past Xmax without problem.  Once you get out past xmax distortion goes up significantly, but whether or not it's audible in the lowest frequencies is questionable.  WRT to your MCM's, the 2mm figure sounds like a typo, so verify that first.  WRT to Linkwitz's spreadsheet, don't forget you get an extra 6db from floor boundary loading plus 4.8db related to perception with OB bass due to the higher direct radiation heard compared to reflections.  Reflections make up a much bigger portion of what you hear from boxed bass, kind of like a higher noise floor.

Stick with Xmax in planning your design.  Any extra is just a bonus.  A full doubling of Xmax only gets you 6db anyway, which is the same as adding another driver driven to the same excursion, or doubling the rear wave path differential.

[tubamark]

WRT to your MCM's, the 2mm figure sounds like a typo, so verify that first. 

Thanks for the tip about the 4.8 directivity gain, John . . . I forgot about that part.  I'm probably also getting a few db via sidewall boundary gain, as my room is long & not real wide -  bass units <2 feet from sidewalls.

WRT Xmax, MCM has been unresponsive to my query for current specs.  They did acknowledge when I purchased the drivers (model MCM 55-1165) that the current catalog specs are wrong.
(For what it's worth, this is a great driver for an OB newbie to experiment with - VERY affordable.   My samples tested:  Qts 0.85, Fs 36 Hz.)

How about this idea:  When I test my alignments' polarity, I use a 9V battery (only for mid-high power woofers. Little drivers get the AA battery).
Of course the cone pops out (or in) a fixed distance until the DC is removed.
Is this displacement a good approx indicator of Xmax?

I'm a thinking man's man . . . Just not a real smart one.

[Bob in St. Louis]

When I test my alignments' polarity, I use a 9V battery

Mark, I may be wrong (it's happened once or twice) but I think that may be a bad idea. 

Bob