Vague Xmax??

Systems

[soapbox]
Not that it's my responsibility, but I first wanted to apologize to this forum for the immaturity of certain visitors from www.caraudioforum.com in the "delihma" thread.
There are some good discussions there, and not only good expertise, but even industry representatives. It's my home base.
www.caraudiotalk.com is another good one, I'm a mod there, I'd encourage people to sign up over there, I assure you that while there is less traffic (although I'm sure more than here), the maturity level is much higher, and we enjoy discussing reasons why something may or may not be, rather than bashing someone for stating a myth they may believe... myths by nature are often intuitive, it's understandible.
Anyway... I wanted to apologize.
[/soapbox]

But I'm looking to leverage your expertise as regards these subwoofers, or rather Polk's approach to reporting it's Thielle/Small specs:
On the spec sheets, for the Xmax listed, is that one-way, or peak-to-peak Xmax?

And do they measure that using a measurement standard like DUMAX (being the shorter of measured Xsus vs. Xmag), or do they use the simple motor-based "overhang" calculation (that can actually come up shorter than measured Xmax, due to real fringe field magnetics)?

If you can answer that, it would be much appreciated.
I see the Xmax for the MOMO listed somewhere around 25mm...
Which would be respectable in today's world of 20mm-30mm subwoofers...
But outside that class, if that's really only 12mm-13mm.
The spec sheets (none of them, that I tried) state one way or the other....

Usually, you would assume an Xmax spec with no qualifier to be "one-way", but assumption is to make an ASS of U and ME... I've seen peak-to-peak Xmax listed without qualifier (and usually on the type of subs that compete with Momo's, unfortunately), which doesn't lend itself to comparison across the marketplace, if you don't know. :rolleyes: urg.

Anyone know the answer?
Much appreciated, thanks!

Jstas

One benefit you have here is if you want specifications and other details about Polk Audio equipment, you merely have to ask a question. Polk Audio makes sure the customer service reps answer questions and offer assistance. We have spoken directly with not only customer service reps like Kim but the in-house installers post here and we have even seen posts from the likes of marketing guys, product line managers, vice presidents and even Matthew Polk himself.

With that being said, Polk Audio genuinely cares about it's customers which is one of the reasons it has an almost fanatical following. So I think that instead of trying to find specs and answering your questions myself, I will leave it in the hands of the very knowledgable Kim Jasper. Besides, he has complete access to all that info right there at his finger tips!

MacLeod

Bravo!

Couldnt have said it better!

You can tell by the fact that this is the most profesional, informative, useful and downright cool website in the car audio industry; and one of the best period!

Systems

Originally posted by Kim
Geolemon,
When we list the our X-max specification for our subwoofers, this is the maximum mechanical excursion one way, not peak to peak that the suspension will allow.


Thanks,
Kim

can you please tell me the coild and gap heights? i dont doubt your subs. what does polk use to test the subs parameters?

Systems

Originally posted by Jstas
One benefit you have here is if you want specifications and other details about Polk Audio equipment, you merely have to ask a question. Polk Audio makes sure the customer service reps answer questions and offer assistance. We have spoken directly with not only customer service reps like Kim but the in-house installers post here and we have even seen posts from the likes of marketing guys, product line managers, vice presidents and even Matthew Polk himself.

With that being said, Polk Audio genuinely cares about it's customers which is one of the reasons it has an almost fanatical following. So I think that instead of trying to find specs and answering your questions myself, I will leave it in the hands of the very knowledgable Kim Jasper. Besides, he has complete access to all that info right there at his finger tips!

ED is totally about customer service too. thats the biggest thing about ED. all of the people that visit soundillusions.net and caf as well as other forums know who the ED owner is. my friend is a rep for ED i have gotton a chance to try alot of there subs and i can tell youthat ED has great service and grea products. the ED subs are just awesome for the price you pay for them. if you heard them correctly installed i bet you would be willing to pay the MSRP all day long. BTW when ben isn't busy hes a cool person to chat with.

PoweredByDodge

Geolemon -- u may want to just give polk a call and ask for Kim... I imagine your inquiry would be better handled on the phone -- don't think I'm saying not to ask here, but I kinda "feel" for you in that I'm sitting here going "he's trying to ask 'such and such', but then once it's answered he's got another question regarding it..." and I totally get where you're coming from... been there many a times...

good luck...

geolemon

LOL...
PoweredByDodge has ME nailed!

Actually, this is just a casual question...
One of the guys who visits CAF chat was inquiring about them, I just wanted the numbers to compare...

Bear in mind, a one-way Xmax number compared to a peak-to-peak Xmax number...
You'd have to divide a peak-to-peak Xmax in half to get a reasonably comparable one-way Xmax number...

And also bear in mind that if all other specs on the sub were equal, and one sub had half the excursion of the other, the longer-throw sub would literally have 6dB (a 2x increase) potential in additional output (SPL)!

It's something important to know, really.

geolemon

Originally posted by Kim
Geolemon,
When we list the our X-max specification for our subwoofers, this is the maximum mechanical excursion one way, not peak to peak that the suspension will allow.


Thanks,
Kim

Kim - thanks for the speedy response!

My only question is with the way you stated "maximum mechanical excursion"..

Worded that way, it sounds like you are talking about Xmech (the point where the speaker comes to a hard stop... cone touching down on the spider landing, or voice coil former touching down on the backplate are the two usual limits... but tinsels could also be, etc.
Is that correct??

This would be in contrast to the "old school" usual calculation of VC overhang (aka: how far the voice coil can travel before the coil count in the gap starts to go down)..
Or the "new school" calculation of Xmax being the smaller of Xsus or Xmag, where Xmag is the point that BL drops to 71% of it's "at rest" value, and Xsus is the point where Cms reaches 400% of it's "at rest" value (DUMAX standards).

Xmech can be dramatically longer than Xmax...
Such as the seemingly bottomless W7...
Xmax is a motor limited (Xmag) 27+mm...
Xsus is a shocking 45+mm...
And Xmech is WAY out there past that even, limited by the half roll's maximum stretch I believe.

So just to be clear, is this Xmech, or Xmax?

Thanks again!:cool:

sntnsupermen131

lol...thats the first time in a long time that anyones accused Kim of having a speedy response, he used to visit this forum almost daily a couple years ago, but we got a few more regulars and his appearances have become less frequent to say the least, if i were you, i would make a new topic, make the title "Kim, Josh, or Thom", thats the installers that come to the forum, and then post ur question their, otherwise, chances are it might not get answered
-Cody

PoweredByDodge

this may sound a bit dumb, but, i have made the improper assumption that when a speaker company (any in general across the board) listed its peak to peak excursion or max one way excursion, that said excursion was the MOST (hence "max") that the sub could SAFELY do -- PERIOD.

this would take into account any limiting factors.

for example, if the surround would travel farther than the spider would allow, then you'd go by the shorter spider's limit. etc etc...


the only exception i ever made to that rule was aknowledging that they probably did not figure in tinsel lead length, and while the tinsel should be designed to be long enough to safely stay attached though Xmax, that sometimes it might not be, and the sub would have to be slightly altered (just lengthen tinsel leads) by the end user.


I would find it mighty disturbing to realize that some companies may actually be listing the peak excursion of the sub in regards to one specific part, when in reality it is much less due to another part of the sub being less forgiving.

on a side note...
i believe a good rule of thumb is "unless specifically listed as 'one way', maximum excursion should be assumed peak-to-peak".

And Geolemon...
why would a company list the point where a speaker comes to a "hard stop" (as you put it). wouldn't that be pointless? however now that you mention it, i realize that things like this are often floating through the audio world -- the consumer world in general in fact, and specs are never quite what they seem.

Don't you wish that we could just redefine the standard of Xmax ?? take every limiting factor... everything that you think is "bad" about a sub having traveled too far... put tolerance limits on those characteristics, and then combine that with physical hard barriers (spider, length of surround) and come up with an Xmax that has been tested with an ultra high speed timed video device (if that's even possible).

Systems

Originally posted by Kim
Geolemon,
When we list the our X-max specification for our subwoofers, this is the maximum mechanical excursion one way, not peak to peak that the suspension will allow.


Thanks,
Kim

Understood...does your suspension allow a longer travel than the motor? I've seen subs that had a motor stroke of say...24mm one way....though the suspension would have allowed for 40 or so. Now obviously the motor will not be able to travel that far before hitting the wall so to speak. Is there anyway to get actual driver excursion rather than suspension allowance?

geolemon

Well, PoweredByDodge... it would be nice to have a set standard... and really it is very much as you mention today, via the most modern standards. It's "the weakest link", whatever is the most limiting factor.
But the problem is, there's still a lot of "old standard" based stuff out there, and there are some manufacturers who really do rate based on how far the sub can move period, before that "hard stop" :rolleyes:

The old way:
Overhang.
If you had a 40mm long voice coil, and a 20mm thick top plate, this would mean 20mm of the VC was suspended "in the gap", with 10mm worth of VC "overhang"... or voice coil sticking out from either side. So the old way of calculating Xmax would have said simply "10mm of Xmax". Just a function of overhang.
The theory being, if the voice coil moved more than 10mm in either direction, the coil count in the gap would start to decrease, the farther and farther it continued to go.
The downside:
This doesn't account for any fringe field magnetics. Maybe more to the point, it isn't a direct measurement of when the voice coil's motor strength starts to roll off, rather it's an arbitrary guess as to when it's supposed to start to roll off.

The new way:
DUMAX (or Klippel).
The subwoofer is measured not using static small-signal analysis like Thielle/Small (which simply measures the sub only "at rest"), but rather the sub is manipulated across it's excursion range and tested with respect to excursion.
So, it generates a BL plot, and a Kms plot (Cms is the inverse of Kms... stiffness vs. compliance).
The BL plot is motor strength with respect to excursion, and the Kms plot is suspension compliance with respect to excursion.
Using this technique, Xmag is measured as the point where motor strength has fallen to 71% of it's "at rest" (0mm of excursion) value.
Similarly, Xsus is measured, as the point where suspension stiffness has increased to 400% of it's "at rest" compliance value.
And then, simply, Xmax is the smaller of the two numbers. :cool:

I'm fortunate enough to have had the honor and priveledge of visiting DLC labs in person, for a small get-together with Steven Ponte (who has since moved on to TC Sounds, looking forward to seeing him at CES this January! :cool: ), who was the primary DUMAX test man at the time.

...anyway, Xmech is simply that point where the sub physically can't move any more, regardless of calculated "old-school" Xmax, measured Xsus, or measured Xmag.

Depending on the particular sub, things can be interesting.

Most subs are motor limited...
That is, the Xmag is reached first, with still some Xsus travel room left, and then Xmech would be beyond that even still. The W7 is the most way-out example of this that there is, because the numbers aren't even close. Xmag is about 27mm, Xsus is about 45mm, and Xmech... who knows, the thing is virtually bottomless. Haven't seen it.

However, with some subs (such as Audiomobile's Evo-R, or Resonant Engineering's ****), the basket doesn't allow for enough rearward travel to fully take advantage of the motor... it's suspension limited. The **** reaches Xsus first, and Xmech is only about 2mm beyond that even. The motor itself would allow for about 7mm or 8mm of travel beyond Xsus, and obviously 5mm or 6mm beyond Xmech even then, if the basket allowed for it.

I'd love to see an **** motor on a W7 chassis, myself. :cool:

But at any rate...
Good news is, things are absolutely moving towards exactly what you are wishing for...
Standards based on real performance, and limit-based-specs (like Xmax) that are based on the weakest link of all the factors in the measurements.

PoweredByDodge

Originally posted by geolemon

But at any rate...
Good news is, things are absolutely moving towards exactly what you are wishing for...
Standards based on real performance, and limit-based-specs (like Xmax) that are based on the weakest link of all the factors in the measurements.

Well THANK YOU for what has been perhaps the most educational lesson in subwoofer mechanics I have ever had put in front of me...

I'm printing this out (seriously) for future reference...

And I'm happy to hear you're seeing them move toward the "just gimme the facts maam" standard of Xmax. Simplicity has its virtues --- somtimes... lol.

However, being as brand biased against some as I am... I would love to see/hear an RE **** any time, any day, but not with anything that came off a W7... it's like "contaminated"

geolemon

Oh, if I could make it myself I would...

I've got access to a blown 13W7...
And I do have an **** in my basement...

Hmm...:D

No, seriously, that would be exceptionally difficult, particularly since having the voice coil nicely centered is really critical with these XBL^2 subs.

But I still want one. To me, that represents possibly the "ultimate" subwoofer.

The W7, for it's king-of-the-hill chassis and suspension package...
The **** for it's long-throw XBL2 motor... although Adire's going to be revealing some much longer, more powerful XBL2 designs at CES this January... I've gotten a preview, and it's really mind blowing. Can't wait to see their working prototypes in person. :cool:

ScottO

Hi,

I'm Scott. I engineered the latest MOMO subwoofer series.

As you've probably already noticed, Xmax is a rather controversial parameter. There are many ways a measuring and stating Xmax and several standards. Determining what method and what standard is used for each manufacturer can be difficult. Even if you can determine which method and standard is being expressed, unless they are the same method, comparing them is not going to bear much fruit.

There are standards being formulated by professional orginizations now, namely ALMA and AES, to try to level the playing field, but progress is slow.

Polk is continually improving our measurement and diagnostic capabilities. We currently use the Klippel Distortion Analyzer to measure the small and large signal parameters of our loudspeakers. We also use a laser and tone burst signals to measure the Xmech of a speaker, because the Klippel Analyzer's LSI (Large Signal Identification) package is not capable of measuring the extremes of a loudspeaker's travel.

Using the MM2124 as an example, the spec on the website reads Xmax(linear) = 19mm. In this case, that 19mm is the one way Xmech of the speaker. Basically, than means that the cone can move 19mm in one direct before damage will occur. This is not a hard or soft bottoming, etc. situation. One designs the driver to prevent those conditions.

If you're looking for the traditional Xmax = (voice coil winding width-top plate thicknes)/2, then the answer is about 7.5mm for the MM2124.

If you're looking for the 70.7% of rest Bl (a la DLC Xmag) then the Klippel Analyzer will tell you than those points lie at about +/-9mm. I can't provide the Xsus as stated by DLC-DUMAX measurements because the DUMAX measures the gradient of Kms and not Kms, so we wouldn't be comparing apples to apples. We currently use 200% of rest Kms as a benchmark (Xsus). That number is also unavailable as it lies outside of the Klippel Analyzer ability to measure, but is greater than +/-12mm.

Xmech, in the case of the latest MOMO series was measured by displacing the cone as far as possible by hand without damaging it and measuring that distance with a micrometer. As I said before, we do this with a laser and tone burst signals now.

I would once again caution the uninformed on using any data without full knowledge of it's origins and usage. The waters of Xmax and excursion data are still quite muddy and contain many pitfalls. And, as always, use your ears!

Cheers,

Scott

sntnsupermen131

wow, an actual engineer, i hope you decide to frequent this forum...its always nice to have someone on the "inside"...i guess you could say
anyways, welcome!
-Cody

PoweredByDodge

ever see that one star trek with those friggin backwards stupid people with the broken ship -- and Geordi goes over from the enterprise to fix their ship -- then they want to keep him cuz the fixes all their problems...

"He is smart... he can make us go... we search for things that can make us go... we will keep him."

lol... that's all I can think of regarding our new found forum member Scott.

MTXMAN

LMAO

PBD you never cease to provide me with a good laugh ;D

geolemon

Thank you Scott, that was exactly what I was looking for! :cool:
That's the kind benefit you WANT when you come to a manufacturer's forum (but don't usually get)!

BTW, if you ever feel the need, DUMAX testing is actually quite affordible... only around $100/test. DLC labs in Detroit (it's actually a suburb outside Detroit) is the place.
I will say the tests can yield inconsistent results at times, it's actually sometimes valuable to have the tests repeated on the same driver, to ensure accuracy.

Thom

Originally posted by geolemon
BTW, if you ever feel the need, DUMAX testing is actually quite affordible... only around $100/test. DLC labs in Detroit (it's actually a suburb outside Detroit) is the place.
I will say the tests can yield inconsistent results at times, it's actually sometimes valuable to have the tests repeated on the same driver, to ensure accuracy.

I'm pretty sure that in the past we've used them for testing, correct me if I'm wrong, Scott.

Quite a few of us went up to DLC 2 or 3 years ago for the "listening technologies" course.

ScottO

Thom,

Yes, we've worked with DLC in the past. We've had some drivers DUMAX'd and had some LT training. I have a lot a respect for what Dave and company do.

I have a fair amount of experience with the DUMAX machine. I recently presented a paper at this year's ALMA convention comparing results between the DUMAX and the Klippel Distortion Analyzer of several different drivers. Its a work in progress, so please no requests for it. Wait until I'm done.

Glad to see that people are interested in this subject.

Cheers,

Scott

geolemon

Originally posted by ScottO
...I recently presented a paper at this year's ALMA convention comparing results between the DUMAX and the Klippel Distortion Analyzer of several different drivers. Its a work in progress, so please no requests for it. Wait until I'm done.

I would absolutely love to see that paper, as my Klippel experience as a whole is relatively thin - it sounds quite educational, in a very practical sense... definitely more exciting than a theoretical comparison.

Any ETA on when (or if) the paper might become available?

It would be for my own personal benefit, so if there is any information inside that is Polk-proprietary, I don't mind signing an NDA regarding it... the comparison is intriguing, I'm sure I wouldn't be the only one interested. :cool:

neomagus00

i don't know why i ended up reading this thread, but that paper sounds pretty interesting... and with a year and a half since the last post, i think it'd be done by now... anyone have an idea how we can access this?

audiobliss

I'm glad you posted on this thread and brought it up to the top; all that information about subs was absolutely mind-blowing. I'll hafta read it 20 more times to understand it! lol

And, yeah...after 1 & 1/2 years....I think that paper should be done.

What's happened to Scott O? Is he still hanging around somewhere? Maybe working on Polk's (forgot what they're calling them) new TOTL subs?

neomagus00

well, those were his only two posts ever, but he could be working on the Signature Series (i think that's right)...

sounds neato

VR3

That gave me one hell of a headache...

Tons of info...something I refuse to read this time of night...

And yes, we need more Polk engineers on the forum...

We need Micah back dangit!