Xmax definition - Are manufacturers fudging specs ?

[OldogNewTrick]

We have learned by now that Xmax is the key spec when considering maximum power / SPL of a bass driver or the suitability of a driver for low frequency EQ boosts.
Manufacturers have learned that this is an easy key marketing spec (single figure - the more, the better), so are manufacturers in fact quoting comparable specs ? A bit of research shows - definitely not !!!

The original Xmax definition is (Hvc - Hg) / 2, where winding depth = Hvc and magentic gap depth = Hg.

Reading the fine print in the spec papers, you find that many are bemoaning the unfairness of this and want to quote "more realistic" figures....

Eminence measures to 10%THD, but offers insufficient data (no Hvc) to relate their quoted spec to anybody else.

Faital Pro quotes based on (Hvc - Hg) / 2 + Hg / 3
JBL, RCF and 18sound quote based on (Hvc - Hg) / 2 + Hg / 4
BMS quote most conservatively on the original (Hvc - Hg) / 2
Beyma and B&C state both Hvc and Hg in their specs, but the quoted xmax spec don't seem to follow any clear methodology and vary also from driver to driver.

Fane's Sovereign series, which is based on Eminence components states both Hvc and Hg, but their quoted specs vary between (Hvc - Hg) / 2 for their older designs and the "better looking" (Hvc - Hg) / 2 + Hg / 3 for their newer Nd drivers....


Bottom line is: You cannot compare Xmax driver specs from one manufacturer to another's.

Eminence's spec seems to be potentially the most fudged, as they offer no basis of comparison to anybody else...
Edit: the 10%THD basis seems to be the currently technically relevant standard, just not universally adopted and the lack of provided data makes a comparison on other methods impossible.

Opinions please !

                                                       Faital                   RCF                      BMS             Eminence
  Xmax definition                           15HP1020             LF15N301             15N630          KL3015LF
  
  winding depth  (Hvc)                       23,00                    19,00                    26,00             
  magentic gap depth  (Hg)                12,00                    10,00                    10,00             9,27
  
  quoted spec                                  9,50                      7,00                      8,00               9,60
  
  (Hvc - Hg) / 2                                 5,50                      4,50                      8,00               
  (Hvc - Hg) / 2 + Hg / 3                    9,50                      7,83                      11,33             
  (Hvc - Hg) / 2 + Hg / 4                    8,50                      7,00                      10,50             
  10% THD                                                                                                                   9,60

[billfitzmaurice]

The coil depth minus plate thickness Xmax calc is obsolete. The current standard is excursion at 10% THD, measured by Klippel Analysis. This is what Eminence uses. Not all manufacturers do, because the measuring gear isn't cheap.

[OldogNewTrick]

Originally Posted by billfitzmaurice

The coil depth minus plate thickness Xmax calc is obsolete. The current standard is excursion at 10% THD, measured by Klippel Analysis. This is what Eminence uses. Not all manufacturers do, because the measuring gear isn't cheap.

Well, I have tried to research that... the AES don't seem to have passed any resolution on that... neither it appears anybody else, except Eminence....

Or maybe, they all didn't get the memo ?


Edit:
Are there any other established Prosound driver manufacturers who work according to the 10%THD standard.
Do you suggest none of the mentioned European manufacturers can afford the measuring gear ?

[OldogNewTrick]

wikipedia:

XmaxSpecified in millimeters (mm). In the simplest form, subtract the height of the voice coil winding from the height of the magnetic gap, take the absolute value and divide by 2. This technique was suggested by JBL's Mark Gander in a 1981 AES paper, as an indicator of a loudspeaker motor's linear range. Although easily determined, it neglects magnetic and mechanical non-linearities and asymmetry, which are substantial for some drivers. Subsequently, a combined mechanical/acoustical measure was suggested, in which a driver is progressively driven to high levels at low frequencies, with Xmax determined at 10% THD. This method better represents actual driver performance, but is more difficult and time-consuming to determine.

"Suggested" seems to be the operative word. However, as nobody else seems to follow this, my basic premise stands.

You cannot compare quoted Xmax figures between drivers from different manufacturers.

[OldogNewTrick]

WinISD Pro uses the original Xmax definition of (Hvc - Hg) / 2.

So when when comparing driver/box designs based on xmax limited power, which is what all of us are doing, the results will always be slanted towards (newer) Eminence drivers....

[OldogNewTrick]

Originally Posted by billfitzmaurice

...measured by Klippel Analysis. This is what Eminence uses. Not all manufacturers do, because the measuring gear isn't cheap.

a quick visit to Klippel's customer list shows a lot of well known names... of course including Eminence.
Here are the applications notes relevant to xmax measuring.

The point of my post is not to argue that the 10%THD method of determining xmax is to be faulted, not at all... it pertains to the comparability of the spec and design evaluation.

[thudfromafar]

interesting thread, I'd like to see more

[Jim C]

+1
Wonder if this is similar to output power ratings methods from back in the day, as in, tell us what number you want to advertise and we will use the spec that accomodates.
From what I read here, regardless of published specs, it seems that Eminence is ahead of the cruve for our applications.

[OldogNewTrick]

Originally Posted by Jim C

...
From what I read here, regardless of published specs, it seems that Eminence is ahead of the cruve for our applications.

Well, they are that far ahead, that they have left us behind....

Not only is it not possible to compare the KL3015LF's Xmax spec of 9,6mm with any other manufacturer's speaker, but also WinISD Pro interpretes that figure wrongly as the original (Hvc - Hg) / 2.

The Xmax of 9,6mm (KL3015LF) maybe as little as 5,6mm based on the original definition, whereas the quoted 8,0mm (BMS 15N630) maybe as much as 11,5mm based on the 10%THD spec.
The point being, we don't know... and neither does our main design tool WinISD Pro.

[Jim Campbell]

Originally Posted by OldogNewTrick

We have learned by now that Xmax is the key spec when considering maximum power / SPL of a bass driver or the suitability of a driver for low frequency EQ boosts.
Manufacturers have learned that this is an easy key marketing spec (single figure - the more, the better), so are manufacturers in fact quoting comparable specs ? A bit of research shows - definitely not !!!

The original Xmax definition is (Hvc - Hg) / 2, where winding depth = Hvc and magentic gap depth = Hg.

Reading the fine print in the spec papers, you find that many are bemoaning the unfairness of this and want to quote "more realistic" figures....

Eminence measures to 10%THD, but offers insufficient data (no Hvc) to relate their quoted spec to anybody else.

Faital Pro quotes based on (Hvc - Hg) / 2 + Hg / 3
JBL, RCF and 18sound quote based on (Hvc - Hg) / 2 + Hg / 4
BMS quote most conservatively on the original (Hvc - Hg) / 2
Beyma and B&C state both Hvc and Hg in their specs, but the quoted xmax spec don't seem to follow any clear methodology and vary also from driver to driver.

Fane's Sovereign series, which is based on Eminence components states both Hvc and Hg, but their quoted specs vary between (Hvc - Hg) / 2 for their older designs and the "better looking" (Hvc - Hg) / 2 + Hg / 3 for their newer Nd drivers....


Bottom line is: You cannot compare Xmax driver specs from one manufacturer to another's.

Eminence's spec seems to be potentially the most fudged, as they offer no basis of comparison to anybody else...
Edit: the 10%THD basis seems to be the currently technically relevant standard, just not universally adopted and the lack of provided data makes a comparison on other methods impossible.

Opinions please !

                                                       Faital                   RCF                      BMS             Eminence
  Xmax definition                           15HP1020             LF15N301             15N630          KL3015LF
  
  winding depth  (Hvc)                       23,00                    19,00                    26,00             
  magentic gap depth  (Hg)                12,00                    10,00                    10,00             9,27
  
  quoted spec                                  9,50                      7,00                      8,00               9,60
  
  (Hvc - Hg) / 2                                 5,50                      4,50                      8,00               
  (Hvc - Hg) / 2 + Hg / 3                    9,50                      7,83                      11,33             
  (Hvc - Hg) / 2 + Hg / 4                    8,50                      7,00                      10,50             
  10% THD                                                                                                                   9,60

fudged i dunno,but it's probably more like the home audio world,where each manufacturer will use a different yardstick to measure his gear in a positive light.....generally the higher end stuff is rated conservatively.....i suppose that people who pay the extra are aware that specs are not always a reliable indicator

[Vege-mity-mite]

Good post - kudos. When people start talking specs you really have to know if they're following a standard otherwise using terms like Xmax becomes just another marketing term. Thanks OP.

[rpsands]

Anyone in the know know the 3015LF's HVC? That'd settle a lot of stuff At least on that woofer.

[Plstrns]

I know sh1tzola, nada, nothing about driver specs, but wouldn't Xlim tell you something as a relation to Xmax?

[OldogNewTrick]

Originally Posted by rpsands

Anyone in the know know the 3015LF's HVC? That'd settle a lot of stuff At least on that woofer.

+1

I am pretty certain at least one TB'er has that information, or, at the very least, could get it pretty easily. Wether he is willing or permitted to release it, that's another thing altogether...

[alexclaber]

Beat you to it!

http://barefacedbass.com/technical-i...splacement.htm

[OldogNewTrick]

Originally Posted by alexclaber

Beat you to it!

http://barefacedbass.com/technical-i...splacement.htm

Interesting !

And now to the Barefaced figures!


By the measurement method:

Big One = 846cc


By the complex arithmetic method:

Big One = 915cc
Complex arithmetic method is: Sd x (((Hc-Hg)/2)+(Hg/4)) = Vd

Vd=846cc is the official Eminence spec for the KL3015LF (Sd 881,1cm² x 9,6mm Xmax@10%THD)

Your complex artithmetic method Vd of 915 cm³ resolves to a Xmax of 10,38mm, which in turn resolves to Hc value of 25,4mm (1").

                                                     Faital                   RCF                      BMS                     Eminence
  Xmax definition                           15HP1020             LF15N301             15N630                 KL3015LF
  
  winding depth  (Hvc)                       23,00                    19,00                    26,00                    25,40
  magentic gap depth  (Hg)                12,00                    10,00                    10,00                    9,27
  
  quoted spec                                  9,50                      7,00                      8,00                      9,60
  
  (Hvc - Hg) / 2                                 5,50                      4,50                      8,00                      8,07
  (Hvc - Hg) / 2 + Hg / 3                    9,50                      7,83                      11,33                    11,16
  (Hvc - Hg) / 2 + Hg / 4                    8,50                      7,00                      10,50                    10,38
  10% THD                                                                                                                          9,60

Everybody agree ?

[rpsands]

What I'd like to see is the 10% THD figures for the other ones. There is a reason people moved to calculation via that method, and I'd be interested to see if those woofers have a failing that makes their harmonic distortion rise faster than the 3015LF

[OldogNewTrick]

Originally Posted by rpsands

What I'd like to see is the 10% THD figures for the other ones.

Me too

There is a reason people moved to calculation via that method, and I'd be interested to see if those woofers have a failing that makes their harmonic distortion rise faster than the 3015LF

Not necessarily so. The other drivers are significantly more expensive (not only in the US, but also here in the EU).
These companies are very highly regarded and very successfull in the Prosound world, where Eminence is mostly regarded as a budget oriented solution.
The Kappalite series, from all accounts that I've read, are regarded as fantastic value for money...
... and more than enough for anything, we can throw at them in a backline application.


Another spec, I'd dearly like to see published are the power compression figures of the Kappalites... and factor that into the comparisions of 15/6 vs 1212/6 vs 410 etc...

[rpsands]

I believe Duke from AK put some rough power compression numbers out for his Thunderchild, and I would bet that woofer shares a lot in common with Kappalites.

* You can also check out power compression figures from 18sound, which one manufacturer that does publish that, to get some ideas. My understanding is that the numbers tend to be fairly similar.

[DukeLeJeune]

Originally Posted by rpsands

I believe Duke from AK put some rough power compression numbers out for his Thunderchild, and I would bet that woofer shares a lot in common with Kappalites.

I don't remember doing so, but if I did the figures were calculated rather than measured. I've put out calculated thermal compression figures for my home audio speakers... some of which use the 3012LF(!), and the Thunderchild woofer is 1.4 dB more efficient (based on T/S parameters), so it goes a little bit louder than a 3012LF before thermal compression reaches a given level.