Do you have links to Klippel results? I've seen the tbw but I'm always interested in looking at more.
Although i would generally agree, sometimes it's just all about xmax.
This is the kind of stuff I've been looking at lately for a more HT style sub - 21sw115 in about 818 liters. Xmax is the killer here while it's not even sucking up 350 watts. Thermal is not any kind of issue here, neither is a stiff cone as cone pressures are pretty low. This driver doesn't have a really strong motor either but in this design it works well. In this case (and pretty much all cases where you want to tune very low) xmax is king.
An externally hosted image should be here but it was not working when we last tested it.
Xmax is a slippery subject. Klippel testing is best but even that can have some issues. Production tolerances, setup, variations on reporting method, etc...
B&C's Xvar was I thought based on some interpretation of Klippel testing on the drivers.
Even comparing 2 drivers both tested by the same Klippel can be interesting. For example one woofer may hit the 50% compliance change limit at 12mm displacement, at the same time that the BL has dropped to 70%, as well as the inductance has started to vary substantially. A different driver may also hit the compliance change limit of 50% at 12mm but perhaps the BL does not drop to 70% until further out at 18mm and the inductance behavior is better. Both technically would be limited to 12mm xmax under the definition offered by typical Klippel testing but in use the first driver would be far more distorted and likely a little bit beyond what would be considered useful excursion while the second would likely be cleaner sounding and perhaps useful a bit beyond 12mm. On paper they would have the same useful displacement but in actual use it may not be that way.
B&C's Xvar was I thought based on some interpretation of Klippel testing on the drivers.
Even comparing 2 drivers both tested by the same Klippel can be interesting. For example one woofer may hit the 50% compliance change limit at 12mm displacement, at the same time that the BL has dropped to 70%, as well as the inductance has started to vary substantially. A different driver may also hit the compliance change limit of 50% at 12mm but perhaps the BL does not drop to 70% until further out at 18mm and the inductance behavior is better. Both technically would be limited to 12mm xmax under the definition offered by typical Klippel testing but in use the first driver would be far more distorted and likely a little bit beyond what would be considered useful excursion while the second would likely be cleaner sounding and perhaps useful a bit beyond 12mm. On paper they would have the same useful displacement but in actual use it may not be that way.
A few months ago I was informed that Klippel actually had two different assessment tests of xmax limits, the performance based test and the parameters based test. i was advised that I should read Klippel's paper on this subject.
https://www.klippel.de/fileadmin/kl...t_of_Voice_coil_peak_displacement_XMAX_02.pdf
I didn't bother reading it until today which was a big mistake. Until reading this I had assumed that the AES 2-1984 standard was similar to Klippel's performance based test (which I didn't know existed) and therefore I thought the AES 2-1984 was a good standard. It isn't.
The paper describes:
1. The standard linear mathematical xmax model (but only to say this paper makes it obsolete).
2. The AES 2-1984 standard.
3. The klippel based performance test. (this is an actual distortion measurement)
4. The klippel based parameters test. (this is a measurement of each parameter, Bl, Cms/Kms, Le, Xd).
The standard linear mathematical xmax spec is useless as the paper points out, for reasons that have already been listed here. These include the BMS driver having 19 mm calculated linear mathematical xmax (or more based on the way all the other companies rewrite the coil/gap formula) but only having 14 mm based on compliance limits in the Klippel parameters test. And many of B&C's drivers have less Xvar than Xmax. These factors make the linear mathematical spec ambiguous if not completely useless.
The AES 2-1984 standard isn't much better. The paper makes quick work of showing how useless AES 2-1984 actually is. In one example they show a driver can have either 0.6 mm OR over 19 mm xmax depending on what you measure. And publishing either of those numbers would be perfectly ok according to the rules set out in AES 2-1984.
The intent of AES 2-1984 is to measure 10 percent distortion which is a noble cause but the various methods that are allowed to do so, and the ambiguity in the wording and the subsequent assumptions about what the test means makes for a test that can be performed in two very different ways giving two very different results. And as the example in the paper shows it's entirely possible that neither of the possible results are even remotely accurate.
So then we have the Klippel parameters based testing method. This is the test we usually see, the test the Klippel reports with all the graphs measured by Stereophile or whoever publish. This has four main categories, Bl, Cms/Kms, Le and Dopper linearity. Each category has it's set limit (Bl = 82 percent for mids, Bl=70 percent for subs for example). The limit for each category is SUPPOSED to be roughly equivalent to 10 percent distortion (for mids, 20 percent for subs) but this is just a best guess. Actual measured distortion might not follow the Klippel defined category limits. First the limits could be a bad guess for any given driver's actual measured distortion performance, second because the driver may be approaching 2 or more of it's category limits simultaneously which could result in higher summed distortion as Ricci pointed out.
This parameters based testing is especially useful for driver designers and manufacturers. The paper clearly lays out what causes each type of distortion and what physical part of the driver to fix or improve if you want to achieve higher xmax.
This parameters based testing is really interesting but maybe not quite as useful as the performance based test for people that actually use the drivers.
The performance based test involves directly measuring the driver output, doing spectral analysis and directly reporting the measured distortion and stating whether it's primarily 1st, 2nd or 3rd order.
Therefore a performance test xmax spec would look like this -
Xmax= 3.8 mm @ d2=10 % (dt, d3 < 10 %)
The performance based test would seem to be more useful to people using the drivers (like us) simply because it measures the actual 10 percent distortion point whereas the parameters test estimates the distortion based on the assumption that any given parameter will meet 10 percent distortion when it reaches the Klippel defined limit for that parameter's category, which is not necessarily true. The performance based test and the parameters based test are supposed to give somewhat equivalent results but as the paper shows, even Klippel's own simplified examples don't net the same xmax value when run through both tests. I would think the performance based test would be more useful to us since it's reporting actual measured distortion in the measured output.
https://www.klippel.de/fileadmin/kl...t_of_Voice_coil_peak_displacement_XMAX_02.pdf
I didn't bother reading it until today which was a big mistake. Until reading this I had assumed that the AES 2-1984 standard was similar to Klippel's performance based test (which I didn't know existed) and therefore I thought the AES 2-1984 was a good standard. It isn't.
The paper describes:
1. The standard linear mathematical xmax model (but only to say this paper makes it obsolete).
2. The AES 2-1984 standard.
3. The klippel based performance test. (this is an actual distortion measurement)
4. The klippel based parameters test. (this is a measurement of each parameter, Bl, Cms/Kms, Le, Xd).
The standard linear mathematical xmax spec is useless as the paper points out, for reasons that have already been listed here. These include the BMS driver having 19 mm calculated linear mathematical xmax (or more based on the way all the other companies rewrite the coil/gap formula) but only having 14 mm based on compliance limits in the Klippel parameters test. And many of B&C's drivers have less Xvar than Xmax. These factors make the linear mathematical spec ambiguous if not completely useless.
The AES 2-1984 standard isn't much better. The paper makes quick work of showing how useless AES 2-1984 actually is. In one example they show a driver can have either 0.6 mm OR over 19 mm xmax depending on what you measure. And publishing either of those numbers would be perfectly ok according to the rules set out in AES 2-1984.
The intent of AES 2-1984 is to measure 10 percent distortion which is a noble cause but the various methods that are allowed to do so, and the ambiguity in the wording and the subsequent assumptions about what the test means makes for a test that can be performed in two very different ways giving two very different results. And as the example in the paper shows it's entirely possible that neither of the possible results are even remotely accurate.
So then we have the Klippel parameters based testing method. This is the test we usually see, the test the Klippel reports with all the graphs measured by Stereophile or whoever publish. This has four main categories, Bl, Cms/Kms, Le and Dopper linearity. Each category has it's set limit (Bl = 82 percent for mids, Bl=70 percent for subs for example). The limit for each category is SUPPOSED to be roughly equivalent to 10 percent distortion (for mids, 20 percent for subs) but this is just a best guess. Actual measured distortion might not follow the Klippel defined category limits. First the limits could be a bad guess for any given driver's actual measured distortion performance, second because the driver may be approaching 2 or more of it's category limits simultaneously which could result in higher summed distortion as Ricci pointed out.
This parameters based testing is especially useful for driver designers and manufacturers. The paper clearly lays out what causes each type of distortion and what physical part of the driver to fix or improve if you want to achieve higher xmax.
This parameters based testing is really interesting but maybe not quite as useful as the performance based test for people that actually use the drivers.
The performance based test involves directly measuring the driver output, doing spectral analysis and directly reporting the measured distortion and stating whether it's primarily 1st, 2nd or 3rd order.
Therefore a performance test xmax spec would look like this -
Xmax= 3.8 mm @ d2=10 % (dt, d3 < 10 %)
The performance based test would seem to be more useful to people using the drivers (like us) simply because it measures the actual 10 percent distortion point whereas the parameters test estimates the distortion based on the assumption that any given parameter will meet 10 percent distortion when it reaches the Klippel defined limit for that parameter's category, which is not necessarily true. The performance based test and the parameters based test are supposed to give somewhat equivalent results but as the paper shows, even Klippel's own simplified examples don't net the same xmax value when run through both tests. I would think the performance based test would be more useful to us since it's reporting actual measured distortion in the measured output.
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JAG,
Of note, many B&C's drivers have less Xmax than Xvar, one should compare the different drivers in their line up for comparison, some hit out of their "weight class".
Art
The problem with B&C's specs is that they're not representative of real world performance across their catalog.
Take their new MBX series for example. The 8" looks like it has better excursion capabilities than the 6" based on physical and measured specs. Voicecoil has measured both in recent issues.
8MBX51
B&C specs - Xmax 6mm Xvar 8mm
Klippel 10% - 2.2mm
Klippel 20% - 4.0mm
6MBX44
B&C specs - Xmax 3.5mm Xvar 3.0mm
Klippel 10% - 4.3mm
The numbers don't add up. Some drivers can be extrapolated to reach B&C's Xvar value and others don't follow at all.
Take their new MBX series for example. The 8" looks like it has better excursion capabilities than the 6" based on physical and measured specs. Voicecoil has measured both in recent issues.
8MBX51
B&C specs - Xmax 6mm Xvar 8mm
Klippel 10% - 2.2mm
Klippel 20% - 4.0mm
6MBX44
B&C specs - Xmax 3.5mm Xvar 3.0mm
Klippel 10% - 4.3mm
The numbers don't add up. Some drivers can be extrapolated to reach B&C's Xvar value and others don't follow at all.
As stated in post #67, some of B&C's drivers hit out of their "weight class", as in your example. The smaller 6" SD allows the very strong motor to push the cone further with less distortion than the larger 8". That said, you don't "get nothing for nothing" the larger cone with less excursion likely makes more SPL than the smaller cone with more linear excursion.
Disclaimer: B&C, Gorilla Tape, and Corrosion X do not pay me for the endorsement of their products, but I will accept offers ;^) )
Cheers,
Art
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