Understanding Anomaly in this Impedance Sweep / Measured TS Parameters?

[recon440]

At the moment I'm looking at a pair of Sundown U-12s for my next build and it's nice to see that they posted some DAAS32 measurements.

 

I'm currently designing a series 6th-order and would like to have the best grasp of the driver in terms of TS parameters. Specifically, I design all BP6-series enclosures in Hornresp and I like to work out semi-inductance parameters from the impedance curve but there are some inconsistencies in this trace along with T-S parameters. This is quite critical to know this the most accurately as the impact on a BP6S is unforgiving relative to a simple reflex design. 

 

To look at a specific example, take the U-12 D4 (although the same effect manifests itself on all woofer sizes and coil configurations throughout the U-series lineup). This QTS parameters are also consistent with what Sundown publishes on their site. There are several key features below which I'm not fully understanding:

 

1) The impedance peak for this particular driver is asymmetric and bimodal (40Hz and ~48Hz). This is actually the first time seeing such an effect as though the woofer has two resonant modes... Is there an explanation for this?
-Whether it's just down to smoothing? Not sure about this as the lower peak is too pronounced to be deemed insignificant.
-Whether there is something inherent to the subwoofer design that causes this? Particularly the alarmingly high Rms value of 24.4kg/s (which effectively damps resonance outside of key nodes, like a high-Q effect).

To me it sounds dubious that the subwoofer would have two resonance modes so close together. Ideally you would expect to have a symmetric singular peak and if we were to extrapolate the "ideal" peak it would probably lie ~42-43Hz, rising to an impedance of ~60-70ohms.

 

2) Ultimately, parameters presented from DAAS32 sweeps are typically bang-on in terms of consistency and yet here there are some strange anomalies.

In terms of TS parameters, there is a strong disconnect between the Fs, VAS and MMS. The Fs cannot be 40Hz with the given VAS and MMS but much higher. I would wager to say that the MMS is realistic for a woofer of this caliber so I'm not sure about the VAS. To me, for the QES/QMS to be agreeable the VAS would have to closer to realm of 20L rather than the published 12.9. I'm wondering if the derived QTS parameters are inconsistent due to the shape of the observed impedance curve or if something else is going on.

 

3) I'm also questioning the very high Rms...

I've generally not seen such high losses, even in ultra-low QTS, high-MMS, SPL-oriented woofers which are still typically in the realm of 10-12 kg/s. For mid-QTS, mid-MMS woofers I would expect this to be in the range of 4-7 kg/s. Is this value correct?

 

I would appreciate any insight to try to better understand this before I decide to purchase these woofers.