MW6500 DATS T/S Parameters Revised (Pics)

MrGeology · April 2023

Measurements in first post were taken in rapid succession.
Measured Piston Diameter estimated 5.25 inches.

3 sample drivers:
Original 1983 M7B driver
Wuhan VC Repaired 1983 M7B driver
1980 Silver Basket MW 6500 with original coil.
For each driver in M7B Cabinet Impedance Sweeps, see my post here:
https://forum.polkaudio.com/discussion/198509/m7b-dats-cabinet-impedance-sweeps-pics

3 pics of T/S Parameters (closeup).
3 pics of Impedance Curves.
1 pic Overlayed Impedance Curves.

T/S Measurements.

Impedance Curves.

Overlayed Impedance Curves.

Gardenstater · December 16

Here's a small spreadsheet that compares the measurements you made with T/S Parameters posted in the forum:

I think it is notable that there is significantly higher Qms compared to our T/S parameters in the forum, which would tend to indicate a loss of damping in the doping perhaps. Something to look into regarding whether or not re-doping or applying a thin coating of "something" to regain the original stickiness might bring this to where it aligns more with the T/S parameters we have in the forum. Also, the Vas is less which would mean less compliance. I would've thought that counterintuitive since I would think that the suspension would get softer over time (may not be true esp. the surround rubber) but perhaps that again is related to the cone getting stiffer as the viscoelastic damping material dries out over the decades.

Anybody have any speculation as to why Qes would be higher?

Gardenstater · December 16

.

xschop · December 16

Bueller....Bueller.

Gardenstater · December 16

Ferris?

xschop · December 16

No eggnog today, so I can't speculate.

xschop · December 16

But I can speculate that 6511's today would be far off from factory specs. Flabbier than Rosie O'Donnell after an Ozempic binge.

Gardenstater · December 17

Another thing to contemplate over a large glass of heavily spiked egg nog and to get the mind off of Rosie O'Donnels lovely countenance. What effect would these altered T/S parameters due to aging have upon the optimal enclosure volume? What changes if any, should one make to the polyfill and/or totality of damping materials? What effect could it have upon the optimal tuning weight for the passive radiator?

Gardenstater · December 17

This is how the Q's are calculated from the open air impedance curve measurements:

Gardenstater · December 18

More background info. on T/S parameters:

Gardenstater · December 18

Unfortunately this is all we have for the T/S Parameters of the MW6500 and notably missing is the Bl, which can be calculated from what we do know if we assume that the avg. Mms obtained by MrGeology applies to the new MW6500. Probably true since why would mass be lost or gained appreciably?

Bl = √(2π × Fs × Mms × Re / Qes)
(with Mms in kg, Fs in Hz, Re in ohms).

That gives Bl (new) = 6.60 Tm.

The others come out to (1983 from 7B, 1980 Silver Basket): 5.78 Tm and 5.73 Tm

So you can see that a considerable amount of Bl was lost due to possibly degradation of magnet strength and less than optimal alignment and increase of Qes with aging (possibly related to loss of cone damping which increases Qms)

And here is a way to measure Bl where all you would need would be a variable current DC power supply. Something to try for sure to see how much has been lost in these vintage drivers:

xschop · December 18

So don't store your drivers in the attic, and don't drop them?

Gardenstater · December 18

MrGeology hasn't been around unfortunately but I'm wondering why the Re was so high. Tinsel leads corrosion? Maybe flux plus resoldering along with DeOxit treatment of the tinsel leads would drop that closer to Polk specs? I'm out of Nog so maybe I shouldn't speculate.......

It definitely would've raised Qe though, although not nearly as much as reduced Bl.

I'm assuming he didn't neglect to deduct the test leads resistance (which was strangely high).

Gardenstater · December 18

xschop wrote: »

So don't store your drivers in the attic, and don't drop them?

And go back in the time machine and make sure the assembler centers the magnet on the pole piece 😬😬 ? And uses a good acrylic adhesive instead of epoxy.

Gardenstater · 8:45AM

Here is a way for people who don't have DATS to find the impedance maxima and minima. It could be done on free air basis with the drivers and on the whole speaker with drivers installed (like to find the tuning frequency Fb and see if if/how much it has drifted upwards with time/aging).

MW6500 DATS T/S Parameters Revised (Pics)

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