Thiele-Small R_ms (mechanical losses) vs. "precise" sound?

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Bernd Timmermanns (formerly editor of Klang und Ton, now of HobbyHifi), has maintained for years that there is a good correlation between the R_ms value in the T-S parameters and the sound of a driver. A woofer should have a low R_ms value to obtain a precise, clearly definded bass and a subjectively good reproduction of impulses. He stated explicitly that this is also true for all kinds of drivers, i.e. also mid range and tweeters. I have seen this statement mainly in connection with vented enclosures, but I believe it is supposed to be valid also for sealed and TML enclosures.

A driver has low R_ms if the voice coil carrier is non-conductive (i.e. Kapton rather than aluminum) and if the magnet has venting holes so the air can escape more easily (+ narrow cage ribs + low loss centering and sealing elements + ...).

Oh yes, and when comparing drivers, one should actually divide R_ms by the effective area of the cone.

Bernd Timmermanns has stood by his statement that a clear correlation is observable many times but he admits that there is no satisfactory theoretical explanation. He offered a tentative explanation that the air currents will have a strongly nonlinear dependence on the cone movement and exhibit hysteresis (i.e. an eddy will need a certain time to subside).

Has anybody made similar observations?

Is there any other explanation?

I think the nonlinear air current explanation could hold some truth.
However, if the voice coil carrier is conductive, eddy currents will have a large contribution to R_ms. On the other hand, eddy current losses will be a linear function of the speed of the voice coil except for very high movement amplitudes. So for this part of R_ms there is no nonlinear loss mechanism. And I can imagine that this highly linear damping might even be beneficial in some cases.

So one should try to separate R_ms_air=R_ms_nonlinear and R_ms_carrier=R_ms_linear and see if there is a general correlation between subjective impression and R_ms_total or rather R_ms_air.

The separation can be carried out be determining R_ms_total at two or more different drive amplitudes.

Hi Eric

I have also read Mr. Timmermann's statement but I don't know any theory behind it either.

But the following statement is from Per Skaaning of Audio Technology Denmark, who also made some experiences in sonic differences between coil former materials (by "standard" he means aluminium hexacoil):

"We can deliver drivers with Kapton voice coil former. ........ If you choose Kapton for voice coil bobbin, you will not experience the "dynamic sound" but a more full bodied bass reproduction, even at the lowest levels."

But maybe this is due to the fact that it would otherwise be the SAME driver and the QTS of the aluminium voice-coil driver would be lower due to lower QMS.


If it were exactly the same driver except for the coil former, this is probably the way to conduct the experiment. One build two identical boxes, one with the aluminum and one with the Kapton former, and adjust the total Q electrically.

Problem: lots of work and money involved just to gain some knowledge. Would be interesting as a thesis for a BSEE degree though....

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