Speaker advice redux?

Thanks Duke -- very interesting tech point. As you may recall, my speakers are Paradigm S8s (v3). The main difference between the v2 and v3 versions is that the 4 woofers are 3 db more efficient.

Paradigm changed the voice coil and the motor throw distance in the woofers to achieve the greater efficiency. I understand that they also "untapped down" the tweeter and midrange driver to match the woofers. Btw, the tweeter and mid both use ferro fluid to cool the voice coils. I suspect that these changes may have helped to mitigate thermal compression, but I honestly don't know if Paradigm had thermal compression in mind when it updated the S8s.

Last point, admittedly unrelated to this OP, so I'll make this short. You may recall that we had numerous on-line and off-line conversations about the electrical match of my ARC tube amp and the S8s. Two tech points came to my attention later that resolved many of my concerns. The first is that when Paradigm changed the woofers on the v3 and "un-tapped down" the tweeter and mid, the impedance peak at 2KHz dropped from 28 ohms (v2) to 21 or 22 ohms (v3). That's a good fact. In addition, I came to better understand that my amp is NOT a pure Power Paradigm amp because it uses 12db of negative feedback. As a result, output impedance off the 8 ohm tap is about 1.1 ohm. Output regulation is rated at approx. +/- 1.2 db. The stats improve even further off the 4 ohm tap. Output impedance may be 50% to 60% as compared to the 8 ohm tap and output regulation may be as tight as +/- .5 db. So, I've been using the 4 ohm tap and retrained my ears. I think the combo is pretty good after all.

Sorry for the hijack. Feel free to continue this off-line if you have any interest.

Cheers,

Bruce

Duke, your premise is fundamentally correct in that power compression will affect tonal balance. However, your assumptions of 1db up to 3db just aren't accurate for well engineered drivers. Modern Klippel testing has shown that for top tier drivers power compression is more like .3-.6db

I think we'd both agree that 0db is better, and that .3 to .6db isn't that big of an issue.

Vapor1, I don't think I've ever seen published Klippel data on thermal compression. An explanatory graph on Klippel's website implies that thermal compression in the neighborhood of 3-4 dB is normal:

http://www.klippel.de/measurements/nonlinear-distortion/compression-of-fundamental-components.html (first graph on the page)

If pushed hard enough, even the top tier drivers you refer to will exhibit significant thermal compression (assuming something doesn't melt first - which I guess would be permanent total compression!). But they may be rated conservatively enough that thermal compression is negligible at their rated input power.

Earl Geddes on the subject, from the text "Transducers", page 241: "Thermal effects in transducers do not generate distortion byproducts, but they do distort the frequency response - i.e. they cause severe linear distortion." He then goes on to discuss both short-term and long-term thermal effects, the latter including the effect of magnet heating on the BL curve.

Here is some further clarification from Earl on short-term thermal effects, from a DIY audio thread: "Awhile back I did some calculations of thermal rise in a VC. It happens almost instantaneously. This means that the VC resistance is being modulated at a very fast rate. The magnet heating and other effects take much much longer. But I became interested in the effect of temperture rise of the voice coil since this happens very fast.... The temperature problems occur almost instantaneously with the large signals even if we don't hear them as loud... These thermal issues may occur even at low levels if the signal's dynamic range is sufficient."

[ubergeek warning] There are two primary heating events at play in thermal compression: That of the voice coil, and that of the motor structure. We find evidence of very rapid voice coil heating in the generation of subharmonics at low frequencies and high power levels. Non-linear theory says that subharmonic generation is impossible in a time-invariant system. So there must be a time-variant factor, and that could well be voice coil heating on a timescale comparable to the period of a low frequency signal. Which is pretty darn fast. Geddes is again my source. [/ubergeek warning]

EighteenSound, a high-end Italian prosound driver manufacturer, is among the few companies that publish thermal compression data on their drivers, and my ballpark estimates of 1 dB @ 1/10th the rated power and 3 db @ the rated power are largely based on their published data.

From JBL's FAQ page: "Some speakers may exhibit 3 to 6 dB of power compression." Tom Danley published a paper a while back showing power compression of drivers operated at their rated power to be generally in the 3-6 dB range. (I'm guilty of using the terms "thermal compression" and "power compression" interchangeably; thermal compression is the primary component of power compression.)

On the other hand, in 2006 Keith Howard published an article in Stereophile which seemed to disprove the significance of thermal compression based on his measurements. (Earl disputes Keith's findings and technique, which did not include either a direct measurement of the voice coil temperature or of the frequency response.) So anyway there are two sides to the issue, but I think the more professionally qualified opinions fall on the side of thermal compression being a potentially significant issue, even if it's not an issue in all cases.

Two conventional but very effective defenses against thermal compression are high efficiency and large voice coil diameter. High efficiency = less wattage needed for a given SPL = less heat, and large diameter voice coil = more thermal mass = greater thermal inertia, and more surface area for better cooling. Of course using multiple drivers helps in both areas.

That being said, it is quite possible for a well-designed (top tier) speaker with a smaller voice coil diameter to incorporate features that give it very good thermal characteristics. I believe that you use Acoustic Elegance woofers in some of your models, and they are superb in that regard (and in many others). That drastically extended pole piece wicks away heat right where it's needed the most - at the forward edge of the voice coil (which is where voice coils tend to burn out because of insufficient local heat sinking).

Duke

Hi Bob,

Good catch! I mis-used (and mis-spelled) the term "vicious cycle"; it only becomes such when a human enters the loop and keeps turning the volume control up higher and higher because he's not hearing the volume level increase as much as he expected.

Duke