Speaker advice redux?

Linn majik 109

Redux great speakers.

Duke- I read through and quite enjoyed the discussion. No worries at all! And Bob, thanks again! I did see Robert Reina's review of the Creek Evolution 50a and am pleased he thinks so highly of it. I hadn't seen Mike Creek's comment but will go look for that now. Really appreciate all the good advice!

My apologies to Margot also... sometimes the nerd in me takes over, and this time it ran rampant.

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

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

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.

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, very informative post, thank you.

So, is there an advantage to use a speaker with high sensitivity for low level listening?

Oh, to the OP. Considering your requirements, perhaps you should also check out the offerings from System Audio. They weren't for me because when I listen to rock, I tend to like it loud. The SA's wouldn't do rock the way I liked it, but for low level listening, they are very nice. I had the 505's but considering that you'll have the vertical space, the 510s would be very nice for what you want. They also have a very small footprint, so narrow stands work nicely.

Sorry I wasn't clear, Bob! Let me try again.

First a bit of background that I left out: Thermal compression is caused by the voice coil heating up which causes its resistance to rise (there are other bad things that happen as well, but this is the main one). As the resistance rises, progressively less wattage goes into moving the voice coil, and progressively more wattage goes into further heating the voice coil, so it's a viscious cycle. Normally thermal compression is negligible until we get up to maybe 1/10th the driver's RMS power handling, then it starts to accelerate rapidly, often reaching 3 dB at full rated power. In the example I gave in my post above, the woofer is subject to significant thermal compression but the tweeter is not.

It might be easier to visualize how this can make the speaker's spectral balance change with level if we start out at low SPL, using the same 83 dB, 50 watt woofer and 90 dB, 50 watt tweeter as before:

Let's say we start out with woofer and tweeter level-matched at 1 watt (83 dB). Since the tweeter is 7 dB more efficient that the woofer, we have padded it down by 7 dB in order to match the levels.

Now if we go up to 50 watts, we would expect both the woofer and tweeter to deliver 100 dB. But the woofer in our example loses 3d B to thermal compression (due to the voice coil getting very hot), so it only gives us 97 dB, while the tweeter is still delivering 100 dB. So the speaker sounds bright at high SPL.

If we wanted to "voice" the speaker to sound correctly balanced at 50 watts, we would want the tweeter to also deliver 97 dB. We do that by padding the tweeter down by an additional 3 dB.

But this additional 3 dB of padding on the tweeter needed to level-match at high SPLs results in the tweeter being -3 dB relative to the woofer at low SPLs, where thermal compression is negligible.

If it's still unclear, let me know and I'll try again.

I first became aware of thermal compression when listening to a well-respected three-way speaker (which cost more than my car) that sounded overly laid-back at at low and medium levels, magnificent at fairly high levels, and forward and harsh at very high levels. In this case, the woofer and tweeter were suffering from thermal compression, but the midrange driver was not (it was the "showpiece" of the system). Since then I've noticed the effects of thermal compression on spectral balance in more speakers than I can remember. It's most noticeable when a speaker gets bright or harsh or too forward-sounding at high SPL, and is most common when a high-efficiency tweeter is paired with a low-efficiency woofer.

Duke

"A speaker is either able to resolve details better than another one or it's not, level isn't going to change that."

That is true in many cases, and would probably be true in all cases if the spectral balance didn't change with volume level. But sometimes that is what happens. Let me give a hypothetical example that illustrates what I'm talking about. I'm going to make some simplifing assumptions here, to keep this hypothetical from getting too complicated:

Suppose we want to design a compact speaker with decent bass, so we choose a beefy little 83 dB efficient 5-inch woofer that has some serious low end going on (honest 40 Hz in about 1/4 cubic feet). Power handling 50 watts RMS, 100 watts "music program", 200 watts peak. And because we want good resolution, we choose a nice little 90 dB efficient 1" dome tweeter, same power handling characteristics as the woofer.

The problem we run into is thermal compression. As a ballpark generalization, a speaker will have about 1 dB of thermal compression at 1/10th its RMS power handling, increasing to about 3 dB at its RMS rated power handling, and going up from there.

The woofer will see far more power than the tweeter, since the tweeter will be padded down to level-match with the woofer. So let's say we "voice" our little speaker to sound correctly balanced at 97 dB, which corresponds to 50 watts input. Our woofer is seeing nearly the full 50 watts, and our padded-down tweeter is seeing a little over 2 watts. So at 97 dB, our woofer is exhibiting about 3 dB of thermal compression (that's why it isn't giving us the full 100 dB we would otherwise expect), and our tweeter isn't exhibiting any thermal compression at all. So instead of being padded down the 7 dB we might expect based on the efficiency difference between the two drivers, we have padded down the tweeter by 10 dB, so that the spectral balance is correct at high sound pressure levels (97 dB ballpark).

So, what happens at low levels, say down at 85 dB or less?

Well, at low levels, neither woofer nor tweeter are seeing enough power to exhibit any significant thermal compression. But remember that we had to pad down the tweeter by an extra 3 dB because we wanted the speaker to sound good at 97 dB. So now, at low levels, the tweeter is 3 dB too soft relative to the woofer! As a result, our killer little speaker sounds dull and lifeless at low levels - you have to hit it with a lot of power before it "wakes up"!

Now this isn't really a change in resolution per se, but it can sound like a change in resolution, because the treble range - which conveys the little details and nuances - is subdued relative to the rest of the spectrum at modest SPLs.

What would happen if we didn't pad down the tweeter by that extra 3 dB? Well it would sound correct at modest levels, but then at high levels it would be too bright, and especially so on peaks. (The correct solution, imo, is to do a better job of matching up the real-world thermal characteristics of woofer and tweeter... but then this hypothetical example would fail to illustrate my point.)

As mentioned early on, I have made some simplifying assumptions here, but the general principle is valid: When you have a large discrepancy in the thermal headroom of the woofer and tweeter, the spectral balance may change significantly as the volume level changes significantly.

Duke
dealer/manufacturer

Oh man, I'd hate to have to make this decision. My wallet made my decision for me.

My knee jerk reaction would be to say get the Harbeths. On the other hand, the Minuets are very nice. I have a pair.

I also listen to a very wide range of varying types of music and I have been on the hunt for the "perfect" speaker for me.

Just to play devil's advocate, I'd say that the Harbeths are a good bit less efficient. Just going by the specs, the Minuets should play about twice as loud for any given amount of power, for the times when you can. The Harbeths are also rated pretty low for power handling, at 50 watts. The Minuets are rated at up to 300. I also didn't like the Harbeths playing rock music, although they did everything else very well.

The Harbeths also don't go as low as the Minuets do. I often run my Minuets without a sub and don't miss it at all. When I do run a sub, I have it crossed over at 50hz, for what it's worth.

I think that I would characterize the Harbeths as being very civilized. They don't play all that loud and they don't go very low, but they sound excellent with what they can do.

The Minuets are a great bang for the buck speaker. I've had Dynaudios, Paradigms, System Audios, etc. and the Minuets have sounded the best in my room. What I've found with them is that they sound good no matter what. I mean, I can set them up to sound really good, but they sound at least good regardless of how I set them up. They are very forgiving.

To tell the truth, I feel kind of silly recommending the Minuets over a pair of Harbeths, but the Harbeths just weren't for me, although I will acknowledge that they are recognized as being a better speaker.

If you can afford the Harbeths, I'd recommend finding a used pair of Minuets. You can get them for $500 and under. Then I'd find a used pair of Harbeths and get those too. If you decide that the Harbeths are for you, then stick the Minuets in storage because everyone should have a pair at their price.

If the Minuets are for you, you can sell the Harbeths.

For their price and the countless number of hours that I've enjoyed the little Minuets, I cannot fault them for anything. Hours literally fly by when I'm listening.

there is a nice pair of Totem Staffs for sale on Agon. I feel they would do the trick for you ant low and med volumes.

Harbeth, if you really have the budget.