Speaker advice redux?

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

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.

Duke, very informative post, thank you.

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

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.