Let's talk Tweeters!

My understanding is that there are two general areas of thermal compression effects. The one that has been studied and documented the best is what we might call long-term compression, which arises from both voice coil heating and magnet heating, the latter inducing a (usually temporary) loss of magnetic strength. Less well studied is what we might call thermal modulation, which as you describe happens very quickly - quick enough to reduce a sudden peak, thereby reducing the dynamic contrast, which in turn reduces the emotional effect of the music because musicians often use dynamics to convey emotion. I would expect thermal modulation to be primarily a voice coil heating phenomenon (with an accompanying increase in resistance)... when the voice coil of a speaker is hit with a 100 watt peak, it’s like touching it with a 100 watt soldering iron. Apparently JBL has patented a voice coil alloy whose resistance doesn’t change much as it heats up, presumably to combat thermal modulation. Others have worked on this too, but far as I know JBL is the first to include it in commercial products (some of their big high-end studio monitors).

A couple of years ago I had an exchange with Floyd Toole about thermal modulation, and he said they had definitely found it with some of their measurements at Harmon, and that in some cases it was pretty bad. He mentioned testing a 3-way speaker whose midrange driver was effectively compressing on peaks by about 7 dB! He’s the one who told me that thermal modulation is an area that needs to be studied more.

My own approach to thermal compression and thermal modulation ended up being the brute force method - high efficiency drivers with big motors and big voice coils that won’t start going non-linear until they reach much higher SPLs than are typical for home audio. This just happened to be a fortuitous side effect of giving radiation pattern control a high priority.

I’d like to learn more about "static compression", but apparently my Googling skills are weak... I couldn’t find a website for speakermeasurements.com, but apparently it is associated with SoundStage. I’m aware of their "Deviation from Linearity" test, is that what you’re referring to? Seems to me it includes thermal effects as well. Anyway kudos to them for running it - Richard C. Heyser used to do something similar back in the day for Audio magazine.

Duke

Thank you for that information and for digging up those examples, Erik!

Interesting that the less expensive speaker beats the more expensive one not only in deviation from linearity, but also in radiation pattern smoothness - look at the 45-60-75 degree off-axis curves. I tip my virtual hat to Paradigm.

Considering how SoundStage makes their Deviation from Linearity measurement, I think it includes not only mechanical effects but thermal ones also, because at 90 dB the speaker is seeing 10 times as much excursion but 100 times as much wattage as at 70 dB. At any rate, given that peaks 20 dB above the average are quite common in recordings that aren’t overly compressed, the deviation from linearity going from 70 to 90 dB may just be the "tip of the iceberg" for real-world effects, if the speaker is driven to average levels higher than 70 dB/1 meter.

My impression is that excursion-related non-linearities decrease gradually with level until they reach a certain point and then they shoot up rapidly.

The same thing is more or less true for thermal effects: It’s not uncommon for a driver to exhibit less than 1 dB of thermal compression at 10% of its AES rated power, often rising to about 2 dB at 50% of its rated power and then maybe 3.5 dB at 100% of its rated power. In other words, in that last doubling of input power, we only get about half as much increase in SPL as we "should have" (1.5 dB instead of 3 dB), Now these ballpark figures come from eyeballing the spec sheets of those few prosound manufacturers who publish compression specs. These numbers are for long-term thermal compression rather than short-term thermal modulation (which has a very rapid onset and then a slower release, unless another peak comes along before the voice coil has had a chance to cool down). My assumption is that there is a correlation between the short-term thermal modulation behavior and the long-term thermal compression behavior.

(Note that the AES rated power is a fairly conservative yardstick; typically the "music program" power rating is double that, and then the "peak" power rating may be double the music program rating, and we probably don't know which of these the manufacturer is using... and real-world, the excursion-limited power handling may be significantly lower than any of these at low frequencies.)

Duke