From Power Compression towards Thermal Distortion / developing on a point of view

[mige0]

I started this thread on request of Earl about "in depth" discussing the topic of Thermal Distortion / Power Compression as I have outlined in the paper linked below.

Basically this is a *my* focused summary of several postings and some additional ideas having evolved in a discussion with nickmckinney, John_E_Janowitz, EarlGeddes, Charles Hansen and others about impacts of voice coil wires used today at Lynn's thread.



Michael

[gedlee]

The main point here is that a thermal change that happens slowly is still a "linear" phenomina because the time rate of the change is slower than the signal rate. The system can only be non-linear if the time rate of the thermal change is within the signals bandwidth. Now the voice coil changes CAN BE within this range depending on two basic factors, the thermal mass and the current. There is no way that anything beyond the voice coil will have time rates fast enough to be nonlinear. The long term thermal aspects like voice coil cooling and magnet temperature will affect system tuning etc, but they will not generate nonlinear distortion of any kind.

Only the voice coils mass and the current through it has any nonlinear component and these factors are almost completely determined by the voice coils size and nothing else. A large voice coil tends to have lower current and a greater thermal mass so it is unlikely that there would be a nonlinear effect. However smaller voice coils (smaller woofer etc.) such as have become popular, need more current, because they have less copper and they have lower thermal mass, hence these speakers would be very likely to have thermal nonlinear effects.

I hypothesis that this is what people hear in the larger speaker as greater dynamics. Its a simple situation - the bigger the voice coil the lower the thermally generated distortion. This is why I like compression drivers over dome tweeters. There is a lot more copper in a compression driver voice coil than a dome tweeter - a lot more! And a 4" voice coil woofer is going to be a whole lot better in this regard that a 2" voice coil woofer. Bigger simply is better in a lot of ways.

[soongsc]

I can understand that power compression needs to be considered in pro audio, but for home audio and car audio, continuous levels can't be that high in a properly designed system.

[mige0]

Originally posted by gedlee
The main point here is that a thermal change that happens slowly is still a "linear" phenomina because the time rate of the change is slower than the signal rate. The system can only be non-linear if the time rate of the thermal change is within the signals bandwidth. Now the voice coil changes CAN BE within this range depending on two basic factors, the thermal mass and the current. There is no way that anything beyond the voice coil will have time rates fast enough to be nonlinear. The long term thermal aspects like voice coil cooling and magnet temperature will affect system tuning etc, but they will not generate nonlinear distortion of any kind.

Only the voice coils mass and the current through it has any nonlinear component and these factors are almost completely determined by the voice coils size and nothing else. A large voice coil tends to have lower current and a greater thermal mass so it is unlikely that there would be a nonlinear effect. However smaller voice coils (smaller woofer etc.) such as have become popular, need more current, because they have less copper and they have lower thermal mass, hence these speakers would be very likely to have thermal nonlinear effects.

I hypothesis that this is what people hear in the larger speaker as greater dynamics. Its a simple situation - the bigger the voice coil the lower the thermally generated distortion. This is why I like compression drivers over dome tweeters. There is a lot more copper in a compression driver voice coil than a dome tweeter - a lot more! And a 4" voice coil woofer is going to be a whole lot better in this regard that a 2" voice coil woofer. Bigger simply is better in a lot of ways.

Earl, thanks for pointing out what is the meaning of "distortion" in a strict scientifically context.

I very much appreciate your input at the topic and I perfectly agree with you that "tings ain't that simple"

My point about TD / Thermal Distortion is that it gives us kind of semantic "alarm" message on the topic.

Leaving semantics aside for a moment, I'd like to know where *exactly* to draw the line on interaction .

As I understand your argumentation – you are saying there is non "distortion" (in the strict scientific sense) happening as there is no bandwidth intersection where the thermal caused resistance change could possibly interfere with the audio signal.

Maybe you are right .

On the other hand - digging deeper into this we can see that both – heat up and cooling down are *immediately* happening.

Not exactly correlated with the audio signal itself - but with the envelope of the amplitude of the audio signal.
And to make things even more complicated – there is kind of asymptotically slew rate limitation behaviour involved for the cooling down part .

From this point of view I could agree that there is no bandwidth intersection at all – as there are no two *frequencies* to compare with and hence calling the effect sort of "distortion" would in fact be wrong - at least in a more strict scientific sense -

- OR - looking at a pulse signal only – I could say - that this pulse is "distorted" (in the strict scientific sense) as there is *immediate* rise of resistance at the raising side of the pulse and also there is *immediate* fall of resistance (at a different scale) at the falling side of the pulse.

Summing together I tend towards looking at the effect of Power Compression from a Thermal Distortion point of view .


We also have to stay clear about two things :

- first, Thermal Distortion doesn't have any lag / delay – though it might not be "linear" over time
- second, Thermal Distortion occurs as well for *any* given frequency – though the effect on a single sine wave swing would be rather small as there is the cumulative effect of envelope behaviour at the raising side and the sort of asymptotic slew rate limitation behaviour at the decay side – nevertheless this is more about quantity than about the quality of the effect


Michael

[forr]

Hi Mige0,
Some good points were raised in this discussion :
Voice coil heating and the performance of passive XO, plus my funny experience at HTG
See some data in Philippe Lasage's paper which is quoted in one of the posts.

[mige0]

Originally posted by soongsc
I can understand that power compression needs to be considered in pro audio, but for home audio and car audio, continuous levels can't be that high in a properly designed system.

If you follow my example in the paper linked below you can see that – from plain calculation – it affects even home and car audio.
But yes - sure – there never will be any 100% consensus about ranking of speaker design goals.

Pro Audio stuff is usually both – more sensitive and equipped with bigger voice coils than standard audio gear.
The background why they are doing so is to optimise first SPL capability and second reliability.

Fortunately bigger voice coils and top notch sensitivity improve *also* Thermal Distortion.

Compared to home-fi, PRO stuff always run at way higher SPL levels – hence home-fi and PRO stuff actually work in comparable Thermal Distortion ranges.


To put things into perspective you can also read about Keith Howard findings

http://stereophile.com/reference/1106hot/

Though I don't exactly agree with his conclusions at the end of his article it nevertheless might put another shade of light onto the subject


Michael

[soongsc]

Originally posted by mige0



If you follow my example in the paper linked below you can see that ?from plain calculation ?it affects even home and car audio.
But yes - sure ?there never will be any 100% consensus about ranking of speaker design goals.

Pro Audio stuff is usually both ?more sensitive and equipped with bigger voice coils than standard audio gear.
The background why they are doing so is to optimise first SPL capability and second reliability.

Fortunately bigger voice coils and top notch sensitivity improve *also* Thermal Distortion.

Compared to home-fi, PRO stuff always run at way higher SPL levels ?hence home-fi and PRO stuff actually work in comparable Thermal Distortion ranges.


To put things into perspective you can also read about Keith Howard findings

http://stereophile.com/reference/1106hot/

Though I don't exactly agree with his conclusions at the end of his article it nevertheless might put another shade of light onto the subject


Michael

You mention a lot about voice coils reaching 120 degC. May I ask how much continous audio power is necessary for it to reach that temperature? I do not see it possible in most home audio conditions. I would probably turn the volume down long before this kind of temperature is reached.

[mige0]

Originally posted by forr
Hi Mige0,
Some good points were raised in this discussion :
Voice coil heating and the performance of passive XO, plus my funny experience at HTG
See some data in Philippe Lasage's paper which is quoted in one of the posts.

What a thread!

Thanks for pointing me there, Forr - I may have saved my breath putting up the Thermal Distortion paper having read it back then.

Not much left to say especially after Brian Ding's postings.


Michael

[mige0]

Originally posted by soongsc

You mention a lot about voice coils reaching 120 degC. May I ask how much continous audio power is necessary for it to reach that temperature? I do not see it possible in most home audio conditions. I would probably turn the volume down long before this kind of temperature is reached.

Not sure what you are asking for – as you can read it in my paper.

Basically its like having guests for tee – you will need more time for preparing tea than at breakfast as you will have to get more water boiled.

Same with speakers. To reach a certain VC temperature it takes the time needed to heat up the VC mass (plus the energy to cover the losses due to cooling – which at some point will balance to a thermal equilibrium *if* VC can handle that temperature).

Your water gets boiled very fast with a beefy stove or slowly on the flame of a single candle
Same with speakers.

In the example given it takes a 10W input to increase VC temperature by roughly 100 °C for only 10 seconds.

Is this something you will have occasionally happen with your home speakers?

Brian Ding of rythmikaudio has given an other example if you like to have a second source.


But again – the discussion about perceptional threshold is a different one than about "is it happening at all" or how should we name it


Michael

[gedlee]

Michael

Your incorrect on one important point. The VC does not change temperature instantaneously. It begins to heat instantaneously, but there is a thermal mass that delays temperature changes and its temperature that affects the voice coils resistance. Further temperature rise and temperature fall at different rates being dependent on two different things.

IF the rise and fall time of the VC TEMPERTURE has a time constant that is significantly lower than any signal frequency then the Dif EQ is linear and there is no IMD of THD ("Nonlinear distorion"). There will always be effects of VC changes on the crossover and output power over the longer term and these things depend mostly on the thermal disidation capability of the device.

But no where that I have seen does anyone look at the effect that could occur with smll drivers where the VC heating and cooling could happen at rates fast enough to follow the signal. This aspect seems to be overlooked.

To me its mostly about these time constants as a very slow time constant of temperature effects would simply cause slow and small variations is the frequeny response and would not be heard dynamically. In fact they would be hard to hear at all. But changes on the time scale of the musical dynamics could seriously affect the perceived dynamics of the music.