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RE: Thermal Distortion Paper

Posted by morricab (S) on March 19, 2025 at 04:04:49

In Reply to: RE: Thermal Distortion Paper posted by tomservo on March 18, 2025 at 07:59:15:

I think this is a problem at all volume levels and not just at high SPL peaks, especially for low sensitivity speakers. I think it becomes a real world problem in home audio and why so many speakers don't have a realistic perception of dynamics.

The reason for that has a lot to do with the hysteresis between heating up and cooling down.

Unless you have active cooling (like a fan or liquid) you will have a MUCH slower cool down period than the rapid heating up of the coil. This was also highlighted in the paper put forward.

What this means is that big peaks, which as you rightly pointed out will be blunted by lower sensitivity speakers, will be further damaged upstream as the heat persists and affects subsequent peaks even worse if they are coming in fairly rapid succession, not to mention probably unbalancing the speaker for a long time after those peaks until the temp comes back down to whatever the average working temp is.

If you can get suitable in-room SPL from a few mW on average and less than a watt peak, then you are not putting a lot of heat into your voicecoil and the driver should then be able to scale dynamic peaks without significant compression or distortion.

If you have a 100dB speaker and you are running with average levels of 75dB and peaks of 90dB then your average levels are only using maybe 10mW at average levels and a few hundred mW at 90dB. Given the calculations from that paper, which of course wouldn't be totally accurate unless for a specific voicecoil, it would mean that the VC is never more than a few degrees above ambient for that speaker and relatively unaffected by the heating of the coil.

An 85dB speaker will fare worse because now we are using well over 1 watt for peaks and probably a few hundred mW more or less continuously. That will simply serve to rob the headroom that it would have if the peak came from the cold (0-90dB in mS). Now the ambient temp of the coil is probably well above ambient and more or less stable due to the hysteresis. That just puts the coils already at a higher than ambient temp, reducing the time and energy needed to get really hot and truncate the dynamic range.

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