# 'Warm' VS 'Cold' Drivers a comparison



## avaserfi (Jul 5, 2007)

On another forum I participate in there was a discussion of if it was needed to warm up a driver before measuring its T/S to derive proper specifications. To dispel this myth I spent some time taking some relatively in depth measurements. Using a calibrated WT3 and a Dayton RS150 I took 5 measurements with a 'cold' driver and 5 measurements with a 'warm' driver each after 30 minutes of a 100Hz sine wave. I averaged the information and found total percent difference as well as modeling the two different averages in WinISD Pro.

The compiled data:









The modeling [Blue is warm - Yellow is cold]:










Upper end frequency response is not shown due to inaccuracy of modeling. It is highly unlikely that there is any _audible _differences between 'warm' and 'cold' drivers even if there are minor _measurable _differences.

Please note even this set of data is far from statistically complete, but should be a good example of real world situations.


----------



## thxgoon (Feb 23, 2007)

Very interesting. I've always wondered if there was any weight to the old speaker warmup phenomenon. Good work!

Out of curiousity, how much power did you feed the driver for the 'hot' test?


----------



## Mike P. (Apr 6, 2007)

Good post, Andrew. Thanks for sharing.


----------



## avaserfi (Jul 5, 2007)

thxgoon said:


> Out of curiousity, how much power did you feed the driver for the 'hot' test?


Not to sure to be honest. I hooked up the driver to my LPA-1 [free air] turned on a 100Hz sine wave and just turned up the volume until the driver was at about half xmax [2mm] which was about 70dB then I let it run for half an hour.


----------



## Bill Fitzmaurice (Jun 14, 2008)

Warming a driver before testing is unnecessary. However, testing at a nominal environmental temperature of 70 to 80 degrees is required. A cold woofer can exhibit an fs as much as 50% higher than normal, due to stiffening of the suspension.


----------



## kyle_k (Oct 10, 2007)

Interesting tests... 

I would first ask what voltage you use for that sinwave? I have personally measured woofers and seen over a 20% difference from warm to cold. I used just the basic Klippel LSI program. Its really a function of how much current you're putting through that coil, basically how high your volume knob is. The more voltage you put on the coil, the more current and the larger differences you will see because the resistance of aluminum and copper both have a temperature coefficient that is not zero, in fact its not even linear, but within our needs it pretty much is. You also don't really need to re-measure the entire TPS's, all you need to do is look at the resistance and that will tell you your losses. 

I have seen Qts's shin from 0.5 to 0.7 within 10 minutes of a large signal. That is VERY audiable. Power compression is a very real phenomena and not well understood or dealt with. A ~0.42% increase in resistance will lead to a 1.5dB loss in SPL. In pro audio, this occurs much more frequently.


----------



## kyle_k (Oct 10, 2007)

I would also like to add, its pointless to claim that warm is ideal, much the same way as its pointless to claim that cold is ideal. Unless we contract a device to linearize the compression, you're not ever going to have a constant set a TSP's to model from. i guess warm is probably a closer to the average, but again, this could change depending on listening habits


----------



## avaserfi (Jul 5, 2007)

This test was not designed to measure power compression of a driver this is an established phenomenon which effects poorly designed drivers more than higher quality drivers. Rather this test was to measure the T/S specification differences between a driver that is 'cold' and has _had _no current run through it and a driver that is 'warm' meaning it has _had _[not still does have] current run through it. To see the difference in T/S within a driver with regard to power compression would require the T/S specifications to be measured at the moment of high power input. It should be noted that this could be far more easily measured by doing high SPL frequency response measurements using a microphone and deriving power compression from a lower SPL reference point.


----------



## kyle_k (Oct 10, 2007)

avaserfi said:


> This test was not designed to measure power compression of a driver this is an established phenomenon which effects poorly designed drivers more than higher quality drivers. Rather this test was to measure the T/S specification differences between a driver that is 'cold' and has _had _no current run through it and a driver that is 'warm' meaning it has _had _[not still does have] current run through it. To see the difference in T/S within a driver with regard to power compression would require the T/S specifications to be measured at the moment of high power input. It should be noted that this could be far more easily measured by doing high SPL frequency response measurements using a microphone and deriving power compression from a lower SPL reference point.


but warm vs. cold is exactly a thermal compression so i'm confused.
:\


----------



## thxgoon (Feb 23, 2007)

kyle_k said:


> but warm vs. cold is exactly a thermal compression so i'm confused.
> :


Kyle, I think in the case of power compression we would be talking about much higher than normal input.



avaserfi said:


> On another forum I participate in there was a discussion of if it was needed to warm up a driver before measuring its T/S to derive proper specifications.


----------



## kyle_k (Oct 10, 2007)

ok, so we're going to claim that power compression is a lot of voltage vs a little... even tho the equations stay the same?

I'm cool with that, but as long as we understand we're taking bout the same thing... big or small. A change in DCR is a change in temperature!


----------



## avaserfi (Jul 5, 2007)

thxgoon is right thermal compression occurs at higher output levels [at least in quality drivers] this test was not done to measure this but rather changes in T/S specifications after a driver has been warmed up in more normal listening conditions. 

Please note transient peaks are far less likely to cause thermal compression [in quality drivers] due to their relatively short duration.


----------



## kyle_k (Oct 10, 2007)

avaserfi said:


> thxgoon is right thermal compression occurs at higher output levels [at least in quality drivers] this test was not done to measure this but rather changes in T/S specifications after a driver has been warmed up in more normal listening conditions.
> 
> Please note transient peaks are far less likely to cause thermal compression [in quality drivers] due to their relatively short duration.



Well, I disagree


----------



## cixelsid (Mar 6, 2007)

Anyone doubting the effects of VC heating needs to have a little chat with Dr Klippel....
http://209.85.141.104/search?q=cach...ers&hl=en&ct=clnk&cd=5&gl=us&client=firefox-a


----------



## Bill Fitzmaurice (Jun 14, 2008)

avaserfi said:


> power compression of a driver... is an established phenomenon which effects poorly designed drivers more than higher quality drivers.


In the relatively low power realm of hi-fi and HT, perhaps. In the pro-sound world, not really. With even the best engineered drivers at power inputs of 400 watts power compression of at least 3dB is a fact of life, while at 800 watts you can pretty much count on at least 6dB. This fact of life should make one take the 'calculated maximum SPL' ratings of speakers with 400 watt Pe ratings with a bag of salt, and in the case of 1kW drivers, a truckload. That same caveat would also apply of course to the highly over-hyped/over wattage world of autosound.


----------



## avaserfi (Jul 5, 2007)

Bill Fitzmaurice said:


> In the relatively low power realm of hi-fi and HT, perhaps. In the pro-sound world, not really. With even the best engineered drivers at power inputs of 400 watts power compression of at least 3dB is a fact of life, while at 800 watts you can pretty much count on at least 6dB. This fact of life should make one take the 'calculated maximum SPL' ratings of speakers with 400 watt Pe ratings with a bag of salt, and in the case of 1kW drivers, a truckload. That same caveat would also apply of course to the highly over-hyped/over wattage world of autosound.


I never said assuming a quality driver would be linear in moderate to high output scenarios was a good idea, in fact it is a very poor assumption. Rather it would take proper measurements of the driver to establish real world performance.

I just stated that higher quality drivers are _less _effected than lesser quality ones. At such high power I would fully expect extremely well designed drivers to have a reasonable amount of compression.


----------

