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WinISD System Input Power vs Voltage

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itskabini View Drop Down
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    Posted: 03 August 2021 at 7:16pm
I've got myself in a state of confusion trying to set up the signal source in WinISD

My amplifier is speced to deliver 1550W into 8 ohms, and therefore should have an output RMS voltage of 111V (V = sqrt(P*R)). My driver is an 18tlw3000 8 ohm (although with a 4.6ohm Re)

when i input a 1550W system input power in WinISD it sets the driver input voltage to 84.4V, however I'm thinking my amp can supply 111V, which would (according to WinISD) give a system input power of 2500W.

Which is the correct value to put into WinISD, and am I just misunderstanding the difference between impedance and resistance?




Edited by itskabini - 03 August 2021 at 7:18pm
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ceharden View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 03 August 2021 at 7:54pm
Be aware that if you are trying to model cone excursion from input power, the values you will get from simulations such as WinISD will be a rough guide at best.

Seems a bit odd that WinISD would calculate the input Voltage from Re.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 04 August 2021 at 7:42am
In WinISD, the power field is calculated from the applied voltage and the minimum impedance that the model predicts, which is usually close to but not exactly Re. 
That power is therefore a kind of "worst case" figure for what the driver might see when that voltage is applied.

If you know your amp is 111V for a nominal 1550W @ 8Ω then that is the figure I would use for modelling.

+ 1 to Chris's comment about the excursion prediction being approximate too - it simply assumes that SPL, Excursion etc scale up completely linearly as the applied voltage is increased, which isn't the case in the real world.

HTH,
David.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote itskabini Quote  Post ReplyReply Direct Link To This Post Posted: 04 August 2021 at 1:35pm
Would excursion tend to be less or more than is predicted by winISD? Or is it impossible to say. 

I'm trying to set my limiters as best I can to not blow any of the drivers in a sealed box, while getting maximum volume. Also trying to see whether it's best to set the limiters at exactly xmax predicted excursion or somewhere between xmax and xlim.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 07 August 2021 at 5:30pm
Hi,
Sorry for the late reply, almost forgot about this thread.

Unfortunately, it's not exactly possible to account for all of the real world variations with a program like WinISD.

In the real world, what generally happens as we push drivers to higher excursion is that the effective BL & Cms both reduce compared to when the driver is not moving so much. You can do some reading on the subject of "Klippel analysis" to learn more about that. Re will also often increase due to the heat produced by the amount of power we have to apply to get the speaker to produce high SPL in the first place.

So, as a first approximation, we could save a different version of our driver's parameters with BL & Cms reduced and Re increased and then see what happens.

Here's an example of that; I've used 70% of nominal for BL & Cms & 150% of nominal for Re.
Both simulations assume the same drive voltage.


We can see that the frequency response is drastically altered, as well as the predicted excursion being different.

HOWEVER, the real world still won't be quite like this, because these parameter changes are going to be happening not just based on how loud we run the system, but repeatedly every time the cone moves. Every cycle of the input signal will have the driver spending some time nearer the centre of it's range of movement (thus with predicted response more like the blue, "standard" model), and some of the time with the driver further from centre, thus with response somewhat more like the red, altered model.

As for setting limiters, people who have blown more drivers than me often recommend setting at least 2 limiters with different time constants.

One has fast attack and release designed to stop excessive peaks, and one is set with a slower attack & release and also a much lower threshold to help reduce the thermal overload that comes from running for long periods at high power.

Ivan Beaver of Danley has written a lot about it, a search on Prosoundweb's forums will bring up lots of posts like this: PSW link

HTH,
David.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote KaphaSound Quote  Post ReplyReply Direct Link To This Post Posted: 10 August 2021 at 3:24am
David those are some very interesting plots. I know B&C give you xvar which is “ Beyond this excursion limit, the magnetic field seen by the voice coil, or the total suspension compliance, or both, drops to less than 50% of their small signal value, producing high distortion levels, strong variations from small signal behavior and power compression. The new technique yields different results from the standard measurement based on THD. B&C Speakers believes that this added information gives a more accurate and reliable description on loudspeakers behavior in actual operating conditions.”

I know you showed a 30% reduction in BL but I’m curious what 50% looks like? Always wondered if xvar was within “acceptable” distortion limits or if it gets truly poor at that point.


Edited by KaphaSound - 10 August 2021 at 3:26am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 10 August 2021 at 1:44pm
Originally posted by KaphaSound KaphaSound wrote:

David those are some very interesting plots. I know B&C give you xvar which is “ Beyond this excursion limit, the magnetic field seen by the voice coil, or the total suspension compliance, or both, drops to less than 50% of their small signal value, producing high distortion levels, strong variations from small signal behavior and power compression. The new technique yields different results from the standard measurement based on THD. B&C Speakers believes that this added information gives a more accurate and reliable description on loudspeakers behavior in actual operating conditions.”

I know you showed a 30% reduction in BL but I’m curious what 50% looks like? Always wondered if xvar was within “acceptable” distortion limits or if it gets truly poor at that point.

Well, 50% reduction would be worse, if both were affected to the same degree.
However, B&C don't specify which parameter is most affected for each driver, or how much the other one may be affected when the worse one hits 50%, so in a sense it's just idle speculation.

Also, remember that these changes only apply when the driver is at it's maximum excursion, which is only a relatively small percentage of the cycle - so for much of the times it's nearer to the mid point of the range of motion so the "simple" model is more applicable.
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