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Originally Posted by WagBoss View Post
when modeling drivers in winISD, and setting up xmax, eq and cone excursion, should I be making it so that PEAK power is hitting xmax, or RMS power is hitting xmax?
RMS. Engineering always uses RMS. Peak is used by marketing departments.
Engineers understand how their tools work first....
WinISD shouldn't be labeling the input amplitude as "input power" because really, under the hood, the program is immediately converting the "power" to an RMS voltage, and then doing the calculations. If you want to know the actual power, then you need to look at the "amplifier apparent load power" which is taking into account the impedance of the resultant system.
Also, WinISD is already taking into account the conversion from RMS to Peak for the cone excursion:
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Cone excursion is calculated by first determining volume velocity through driver. Then, volume velocity is integrated, and the result is then divided by Sd to get cone excursion. Still, we need to scale result by sqrt(2) if we want to get peak value and 2·sqrt(2) if we want peak-to-peak value.
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Cone excursion shows how much driver cone moves with sinusoidal excitation at chosen powerlevel. The powerlevel is controlled in "plot"-tab. The power applied can be related to excitation voltage with following relation: Eg=sqrt(P·Re), or P=Eg²/Re where Eg is the RMS voltage applied to driver's terminals, P is the input power in watts and Re is the DC resistance of the voice coil. Please note that there is few different ways to express this value. WinISD can be configured to show RMS, Peak, Peak-to-peak (p-p) values of the cone excursion. RMS value is defined just as RMS value of any sine waveform. Peak value is the difference between zero and the maximum value of sinusoidal waveform. Peak-to-peak is twice the peak value, i.e. difference between minimum and maximum point of waveform. The peak value is perhaps a most practical expression, because driver parameter Xmax indicates how much cone can be deflected from it's rest position linearly, in either direction.
If you want to maximize power handling of any box, then adjust the box parameters so that cone excursion is kept at minimum value possible. Of course the transfer function magnitude graph should be taken into consideration also. In closed box, the minimum excursion is obtained, when enclosure is as small as possible. Same basically applies to vented box, but there is a local minimum at port tuning frequency.
When comparing graphs between programs, please note that many programs give the RMS excursion which is "wrong", in my opinion. I have seen some programs, where the calculated excursion is RMS value, and limit is shown as peak. That gives over-optimistic power handling impression. Please also note that this graph doesn't take nonlinearities into consideration. But it let's you see when the nonlinearities are becoming too great.
All of these descriptions came directly out of the WinISD help file.
Btw, the Maximum SPL plot takes into account the Power and Excursion Limits of the system - just the same as Hornresp:
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Maximum SPL graph is a combination of SPL and maximum power graphs. It tells how loud particular design is able to play, considering Xmax and Pe restrictions. If calculated power to reach Xmax is greater than Pe, then Pe is taken as input power to the driver. With this, you can easily visualize potential of particular design. Please note that it doesn't apply to room or in-car environment, what could change things quite a bit. It is useful as an comparison tool, e.g. comparing same driver in closed or vented box.
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What I've been doing is setting a transform to basically put the driver to xmax at all frequencies below 20 hz, then checking the apparent power to make sure it's not going way over the power handling of the driver. Is this the best order to do it? I assume power handling is more forgiving than xmax, so it's better to design based on xmax?
With power handling, you just turn down the volume to reduce the power....keep in mind that a 3dB change in output level (which is really quite small) will cut the power needs in half....so don't get too hung up on the power numbers.
The way I would approach your problem is take a look at the Maximum SPL plot, and you will see a line decaying at 12dB/octave starting at the point where the maximum power reaches the maximum excursion of the system....and then below that point, you will always be excursion limited.
In the excursion limited region, you can pick the lowest frequency you want reproduced, and then the maxSPL plot shows you the maximum SPL you can have with a flat frequency response. Then you would design your filter such that your frequency response is flat to that frequency, and you've now set the safe bandwidth/maxSPL tradeoff of your system. In the process, you're giving up maxSPL at the higher frequencies unless you set the gain structure of your system such that you can exceed Xmax at the lower frequencies.
Now if you wanted to minimize the solution size of your system, then you could decrease the size of the enclosure until the maximum power achieves the maximum excursion at the lowest frequency you want to reproduce...