Midrange enclosure design, WinISD, Fsc

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smprather1

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Joined 2014
#1
I can't believe I'm asking this question. I've scoured The Google, and I can't really find an answer (maybe it was there and I just didn't grab it).


Scenario:

  1. I have create the wdr in WinISD (entered my midrange TS params).
  2. I created a new project. Sealed. 2 drivers (will be MTM)
  3. I am trying to hit a Qtc of ~0.55'ish based on forum reading (Speaker Builder magazine did some testing and found this to be the subjective listening best)
  4. Of course, I can change the internal volume easily in WinISD
  5. I enter a volume of 0.4 cu-ft
  6. But there is a gray'd out box with a number in it - Fsc. I can't change this number directly, but if I mouse over the box image, hold left mouse button, and sweep left and right, I can make the number change.
  7. I sweep left until Qtc goes to 0.55.
  8. So box volume is 0.4 cu-ft and Fsc is 78.4Hz.
Ok, great. Now how do I build this box? Of course, volume is easy. But what about the Fsc value? How do I design the box to hit the Fsc that I mouse-swept to??


Thanks so much!
--Myles
 
#2
Ok, I think I have a clue now. I added a high-pass filter (LR 4th order, 400Hz). Now when I mouse sweep over the box, the box volume changes (only Fsc changes if you have no filter). Now I can dial in a target Qtc by changing the volume. So I think I am good to go, but I would still be interested in an explanation if anyone has one. Thanks again!
 
#4
But I can't set Qtc. It is only calculated. The only thing I can enter is box volume and filter settings. Once I set a filter, Qtc becomes a strong function of box volume.


Yes, the peak impedance is always at Fsc.


I guess the open question is: If I don't have a filter set, the what is WinISD sweeping when left-button-hold sweep over the box? The box volume stays fixed. But Fsc and Qtc change.


Like I said, I'm good to go now that I have a filter set. So it's just an academic question.


Thanks GM,
--Myles
 
#5
smprather1 said:
[5]I enter a volume of 0.4 cu-ft
[6]But there is a gray'd out box with a number in it - Fsc. I can't change this number directly, but if I mouse over the box image, hold left mouse button, and sweep left and right, I can make the number change.

Ok, great. Now how do I build this box? Of course, volume is easy. But what about the Fsc value? How do I design the box to hit the Fsc that I mouse-swept to??
Click to expand...


Since Qtc, Vb, and Fsc are interrelated, when you select Vb, the other two values are determined by the volume you've selected. They can't be changed without changing the driver's parameters, or making changes to the box.

If you want to choose an Fsc instead, then Qtc and Vb will be simultaneously determined by your choice.

smprather1 said:
But I can't set Qtc. It is only calculated.
Click to expand...

I downloaded WinISD to have a look. After choosing closed box, it asks you to choose the alignment. You are choosing between discrete values of Qtc in this step. For a value between these choices:

Vb = Vas÷((Qtc/Qts)2-1)

Use the Qtc desired, and the T/S parameters to estimate your target Vb. Note that actual driver parameters will vary from manufacturer data, so there's no point in getting too accurate if you're not measuring drivers and end results yourself.

Fsc will increase over Fs in proportion to Qtc/Qts. So, if your Qtc is 1.5x higher than Qts, then Fsc will be 1.5x higher than Fs. See the disclaimer above about accuracy.

smprather1 said:
I guess the open question is: If I don't have a filter set, the what is WinISD sweeping when left-button-hold sweep over the box? The box volume stays fixed. But Fsc and Qtc change.
Click to expand...


Sweeping changes Vb, simultaneously changing Qtc and Fsc. It seems to glitch and not show a changing Vb, but that *is* what you're changing with the mouse. Simply exiting the "Box" window and returning to it will cause it show the Vb changes again, it doesn't require setting a filter or any other modifications. At least, not for me.
 
#7
Yep. I got bit by a GUI bug. Adding a filter spoofed me into thinking that was part of the Fsc/Qtc calculations, when all I was really doing was un-triggering the bug. I didn't think that there was a connection between filtering and Qtc, but while trying to figure out what was going on, I read some Qtc threads about the "series capacitor trick". So that made me think maybe the signal filtering *could* impact it. And I certainly couldn't figure out why sweeping the mouse would change anything other than box volume (and/or tune, in the case of vented).



Anyway, thanks so much guys!
 
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