# port length calculation

slipitysmooth
Member - Posts: 24
Joined: November 24, 2003
Posted: March 14, 2008 at 5:36 PM / IP Logged
Hey...stupid question, just want to make sure im doing this right in winisd/my scratch paper.
If i am trying to sketch out the port with dimensions of 1.5w x 12h x 36.75l Would I add that extra 1.5 inch of length for the green area in the picture to only one side...correct, so if you add the 25.75+9.5+1.5 you would get 36.75 correct?
sedate
Silver - Posts: 1,173
Joined: July 03, 2004
Location: Colorado, United States
Posted: March 14, 2008 at 10:12 PM / IP Logged

Uh - yes.  That's what I get as well.  Correct.

"I'm finished!" - Daniel Plainview
stevdart
Platinum - Posts: 5,816
Joined: January 24, 2004
Location: Pennsylvania, United States
Posted: March 14, 2008 at 10:21 PM / IP Logged

Your answer is correct but it's not added "to only one side", but added only to the short side.  Put in all the measurements in your sketch so you can double-check yourself:

You can double check your results this way:

Sum the long side measurements, then sum the short sides.  Find the difference (which is 3 in this case), then divide the difference in half (as if you are measuring through the center of the port).  Add that half (1.5, here) to the short side for a true measurement of the port.

Here, we have a long side of 38.25 and a short side of 35.25.  The difference is 3, so that is divided in half and you either add that half of the difference to the short side or subtract it from the long side.  You will find the center line value that way.  The length comes to 36.75.

Note:  When you do the build, make sure you include the thickness of the front baffle of the box as part of the port length (if the port runs through the front).

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
slipitysmooth
Member - Posts: 24
Joined: November 24, 2003
Posted: March 15, 2008 at 7:14 AM / IP Logged
Thanks for the help there, also, here is the graph of this sub in winisd...
Does that look about right...the port size is 1.5x12x36.75 to get the tuning to 24.5 Hz and the graph as shown. The port velocity maxxes at around 45 fps rms or 65 fps max at 25 Hz. Also, the cone passes its max excursion at or below 20 Hz so I would use the filter that the amp has on it. Does all of these things sound about right? I drew out some plans last night for a box around 2.6 cubes, which after the port, sub, and bracing, should be right around 2...thanks for any help. Also, if you need to see any other graphs just let me know and I can throw them up here.
stevdart
Platinum - Posts: 5,816
Joined: January 24, 2004
Location: Pennsylvania, United States
Posted: March 15, 2008 at 11:46 PM / IP Logged

I took a glance at the RE website, at the parameters for a SE series 12:

Vas is said to be 72.9 liters.  Your program has the information that the Vas is 1.26 cu ft, which is exactly half of what is should be.  Make the change in the database editor.

Entering parameters is tricky.  You have to look at the various parameters given by the manufacturer and figure out how the particular sub in question has been tested.  This sub is given as available in either dual 2 ohm or dual 4 ohm configuration.  I know that testing is done with coils in series because best Bl is achieved in that manner.  So...after entering all data with the program set to the default mode of 1 voice coil in parallel, change to 2 voice coils in series.  Your goal is to mimic the tested parameters in your program.

You will find that Re changes to something that looks extraordinary.  You will always have to make another change to the Re parameter.  I just use reason for this:  if your sub is a dual 4 ohm and you have it parallel wired, you still want to see what the program says it should look like as if it were series wired, because that is how it was tested to arrive with the published specs.  Two coils at Re 2.8 ohms will make 5.6 ohms in series.  Use that and watch the Bl number get calculated.

You also want to always make sure that the auto-calculate box is checked.  Bl almost never arrives to the same figure as published in the specs, but your program shows the result as the same to the tenth of a point.  That leads me to conclude that you unchecked the auto calculate so that you could get through the process of entering data without errors.  Let the program figure T/S and it will give you a "second opinion", so to speak.

Back to Re:  I know that the Re of a 4 ohm coil is in the range of 2.8 ohms to 3 ohms (always less than nominal impedance).  So by reason, two coils in series is additive and the Re will be twice that amount.  Watch the Bl as you fiddle the Re factor and it should auto calculate to a value that is close to the published specs.  I arrived with 18 Bl while the parameters show it to be 17.7.  If it is as close as that, you will know that you have allowed WinISD to perform the true T/S model calculations with the proper input to work with.

And bear in mind:  even though you may be paralleling the coils for your application, you want the parameters in your program to mimic those that are published.  You almost have to have inside information that testing is always done with coils in series.

Here are some file snapshots showing a vented configuration that I think would be good:

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
slipitysmooth
Member - Posts: 24
Joined: November 24, 2003
Posted: March 16, 2008 at 8:43 AM / IP Logged
Hey thanks for the help man...I restarted the entry carefully and got them to match what you show...very tricky stuff. Also, I think Im going to go with a port tuned to 25 Hz, it gives a smoother curve with less then a 1 db hump right around the 50 Hz range, so it should be able to play a broader range without getting boomy, is this logical thinking? The port length is longer, but I'm building a very long and skinny box, so I should have room...
I was thinking of going with this amp... Alpine Mrp-m500
Now it says subsonic 15Hz filter...is that adjustable to say 20 or 25 Hz or not?
stevdart
Platinum - Posts: 5,816
Joined: January 24, 2004
Location: Pennsylvania, United States
Posted: March 16, 2008 at 1:05 PM / IP Logged

I generally would look toward a slightly lower Fb too, but in car audio there are all the compromises to consider.  That sub's parameters show it to be well suited for a sealed box, so when you build a vented for it the port size sometimes becomes dramatically large.  I don't particulaly like 3 foot port lengths, so I look for some compromises to achieve a well balanced sound and reasonable port length.

You also look at the manufacturer's recommendation for Fb, which for this sub is 33 Hz.  There's some good reason why they stated that tuning frequency (being a reputable company), so consider that in your design.

Check the amp manual for that subsonic filter info, but in all reality a 15 Hz filter should be fine.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
slipitysmooth
Member - Posts: 24
Joined: November 24, 2003
Posted: March 16, 2008 at 6:05 PM / IP Logged
thanks for all your help...got my final box design...gonna start it this week.
stevdart
Platinum - Posts: 5,816
Joined: January 24, 2004
Location: Pennsylvania, United States
Posted: March 16, 2008 at 8:23 PM / IP Logged

Happy to lend a hand, and please let the forum know how it turns out.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

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