Originally Posted by williamhung
Hey Fade quick question, How do you determine proper port area by using displacement and xmax numbers? always wondered about this and nobody seems to be on the same page about it.
Minimum vent diameter is determined by displacement (Sd * Xmax) and tuning frequency (Fb).
There are lots of handy dandy calculators online, but honestly... they're grossly unrealistic. If you plug in an 18" with 30mm of Xmax and a tuning of 30hz... it'll spit out a minimum
port diameter of 22". I don't have to tell you that in an average size box for an 18", a 22" diameter port would be like 15 f'n feet long.
(This one is a bit more forgiving) http://www.carstereo.com/help/Articles.cfm?id=31
But here's the thing... vent mach (how fast the air in the port is moving) is highest
at tuning (actually, just below). But you know what is lowest
at tuning? Excursion.
Here's an Ethos 18" tuned at 30hz on 2,000w. Remember, power is also a factor insomuch as it dictates excursion.
The whole principle behind a minimum port diameter is to avoid port noise, maximize output and minimize compression. When you input these values, they're assuming the Xmax number is what it's going to be doing at tuning. But as evidenced above, our subs don't get to max throw at tuning where vent mach and the chance for chuffing is highest.
With that said... if we entered the number from above- which looks to be ~ 5mm, we get a minimum port diameter if ~ 9". That's doable.
So where does that leave us? Modeling. I know WinISD and Bagby's software can calculate vent mach.
Below is the Ethos 18" again. Again on 2k. First let's do a 6" round port.
Now... it's generally accepted that 5% the speed of sound is the threshold for port noise. That's ~ 17 meters per second. A lot of guys will use a max of 30m/s and still consider that "acceptable". Either way... you can see that a 6" is way too damn small for a long stroke 18" on high power. Now that peak of ~ 55m/s is well below tuning and would be mitigated by our SSF anyway (hopefully), but even at tuning we're still looking at ~ 42m/s. Still too high.
Now if we bump up to an 8" port...
Bingo, bango... we're now below 30m/s to ~ 23m/s at tuning and probably 26-27m/s before our SSF catches it.
How big do we need to go to get to < 17m/s? About 10"
Let me close with this...
This is the Ethos (again), but this time on lower power (500w)
Now let's go back to the 6" port that was way too damn small...
HUH!?!?!? But, T-Fade... I thought you said a 6" was too small for an 18"!?!
Nope, I said it's too small for the displacement of a 30mm Xmax 18" on 2k. Looks to be fine for low power or less stroke.
It's important to note that nowhere in this diatribe did I mention enclosure volume. Displacement, people!!!