You also loose playing range the less your vent mach is. NOt every box is made for maximum output.
And I did a 2 sundown x-15's 4th order with vent mach like 35 m/s and he did over a 150-151 @ 28 hz. Most people don't want straight SPL.
Port area!!!!
6
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.
Links...
[url]http://www.ajdesigner.com/phpvent/subwoofer_vent_port_equation_minimum_diameter.php[/URL]
[url]http://www.diysubwoofers.org/misc/portcal.htm[/URL]
(This one is a bit more forgiving) [url]http://www.carstereo.com/help/Articles.cfm?id=31[/URL]
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!!!
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Box be too big
Yes the port has its own frequency response and dampening but if you have any fall off above tuning you need to shrink your box a little bit
There wasn't a huge fall off above tuning. Box was tuned @ 43 hz front chamber and played up to 55-60 hz strong from what I understand. But keep in mind the box was made to play like 25-35 hz strongly as requested by customer.Box be too big![]()
Yes the port has its own frequency response and dampening but if you have any fall off above tuning you need to shrink your box a little bit
Did you try more port area? You might actually gain unless the dampening isn't high enough.There wasn't a huge fall off above tuning. Box was tuned @ 43 hz front chamber and played up to 55-60 hz strong from what I understand. But keep in mind the box was made to play like 25-35 hz strongly as requested by customer.Box be too big![]()
Yes the port has its own frequency response and dampening but if you have any fall off above tuning you need to shrink your box a little bit
I've built an tested a hand full of boxes that had to much port area I agree with you there is a limit before the damping is to low to have cone control.
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