port tuning

Let's see...ooops my crystal ball just broke.  I got a vision of a few things though...you drive a honda hatchback, an older model...it's maybe a '91...you were born about 6 years before the car was...you listen to Eminem and Young Gunz...but there's something missing.  Oh yeah, the subs!  I can't visualize what subs you have!  How can we help without knowing that??

Really though, that might help.  But it could help you, too, because almost every sub manufacturer lists some recommendations for vented boxes in their manuals.  Most are tuned to about 40 Hz.  That's not necessarily where a box designer/builder would end up tuning the same subs...but it seems a pretty good choice for rap and hip hop.  The designer might try to tune a little lower in most cases for an everyday listening situation like you have.  It all boils down to the space you have to work with and the T/S parameters of the sub.

Remember:  you're not going to get much output below the tuning frequency.  The sub becomes unstable under that frequency and so the lower freqs have to be filtered out. 

Here's two examples of low bass freqs that I've tested:  the low bass in the live version of Hotel California is at 40 Hz.  And this other one is a pop song but I know you're familiar with it - Doncha wish your girlfriend could be...by that all girl dancing group.  The bass that finally booms in that song is at 41 or 42 Hz.

Um, that's just the way I always talk.  For an old white guy did I get the Young Gunz part right?  The 40 Hz thing is what I was getting at if that's anything close to what you were thinking.  And, oh yeah, I design vented boxes so I know a thing or two about it.

i know you know what you talking about cause i read some of your post and i was hoping you be the one who answer my question and i wasn't being mean im just getting a big ass head ache from this box.  I want this one to be the last and i want everything to be pefect  since im put alot of work on this one.

Here's a Unibox workup for two of these subs wired in parallel (2 ohm load):

Assumptions:  the amplifier is producing 700 watts for this pair of subs;  the box as you have already built it will have a net airspace of 3.88 cu ft after accounting for the slotted port dimensions I will give you;  you are willing to give up the round port and change to a slotted opening.

parameters link on this page

I inputted the parameters as given and found Sd by looking to see that the Mms and Bl matched the given specs.  Then I connected two drivers in parallel in the program.  You can indeed obtain a much higher SPL in a vented box than a sealed, and as long as the tuning frequency doesn't go below about 40 Hz the port length remains reasonable.  This box wants a larger port opening than you can achieve with a 4" round port;  the round diameter needed is 7.87".  So this calls for a slotted port.  The equivalent sq inches of opening is 48.6" so I recommend designing the slot to be 4" X 12".  You can change the slot opening to fit your box as long as it maintains a 48 square inch opening area.  The length of the port will depend on the desired tuning frequency.

Either of the following two charts shows good output for rap, with box tuning shown first at 45 Hz and second at 40 Hz:

• With box volume for 2 subs at 3.88 cu ft net and slot port  4" X 12".  This Fb of 45 is obtained by using a port length of 10.75"

vb_response_xplod_xs-l122p5atfb45hz.gif

• This next one increases the port length to 15" to yield a tuning freq of 40 Hz

vb_response_xplod_xs-l122p5atfb40hz.gif

Tuning to a lower freq than that yields less SPL and results in (possibly) unmanageable port lengths.  The workups as shown assume a well-built box of rigid structure, and a bit of polyfil stuffing glued to the walls (but not within or impeding the opening of the port structure).  The end of the port inside the box must be at least 4" away from any part of the structure and the port itself can be elbowed to fit inside the box if necessary.  Apply a subsonic filter at about 30 Hz to 35 Hz if possible.  As long as you give these enough clean power they should be quite loud enough for you.

Take two Excedrin and let us know how it turns out.

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

12 x 4 slot port is great, just what I was figuring on.  Put in some extra bracing in that box that will take up approx. 1/10 cu ft to get the net air volume down to about 3.9 and use one of the port lengths I spec'd above.  Tuning 40 to 45 is optimum for these subs and will work very well for bass-heavy rap.  The port length would grow substantially longer trying to tune to a lower freq and meanwhile the SPL falls.  The little bit of low end extension gained from this is not at all worth it.  So yeah, 40 to 45 is my final answer.

In fact, look at those charts again.  The F3 shown is the -3db level and is quite good at 33 to 36 Hz.

From the port lengths I showed above...10 3/4" and 15" for the respective Fb's...you can see that any length between and including those will result in a tuning freq between 40 and 45 Hz.  Use whatever length works best to displace the right amount of air space.  (The port air space is not counted with the 3.9 cu ft needed for the subs).  So you have to add and subtract back and forth a few times, add extra bracing if you need more adjustments, etc.

.......oh hell I read your post again about the box volume.  I see that the port structure will take up some of that 4 cu ft air space.  I had thought that you had already displaced some air with whatever port size you were working on....I'll look it over again.  But I'll note right now that putting the port against a side wall will save a little bit of volume because you only have to add one MDF baffle for the port side....but I'll look at it with the port centered per your design.

I'm not really following you as far as how much volume this box is going to end up with...but here is what I came up with.  For a box that when empty except for the two subs has an internal volume of 4 cu ft :  to get a tuning freq of 40 Hz the port needs to be 4" X 12" X 18.3".  That port structure, if you figure 3/4" wall on each side, is 5.5" wide and 12" tall from bottom to top of the box.  The total displacement of this port is .698 cu ft.  Deducted from 4 cu ft leaves net air volume of 3.35 cu ft, or 95 liters.  Here is what the output would look like:

vb_response_xplod_xs-l122p5at40hzfb18long.gif

When I want to do quick and easy calculations... and you have to do this repeatedly while you're finding box net air volume vs. what the port takes up....I use the box rectangle calculator on this site.  https://www.the12volt.com/caraudio/boxcalcs.asp  I usually just figure in the side walls' thickness and then put 0 in for thickness when I use that calculator.  But here's the thing with your final build:  just keep in mind that you have to add a little length to the port if the box air volume gets smaller, and conversely you have to subtract a little port length if you end up with more net air volume than the 3.35 that I arrived at here for an 18" long port.  But you got the idea of the relative play in port lengths you have in order to keep the Fb at around 40.  Shorten the port length by 3 or 4 inches to get it closer to 45 Hz.

.....and glad to help, you're welcome.  Keeps me up on it so I don't stress out too much when I do these for my own projects.

My guess is the two subs together mounted to the box would displace about .15 cu ft.  You might try to search online through some of the new line of xplod subs and see if there is a displacement value shown on any of them.  But you should be pretty close using the total of .15 cu ft.

So if you're modifying the box to have 4 cu ft left over after all is put into it, make the port length 14.33" long to get 40 Hz.  10.25" long to get 45 Hz, which is just a tad louder SPL. Or use any length in between.   And keep the 4"x 12" slot opening, which doesn't change.