Slot Port Freq

A friend of mine just got a slot port box built from a car audio shop. The problem is that on low notes it does not hit as hard. The freq is suppose to be @ 32hz. The outer box dia is 31"x16.25"x12.5" with a port dia of2" wide by11" high by 14" deep. The subs are rockford 10" t1's, 2 of them. Is this box tuned right or not. I don't have much knowledge of slot  port designs,so any help would be great.

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furflier, the T1 series woofers are awesome woofers (I have 2 T1 12s). I guess we need a reference point on loudness. The box could definitely have been built to the wrong specs. Rockford's website has slot ported boxes specd out by their own techs that are supposed to be "very loud", but they do not say how well they perform at what freq. Also realizing that 10s are not going to produce the lows as well as 12 or 15 inch woofers. The 10's should give you great bowel disrupting hard bass with some decent low end. I just dont thing there's enough surface area to have 32hz keep up with the volume that the 10s will produce at 50 and above.

IMHO, a really good system (especially with subs in the rear) would have 12s running crossed over around 35-40Hz (at that low of a frequency, the human ear cannot tell what direction the sound is coming from). Then use 8s or 10s somewhere up front for the hard hitting 50-120(or 150).

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The Flakman
I feel strange. I have deja vu and amnesia at the same time.

John | Manteca, CA

Use some of the calculator help that is part of this website first.  You know about the section "Woofer Calculators".  What you should really do is pull one of the subs and look into the box to get a better feel for the type of construction used, so you can make a determination of net inside volume.  There is one port which indicates these subs share a common chamber.

Use the rectangular volume calculator.

Now, once you determine how much air space these subs have, use the port volume calculator to find how big the port diameter should be if the length is 14".  I know, it is a round port calculator but you can make a conversion.

When you find what port diameter it should be, you then find square inches of port opening by using pi R squared.  pi is 3.14, R is radius (half of diameter), squared means that the radius is multiplied times itself.  This will give you square inches.  The math works like:  square the radius, then multiply times 3.14.

The current port is 2 X 11, which is 22 square inches.  Make your comparison.

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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.

This is what I came up with: internal box volume 2.39218  port dia 2" freq32 port length 1.99325. Imultiplied 1.99325 times itself for 3.97304 then multiplied that by 3.14 and got 12.47536. Did I do this right or not?

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No, the port diameter in the calculator is for round ports.  You are looking at a 2 x 11 inch opening, which is 22 sq. in.  You want to find out what that 22 inches translates to in a round port.

pi R^2 tells you how many sq inches are in a circle.

Do a little trial-and-error:  use a round port diameter of, let's say, 6".  R is 3" (half of diameter).  Using the pi R^2 math:  3 X 3 = 9 (that's the R^2 part).  Multiply 9 times pi (which is always 3.14).  9 X 3.14 = 28.26 sq in.  Compare that to the 22 inches your slot port has, and you can see that the round equivalency of a 2 X 11 slot will be LESS than a 6" circle.

This is where you just try some different numbers going down from 6".  Just use your regular calculator on your computer to try some different numbers.

5.5 inch circle is too small, but 5.7" hits it just about right.  Input 5.7" into the port length calculator with a box volume of somewhat less than what you came up with (to allow for driver displacements), and a tuning frequency of 32.  Find the proper port length.

Using 2.2 cu ft as net volume, and 5.7" as port diameter (because that is the equivalent of a 2 X 11 slot), you will see that the port length should be 26 1/3 inches to get a tuning frequency of 32 Hz.  The port as built does not give the proper tuning.  So what tuning freq was this box built for?

To find out, use the calculator and try some different tuning freq numbers until you find a length of 14".  Eureka!  That box was built with a tuning frequency of 41.5 Hz.  No wonder the lows aren't there.

Fixing this is more than a matter of just increasing the port length.  Doing that would only make the box volume that much smaller, and I suspect this box is too small to begin with.  It's possible that this was the solution to go with because of the limitations of box size.  In this case, if it were me looking for a fix, I would use WinISD to find out what should be done to get deeper extension.

And it appears that you found net box volume correctly.

I found the specs of the TI10 here.  Let me know if it's the D4 or the D2 model.

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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.

I looked at this in WinISD.  It looks like it favors the smaller box after all, but it needs a long port.  This box should be adjusted so that the slot length is about a total 25" long.  See if this is manageable by working through the speaker cutouts.  The reduction in box volume looks acceptable, and the low end extension will increase...probably dramatically.  At least it will appear to be dramatic because there won't be such a hump in the response.  Here's a screenshot from WinISD showing the difference...the gray is the way it is now, the yellow shows what it would look like if you increase the port length with that box:

ZE8_rft110.jpg

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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.

I got to looking at the specs for the subs model T1 10D2 and the freq is 35hz. If the box is tuned to 32hz like he wants it , will it make the sub sound worse? Should it be tuned to specs to get optimum sound?

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Fb at 32 Hz looks good for that, like I was saying in the post above. This box should be adjusted so that the slot length is about a total 25" long.  Box tuning doesn't mean that the loudest sounds will come at that tuning point...they are usually well above it.  As you can see in the graph above, the yellow line is a (net) 2 cu ft box with a port length of 25", tuning is at 32 Hz, and the loudest output is 50 to 60 Hz.  As with all subs, you have to look at various scenarios and pick the best one.  

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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.

THanks Stevdart youhav giving me a better understanding of what to look for. I printed the graph out and showed him. He said that is what he is after.Now he wants me to attempt to build this box for him. I will give it a shot . Hopefully it comes out right.

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You're welcome  :)

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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.