Advanced question on box build for RF T115D4

[emilimo701 10+ year member]

This is a multi-part question.

I have decided that I will invest in a Rockford Fosgate T115D4 and power it with my Infinity 1600a (600 watts @ 2 ohms).

before I buy it, I wanted to make sure I could build an enclosure that would fit in my Suby wagon. I have plenty of space in back, but you never know.

In my effort to learn more about subwoofers, I have tried my best to understand the Thiele/Small parameters and their relationship (or lack thereof) to a sub's sound output.

My first question, which has less to do with the T115D4 and more to do with woofers in general, is:

1) To what extent does QTS (and Vas) dictate what the ideal enclosure size should be?

I have used a few enclosure calculators: a couple online resources, and WinISD. All calculators show that there should be a significant difference between box volume between manufacturer's woofers that have 2-ohm impedance vs 4-ohm impedance. Yet all of these manufacturers recommend the exact same box volumes for either sub (with the lone exception of JL Audio).

For example, the T115D2 vs the T115D4:

T115D2: Qts -- .43, Fs -- 26Hz

T115D4: Qts -- .51, Fs -- 27Hz

Rockford Fosgate recommends a sealed box size of 2.0-2.5 cubic feet for either woofer. But according to this calculator, the "ideal" box volume (with Qtc at .707) for the D2 is 1.85 cubic feet, and as for the D4... 3.42 cubic feet! Not only are both volumes outside of the recommended range, but they are on opposite sides! I got about the exact same results for 3 or 4 other manufacturers of woofers within the same RMS/price range. Same thing for ported boxes.

Anyway, if I get the T115D4, I plan on putting it in a ported box, tuned to somewhere between 30 and 32 Hz in a slot-ported box. This is what Rockford Fosgate recommends for slot-ported enclosures for their 15" Power-series subs:

So my second question, in less words, is...

2) What gives?

First of all, I did the calculations half a dozen times. The internal volume is 3.35 cubic feet, not 3.25 which is what they printed. Also, the port length they list is 22.5 inches. But that length does not include the corner -- at all. I'm not a physicist, but i'm sure the corner makes a difference in the tuning frequency.

I used this calculator and gave RF the benefit of the doubt, entering 3.25 as the box volume. Lo and behold, a port length of 22.5 inches yields a tuning frequency just under 38Hz, just as specified in the image above. So does the corner really not affect the tuning, or are these calculators simply too naive the accurately predict performance?

Lastly, what would you (directed to people with experience in this area) recommend for box design for the T115D4? Definitely go higher than 3.25 cubic feet (RF dictates 2.25-3.25 for both the D2 and D4)? Definitely tune it lower? I sketched up a mod to RF's design above; which entails decreasing the port width from 3 inches to 2.75 (lowering the tuning frequency), increasing the box depth by a few inches (which would simultaneously increase box size and lower the tuning due to extending the port), and decreasing the box width by an inch or so (so that the volume isn't TOO great). Would this be a good solution?

Thanks for any responses! This is my first post here, and i really did try to do as much research on my own in order to not ask obvious questions, so take it easy on me. //content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif

EDIT:

I should probably specify my goals. At first, I was dead-set on SQ, SQ, SQ. But after a year of driving around, I realize that I use my woofer mainly for rap. And I really want to get those low-lows that i'm not getting from my Kenwood in its sealed box. So that's why I'm going ported. And from what I read, all those "ported boxes aren't for SQ and don't sound as good" rumors are, well, rumors. I'm sure a ported sub would do just fine if I decided to pop in a Pat Metheney or Miles Davis cd. Hitting those low-lows when listening to Flo-Rida or Young Jeezy and getting a flat response are more important than SPL and efficiency. After all, 600 watts in a ported enclosure will give me all the SPL I need. So that's why I'm planning on going with a bigger box and tuning it low.

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[hispls 5,000+ posts]

Not sure what gives. I didn't think TS parameters changed that much. Of couse they don't tell you whether the new specs are with coils in series or parallel either which is a whole other thing to worry about.

I will say that I expect the box you have planned will do great if you make it tight and brace the large sides.

If you play with your calculator you should find that being off even 4" in port length won't do too much to tuning at that length/freq anyway and I'd say err on the side of lower if you have to and if you can, try to round off the internal edge of the port (where it makes the L). Also a slot port extends half a port length along the wall if it's to the wall SO whatever you measure, subtract 1.5" if you really want to split hairs.

Port area is pretty good now, I wouldn't shave too much off of that for the sake of low tuning. Box tuned to 35 should extend comfortably down to 30 and there really isn't much below that in 99% of music (even chopped and screwed stuff)

 

[emilimo701 10+ year member]

Thanks for the reply.

"Also a slot port extends half a port length along the wall if it's to the wall SO whatever you measure, subtract 1.5" if you really want to split hairs."

I don't understand what you mean by "along the wall." Could you explain further?

My plan was to change the box dimensions from 30" x 17" x 18" with port extension of 9.25" to:

28" x 20" x 18" with port extension of 15.25"

I estimate this to decrease the tuning from 38Hz to about 33Hz. This can't hurt, can it?

 

[Wildbillpb 10+ year member]

What equation are you using to determine box frequency

 

[hispls 5,000+ posts]

If you have a slot port with a wall as one side, the wall acts to extend the port (this can happen outside the box too depending on placement). Technically if you want to split hairs, picture a 45 degree angle from the top of the port to the box wall...that's what the system "sees" as port volume/length.

5hz difference in tuning is a noticeable difference. The loss of some internal volume will also affect low-end extension and response. I have a box tuned to 33 and I think it's a good middle ground with low end extension and some output in the higher stuff.

Honestly the largest factor in how it's going to sound is the vehicle anyway. Really depends on what you like. I've tuned everywhere from 45 to 25 and I even go through phases where I'll miss one or the other.

 

[hispls 5,000+ posts]

Oh yeah, there's a TNESPL show in Roxbury tomorrow evening, you ought to come check it out.

 

[emilimo701 10+ year member]

Thanks! I think I'll come check it out.

 

[emilimo701 10+ year member]

Okay; I think I get it. If the wall is flush with the port opening, some air along the wall and close to the port will move along with air in the port due to friction (is friction the right term?) between air molecules in and around the port? I suspect this same thing happens with isolated ports but to a much lesser extent.

It's funny, even my brother who was a physics major in college couldn't wrap his head around this. Sometimes, you just need to think outside the box (no pun intended... ha)

WildBill: I was mainly using the bottom half of this page PORT Size Calculations and Formulas for WOOFER and Subwoofer BOXES

I guess I could reverse engineer to figure out the equation... unless anybody knows a place I could find it.

But I'd still like to know... what effect does a corner have on a slot-port? Since ports are all about air-volume displacement, wouldn't you just (assuming 3" wide port) add 3 inches to the effective length of the port? (for example, in the RF diagram in my post above, the port would act the same as a 25.5 inch flat port)