# a thread for my small questions

Discussion in 'Archived Threads 2001-2004' started by Fong, Oct 19, 2002.

1. ### Fong Stunt Coordinator

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1). is the equation for using precision port (flared) correct? how do you manipulate this equation if you are using 2 ports? Chris Tsutsui, maybe you can help me here because they are what you used iirc. im using a pair of 4" flared kits, and dont know how long to make them.
the equation is:
8466.4 R^2
---------- - 1.463 R = Lv
(Vb) Fb^2
Lv = port length in inches
Vb = volume of box in ft^3
Fb = tuning frequency
2). i have a 1-input (1 red and 1 black) binding post, but the tempest has dual voice coils. how do i connect them? im thinking that after i attach + to + and - to - (for parallel connection, right?), i could strip the wires so i can tape or sauder (sp?) them to another wire that would be going to the binding up. will this work?
im feeling pretty good about this project. i have everything i need!...except for the sonotube : ( i'll get one after i get some money. the only reseller in town wont sell me anything smaller than a 12' section.
p.s.
3). a quicky: a 24" sonotube refers to the outer or inner diameter?

2. ### John E Janowitz Second Unit

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Hi Fong,

If you're using two ports, all you do is double the length of each of them to get the same tuning. So if you come up with your Lv = 10", if you're using 2 ports, each of them will be 20".

For your terminal, you will connect + to + and - to - to parallel the two coils. Then you can go from one set of the terminals on the woofer to the binding post terminal. So on one set of terminals you'll have to solder two sets of wires, the ones going to the other terminals on the woofer, and the ones going to the binding post.

The 24" sonotube refers to the inner diameter.

John

3. ### Fong Stunt Coordinator

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hello John. thank you for answering my first questions.

being my first diy sub, of course i run into some problems. i asked for a sheet of 3/4" thick mdf at my home depot, but got 5/8" thick on accident (the HD guy just grabed one of the sheets). so my question is if 1.25" is thick enough for the top end cap (2 pieces of 5/8" mdf glued together)? the bottom will have 1.25" + 1/4" of plywood, which i think should be enough. if 1.25" isnt enough, would there be any problems if i glued 3 pieces of 5/8" together (1.875")?

heh another mishap was the sheet of mdf was only 49" x 92", so i had 4 pieces of 24.5" x 25" cut out. hopefully the cuts were exact, because there is no room for error if a 24" sonotube refers to the inner diameter.

4. ### Greg Monfort Supporting Actor

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>If you're using two ports, all you do is double the length of each of them to get the same tuning. So if you come up with your Lv = 10", if you're using 2 ports, each of them will be 20".
====
Hmm, with round vents, cross sectional area is the primary factor so increasing it means a larger end correction due to their proximity WRT WL size.
The area of a single 4" i.d. vent = ~12.566"^2, so two = ~25.13"^2, which in turn = an effective 2.828" radius Vs a single's 2" to plug into the formula. This gets you the new, longer, length that's then doubled.
Some folks instead take the Vb and divide it by the number of vents and plug the new, smaller, Vb into the formula. The results are ever so slightly different, but either one will get you closer to the target Fb than simply doubling the length of a single vent, which yields a somewhat higher Fb.
That said, as Vb increases, the difference between just doubling the length Vs allowing for the increased end correction has less impact on Fb, to the point of inaudibility on a typical low tuned sub. Still, I think it's always best to have the facts to make a design decision rather than just a 'rule of thumb' where possible.
GM

5. ### John E Janowitz Second Unit

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Greg... you're right. I was just in too much of a hurry to do the math last night. :b

John

6. ### Fong Stunt Coordinator

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hehe thanks greg. the lengths were off by several inches if you doubled the length of the single port equation. im sticking to the EBS alignment w/ 340L and a pair of 4" ports. i went with flared ports to help reduce port noise (even if it doesnt, it gives me a little peace of mind ).
question #1: is there a reason for 4" legs on the EBS alignment? obviously it is for the tempest to breath a bit, but would longer or shorter legs affect the sound?
question #2: my endcaps are going to have 3/4" mdf sticking in, and 3/4" mdf sticking out. is 3/4" enough for me to nail and glue the caps on? i can use a nailgun and shoot 1 1/2" nail from the sides and the top if its not. bah. i'll prolly do it anyways, but would still like to have opinions on it.
this is really hard without a circle jig for the router. my circles have very rough/bumpy rims, but i think i can sand them down a bit, and use wood filler on the parts that i really really screwed up on :b . i've only done the four main pieces for the end caps. im gonna try to get a circle jig tomorrow so i can make good cuts for the tempest and ports.

7. ### Fong Stunt Coordinator

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for the equation
8466.4 R^2
---------- - 1.463 R = Lv
(Vb) Fb^2

my Vb = 12 ft^3, Fb = 16.5, R = 2". but for my 2 flared 4" ports, the effective radius is 2.83", giving me 16.6" for 1 port, and 33.26" for 2 ports. this is in direct violation of the 5x the port diameter rule-of-thumb. i was just wondering if the rule is different for flared ports, or did i just enter the equation wrong?

8. ### Patrick Sun Studio Mogul

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Why are you doubling the length? You've already found the port length for 2 flared 4" ports by using R=2.83".

9. ### Fong Stunt Coordinator

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im doubling the the length because it's what i thought Greg said

10. ### Fong Stunt Coordinator

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ok. stupid question time. how do i connect the amp to the sub? i went to every page on the compiled sonosub list, but i didnt find anything saying how to make the connection. there is a red and a black wire coming from the back of the amp that i think is supposed to be connected to the sub, but it has female quick disconnects, which are are really throwing me off. should i just remove the quick disconnects and attach the bare wire to the binding post?

11. ### Greg Monfort Supporting Actor

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> this is wrong, please tell me because i am about done, so i need to attach the flared end to the pvc.
====
S***! I hate it when I do this! Here I was lightly chastising JJ for being sloppy in his generalized response, all the while inadvertently doing the same thing when I was editing my response and didn't delete all of the rewritten sentence. :b
Like Pat said, and the point I was trying to make, is when you use the larger radius it's factoring in the doubling of length, plus the extra end correction, IOW if you double the length for a single 4" vent it's shorter than what's needed by ~1.76", bumping Fb up to ~17.23Hz. So for 12ft^3/16.5Hz Fb I get ~16.62" long, well within my personal length limit of 24".
====
>question #1: is there a reason for 4" legs on the EBS alignment? obviously it is for the tempest to breath a bit, but would longer or shorter legs affect the sound?
====
Longer shouldn't, unless the distance is long enough for standing waves to develop, which isn't going to happen at low tunings unless you're planning on spacing it up many feet.
As for shorter, I use a formula based on slot loading area, and I've posted it enough that DW and others have had a chance to see it, and if any of them had a problem with it they would have expressed their thoughts by now so I'm taking the 'silence' as a tacit approval.
Anyway, I use the reasoning that the max 'slot' area between the perimeter of the baffle/floor be ~the driver's useable Vd (2*Xsus*Sd) to ensure free flow up to this point, with some acoustic loading above it to help protect it if pushed harder. I normally use Xsus = 1.5*Xmax since it isn't usually readily available.
I haven't done any elaborate testing, but FWIW haven't measured any obvious impedance increases until the driver's pushed, nor heard any 'vent' noise.
====
>question #2: my endcaps are going to have 3/4" mdf sticking in, and 3/4" mdf sticking out. is 3/4" enough for me to nail and glue the caps on? i can use a nailgun and shoot 1 1/2" nail from the sides and the top if its not. bah. i'll prolly do it anyways, but would still like to have opinions on it.
====
I believe this is what most folks do. Back when I was building sonotube speakers/subs, I used 3/4" plywood and it worked just fine.
====
>ok. stupid question time. how do i connect the amp to the sub?
====
I'm not familiar with the amp, but if these are the driver's connections, then yes, clip off the terminals and solder it straight up with 2-5% silver bearing solder.
GM

12. ### Chris Tsutsui Screenwriter

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I agree with Greg,

If it's a plate amp it's intentions were to be mounted on the box making the disconnects convenient to connect to the driver. (But you'd still have to solder or use disconnects for parallel wiring the VCs)

What I did was cut the disconnects off and splice/add some 12 gauge speaker wire to extend the black/red wires. Actually on one of my subs I am so lazy that there are still bent disconnects squished in the binding post of my speaker that works fine.

4" legs is for a tempest, while a tumult needs 5" legs or else the driver can lift the box off the ground during xmax. (JK) As for driver position and in room response, raising the speaker 1" (Which I have done) won't have any effect on sound but the legs will be more fragile to break off (Which I have done). I think 4" is a good combination of strength and air flow.

Anyways, So how does it sound?...

13. ### Fong Stunt Coordinator

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you guys know that a circle jig would make the job much much easier? hehe i had to use freehand with a jigsaw and a router. so looks a bit sloppy, but i think if i can make a good seal, everything will be ok, right?
the progress has come to screeching halt due to school. i will work again on thursday and friday. im pretty close to done. i have to recut the tube and pvc now that i got the correct port length (i got 16.62" as well Greg). then i just have to attach the pieces of mdf together, then to the tube, and that'll do it. i'll try to get some pics up of the quasi finished sub (no sock or finish on the endcaps due to me being a poor college student). i have to borrow a digital camera, otherwise i would have had pics of the hole ugly process. heh im hoping that wood filler and some good ole F-29 construction adhesive will make the sub look a bit better at least. im told that the F-29 is supposed to make a great seal and harden to the point where you have to chisel it off
will be able to do a direct comparison to my friend's svs 16-46 pci, but i dont know when we'll have time (both have school). would this be a good/fair comparison? its the only decent sub that i've heard extensively, so it will be my reference point.

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