Box design help, winisdpro and rockford fosgate. - AVS Forum
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post #1 of 29 Old 02-12-2013, 05:45 PM - Thread Starter
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I traded a compound bow I no longer use for 2 Rockford Fosgate T112D2 subs. I'm too old to put them in my car, nor do I have the desire. I want to add them in separate enclosures to my home theater. I'm trying to use winisdpro to calculate box dimensions and figure out proper port lengths for good low frequencies but am obviously doing something completely wrong. If I calculate the box at around 2 cubic feet it shows it playing strong at 130hz and falling sharply off below that. If I plug in a request of 25hz it says the box needs to be around 50 cubic feet! If these subs will work I plan for them to be a permanent addition to my theater so I want to build the right enclosures once. Thanks in advance for any help or even a pointer in the right direction for a program I can use. Attached are the specs for the subs.


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post #2 of 29 Old 02-13-2013, 09:35 AM
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hi loco, that driver will work fine for what you are trying to do.

here is a 3 cubic footer with a 4" diameter port that is 24" long. it tunes the cab to 20.5hz. ignore the frequency response above 80hz.

t/s below if you want to play with them. enter the values in green and hit tab to let winisd calculate the values in blue or you might get errors when trying to save the file. also, make sure the units are correct. to change the units, click on them and they will cycle through different units.

air velocity on the port goes a little high with a single 4" port, but in a smallish cab that is what you get unless you want half the cab to be port volume. driver is right around xmax with 500 watts, so no over excursion problems.

most plate amps will have a protective high pass filter around 20hz, so don't worry about overshoot below tuning.

what amp did you plan to use?




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post #3 of 29 Old 02-13-2013, 10:15 AM - Thread Starter
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Using either a Crown XLS 402 or 202. Probably the 202 which in 4 ohm will produce 300WPC. Any pointers on what I was doing wrong? I entered all the numbers I posted and got crazy results back. I'm at work now so ill compare what you put in to what I put in. Thanks. Another question. would 2 2 inch x 12 ports do the same as 1 x 24?
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post #4 of 29 Old 02-13-2013, 03:17 PM
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my guess is that one of your units was off. mm vs. um or something like that. that can cause the error that you describe.

as for ports, the total cross sectional area and the length are what tune a cabinet. using smaller ports produces more air velocity which causes compression and chuffing. one 4" port with 300 watts in your cab is already kind of small. two 2" ports have only half the cross sectional area, so the air speed will be twice as fast and chuffing twice as bad.

since you will not be using a plate amp, some sort of high pass protective filter would be a good idea as below the tuning point of the cab, the driver will be flapping as though it is in the open air.

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post #5 of 29 Old 02-13-2013, 03:42 PM - Thread Starter
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I have low pass cutoff abilities on both the amp and the EQ I'm using. I'll have to do some more reading to better understand ports. I could probably go up to 6 inch if that would help. No limits really other than the box would have to be quite deep to fit a 24 inch long port. Maybe I need to figure out how to convert that into a slotted port. All simple math I'm sure, just need to sit down and do it. I also assume I need to subtract the port and speaker volume from the overall box and shoot for a "true" internal volume even with those items installed. Thanks again for your help.
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post #6 of 29 Old 02-13-2013, 04:46 PM
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the reason that i suggested the 4" port is because of the tradeoff that you mention--the larger diameter the port, the longer that it has to be.

a 6" port would have to be really long to keep the same tuning, though it would certianly flow better.

a slot port works exactly the same way--cross sectional area and length. it doesn't matter if the port is round, square, rectangular, or a triangle. they all pretty much work the same way.

putting a flare on the port will help it perform better.

here is a good read: http://www.subwoofer-builder.com/port-flares.htm

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post #7 of 29 Old 02-13-2013, 05:32 PM - Thread Starter
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So I take it it's not as simple as matching the area within the port. I was thinking if so, that a 6 inch port could be shorter. How do I figure out what shapes and sizes will have the same effect as the 4" X 24" port? Going to read your link now... Thanks.
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post #8 of 29 Old 02-13-2013, 05:52 PM - Thread Starter
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So I getting some great ideas from the link you posted. I'm falling in love with the external port idea. I really like the "industrial" look and think this is the way I may go. Would basically have no limits on port length or size.

http://www.subwoofer-builder.com/sidewinder.htm

I do wonder however how bad for performance the connections In PVC pipe would be. I would have to assume that turbulence would occur at any 90 degree turn or anywhere the port passed through a connection.

Hmmmm, maybe the internal slotted port is the way to go, I have klipsch towers that need to sit on top of whatever I build so slotted may be easier and cleaner. Your model was for 3 cubic feet. Should I assume that's 3 cubic feet after what space the driver and port takes up?
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post #9 of 29 Old 02-13-2013, 06:13 PM - Thread Starter
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here is what happens when I try to load the parameters you posted. Originally, most of the stuff in the second box auto filled. Not sure what I'm doing wrong. Where did you come up with all the missing numbers and why would they fail when i enter them as you have?

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post #10 of 29 Old 02-13-2013, 06:32 PM - Thread Starter
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And here is what happens when i just enter the values you had in green and tab through so the rest autofill. would really love to figure this out. my son and i are planning on trying to build a prototype this weekend.

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post #11 of 29 Old 02-13-2013, 06:50 PM - Thread Starter
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Ok, so it turns out that if you follow directions, things work out fine. I forgot to change the parameters. All is good now and I have the driver exactly as you do. Thank you.
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post #12 of 29 Old 02-13-2013, 07:27 PM
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yeah, the units on the vas are in liters and sd is in cm^2 in my earlier post. that is what i was talking about as something not obvious that can really goof things up.

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post #13 of 29 Old 02-13-2013, 07:36 PM - Thread Starter
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I got everything entered correctly and am now able to play with the box size and see changes. Figured out that I have to set the input wattage correct and feel like I'm getting somewhere. I still can't get nearly as flat of a frequency response as you posted. I get confused when it asks what type of vented box and have tried them all to no avail. If you don't mind, I would love to know what the rest of the parameters you used are (and why) so that I can do this on my own.
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post #14 of 29 Old 02-13-2013, 07:42 PM
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oh, i'm sorry, set inductance (Le) to zero. i always zero it out for modelling, but leave it in the driver file for information purposes.

that will flatten the line out. in practice, the inductance will cause a hump in the frequency response in the upper end of the bass, but winisd does not model it very well, which is why i zero it out.

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post #15 of 29 Old 02-13-2013, 07:45 PM
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also, it shouldn't matter which vented box you choose initially. just change the volume to 3 cubic feet and the tuning frequency to 20.5hz to get what i posted, then adjust the values however you like.

there is also a "vent" tab where you can adjust the size of the port and it will update the tuning frequency. by switching back and forth, you can see how changing the port size affects tuning and vice versa.

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post #16 of 29 Old 02-14-2013, 04:51 AM - Thread Starter
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Thank you, I'll get into it more tonight.
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post #17 of 29 Old 02-14-2013, 04:01 PM - Thread Starter
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I'm getting it to now work just like yours, however, under signal, if I change the resistance to 4 ohm the curve goes crazy. I'm now assuming that this is not the voice coil that its asking about? Also, am I simply after port volume? Meaning that. 4 x 24 port has 301 cubic inches of volume and I need to come up with that either through tubes or slots? Or is it not that simple?
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post #18 of 29 Old 02-14-2013, 04:21 PM
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under signal, you should leave it at 0.1 ohms or something like that. that is the resistance of the speaker wire not the driver.

the driver resistance, re, was entered on the t/s page.

the driver has dual 2 ohm nominal coils. the resistance is 3.1 ohms. just leave that as is.

if you choose to wire the driver in parallel, it will perform the same in the cabinet.

port cross sectional area and length is what tunes the cab. the port volume will not stay constant.

on the "vent" tab, you can change the diameter and length, then go back to the "box" tab and see how the tuning changed.

or you can change the tuning on the "box" tab and go back to the "vent" tab in order to see what length is required for that tuning.

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post #19 of 29 Old 02-14-2013, 04:39 PM - Thread Starter
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Got it, but wouldn't the voice coils in series be 4 ohms? I take it there is no way to figure a slotted port wider than 5 inches which is the most it will let me do. For Instance, if I wanted a slotted port the width of the cab.
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post #20 of 29 Old 02-15-2013, 06:21 AM - Thread Starter
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Also, is it best for the port opening to be on the same surface ad the driver face? Does it matter? Any advantages to other locations?
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post #21 of 29 Old 02-15-2013, 09:27 AM
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strange. my version of winisd will allow me to enter any shape of slot port. i use winisd pro alpha 0.50a7. it is an older version i believe.

it generally doesn't matter which way the port aims. to the extent that it makes it a little less noticeable if it chuffs or allows high frequencies to slip through, pointing it back or down is probably ideal, but there are successful builds with it pointing any direction.

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post #22 of 29 Old 02-15-2013, 10:57 AM - Thread Starter
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Played some more today while I should be working and noticing the the larger the port the longer it has to be. Yet if you use a much smaller port it can be much much shorter with the same end net result. Does the problem become air speed? The model shows the same end result with a 2 inch port that is half the length of a 4 inch port.
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post #23 of 29 Old 02-15-2013, 02:46 PM
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you've got it. the larger the diameter the port, the longer it has to be for the same tuning frequency.

if air speed gets too high, the port will make noise called chuffing and eventually fail to function properly.

if you flare the port, a higher velocity air can go through before turbulence sets in.

a rough rule of thumb is to try to keep the port air speed at less than about 17 meters/second.

here is a primer on port air velocity and flaring. http://www.subwoofer-builder.com/port-flares.htm

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post #24 of 29 Old 02-15-2013, 03:23 PM - Thread Starter
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Which is why you said above that 4 inch is nearly too small. I'm probably getting close to being over 17 m/s.
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post #25 of 29 Old 02-15-2013, 06:32 PM - Thread Starter
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Unfinished prototype. Thank you for your help and guidance, it is greatly appreciated...






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post #26 of 29 Old 02-15-2013, 07:08 PM
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prototype 1... :-)

your cab is around the size where you might benefit from adding a little bracing, so no energy is lost to vibrating panels that should go to vibrating the air. bracing can also add some weight. if the cab is too light, it will tend to "walk" when you hit it with some big bass.

the other thing is the slot port. the further that it gets "flattened", the less that it will perform like the model. i don't have a good rule of thumb on that one though. some say try to keep it no more than 8:1 (width to heighth) but i have seen some folks report success with narrow slot ports like yours, so you'll just have to see how it performs.

also, it looks like you took extra care to ensure the seems are air tight. nice work.

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post #27 of 29 Old 02-15-2013, 07:24 PM - Thread Starter
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1 inch is as big as I could make the slot without it going around 3 walls. Probably too late to brace it now unless I work from inside the speaker cutout tomorrow. Silicone between all the seams and then along all the edges. It's quite heavy already and the speaker is nearly 40 pounds as well. Going to fill all the screw holes and sand it smooth and then I need to decide a finish. Either veneer which I've never worked with, carpet which I'm unsure of the results, or textured paint which my son suggested. I'm pretty happy so far seeing as how the last "real" boxes I built were in the late 80s to early 90s to stick in the back of everyone's cars.
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post #28 of 29 Old 02-15-2013, 07:33 PM - Thread Starter
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My son found this and it was a great help as well.




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post #29 of 29 Old 02-16-2013, 05:22 PM - Thread Starter
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Prototype 1.0.0 is finished. I'm fairly pleased. Once again, your help was greatly appreciated.



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