Box design help, winisdpro and rockford fosgate.

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?

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.

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

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.

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.

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.

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.

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.

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

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.