6th Order Bandpass Design

[Slo2g]

Hello all, just wanted to know what you all thought about the 6th order bandpass that I designed. I will admit it is actually a 6th order dual reflex box but down to the details. I have two 8" drivers that will be covering the midbass but I wanted more lows so I decided to pull the trigger on a 12" woofer. I put the dimensions of the box into sketchup and came up with a basic design that will let me keep as much room as possible but still "theoretically" hit the notes I would like. After I plugged the basic layout I went over to the winISD program and started to tune the ports and I came up with 36 for my low end and 69 for the upper end. Now lets try this whole uploading pictures part...





Subwoofer is a Rockford Fosgate T2 12" D4,

I know transient response will be limited but I'm worried it may be a bit TOO peaky. Any thoughts? To be honest I want to get as loud from this as possible but I don't want it to be a SPL only box. I would like for it to have some music ability! Thanks again!

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[Beatin' 10+ year member]

it's gonna sound like shit. I bet your group delay is all over the place with that kind of response shape. i would suggest you don't even bother with a 6th order box.

 

[Slo2g]

Here is the Group Delay, the green is the subwoofer box that Rockford recommends for this subwoofer, the purple is the 6th order. So Thanks for the reply and suggesting that I give up on a 6th order box....now any suggestions as to what I could do to improve the box rather than give up? Is there a different design you recommend to give me the output that I'm looking for or should I just say eff it and build a standard slot port?

 

[T3mpest 10+ year member]

Group delay is a bunch of ****.. Don't believe me, go add some electrical HPF and LPF to ANY subwoofer setup and go look at your transient response, you'll find it's just as ragged. Add a

4th order filter to your sealed box in WINISD at both tuning frequencies of your 6th order, it'll look just as nasty. Transient response is the 2nd derivative of frequency response. When you add a hpf, either through the amp OR through the box, you effect it's "transient response" because your changing it's frequency response, which is the ENTIER POINT of having a hpf or lpf applied. Does ANYONE here run subs without a hpf or lowpass filter, fullrange for maximim "SQ" didnt' think so. What most people attribute to bad transient response is 99.999% of the time simply bad frequency response which makes the sub sound sloppy. Typically peaks in the 40hz region, especially in cars since we have lots of gain in that area.. Thankfully your box is quietest there, which is what you want if you looking for a flat response once it's in the car

Anyways with gain applied that box would be pretty loud and pretty flat in most cars. I'd want a bit less peak down low and a lower peak, if it were me, so I'd drop tuning a bit. The issue here is twofold though..

1.WINISD is not that accurate in designing 6th order bandpass

2.Even if it was, the shift in t/s parameters over excursion, when not taken into account will greatly mess with your actual response at the levels you'll likely listen ot it at. 6th orders are all about testing and changing based on what you hear/measure. It's very hard to get one right the first shot, especially with cheaper subs since they have more shift in parameters over excursion in many cases.

Expect to have to adjsut the box several times to get what you want out of it. Use threaded inserts and bolts instead of screws so the sub can be removed as many times as neeed to fix it's response. Add and subtract volume from each side, chop ports etc, until you get what you want out of it. WINISD can be a guide as to what change will do what, but you'll have to use your ears to verify things are going the right way and to compare what the program says you should have,vs what you have in real life... Once you get the response you want, you rebuild a new one to those specs and it should be right.. Def not a one day process.

Start with a standard ported box, if you can't get good results from that in the top end of your bass THEN start messign around with a 6th order.

 

[Slo2g]

I just wasn't too sure how the woofer would react much below FS. The FS is is at 39.9. I appreciate your insight and may just build the box, worst case I'm out about 40 bucks.

 

[T3mpest 10+ year member]

FS is measured in free air.. Unless your going WAY below tuning, it won't effect the response much. Drop tuning to about 32 or 33hz. If it's too peaky up top or down low, add something like bricks into each chamber that is too peaky and try it again. Start with ports that are longer than you want as well, so you can chop them down to raise tuning. What size ports are you using, that has a large impact that WINISD Cannot measure

 

[Slo2g]

Plan was to use square ports, front chamber is 2.5" x2.5"x2.5" the rear chamber is 4x4x8. A round port may be the way to go for adjustability.

 

[Immacomputer 10+ year member]

Those port area figures seem pretty small. You will probably have substantial port noise.

Also, I agree that group delay is a bad way of checking for "SQ" in a box design. The inductance of the driver and the frequency response of the sub+box+vehicle are going to be the biggest factors in transient response (inductance taking the cake really).

 

[Slo2g]

I have changed the ports in favor of a 3" round port for adjustability. Will let you guys know how it turns out..

 

[dbjunior 10+ year member]

Parallel tune 6th will be easiest to tune the series will be louder though. Tune the low side to 28hz and slowly raise it, use about 5-8in of port per cube for this side. The high side tune to 50hz and slowly raise, use 16-20in of port per cube this side. With 6ths a1:1 is fine but I recommend 1:1.5 low to high side if you want a really loud daily.,

 

[Slo2g]

Sounds good, I didn't have too much time to get to work on it. I go the basic ports cut out and left some room for tuning. Hoping to get it completed with in the week as I don't have a ton of free time to devote to it! I will keep this updated.