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Basics of a 4th order bandpass enclosure


86 replies to this topic

#1
donellk

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This is my quick write up on 4th order bandpass boxes. I want to keep it simple and sweet! Hope this helps a lot of new people!!! I'm not a expert but from my research this is what I believe is best. I wouldn't mine if someone has more to add!

A 4th order is simpler then it seems... You have a sealed section witch develops the lower frequencies and a ported section that develops the high end frequencies. The advantage to this type of box is you get the tight clean bass everyone loves from a sealed box but with a wider range of frequencies and higher output!

There are a couple of awesome programs out there that help with the design of a 4th order. My personal favorite is bass box pro 6. You can get it for around $150 at parts-express.com. If you do a lot of car audio or home audio this is well worth the investment!!!

Main thing to remember is the ratio of the box. Most 4th orders are a 2:1 ratio this means for every 2 cubic ft of ported section there is 1 cubic ft of sealed. So if your sub needs 1.5 cubic ft sealed then your ported section is going to be 3 cubic ft.. Some people do 3:1 but most commonly you'll find 2:1 works out best. At least I have so far.

I start the design with the sealed section of the box. I normally design the sealed section around the recommended specs from the subwoofer manufacture! A 12" will normally need between .8 and 1.5 cubic ft in the sealed section! remember the sealed portion makes the low end bass so the lower the f3 the better lol but normally I set it near the fs of the subwoofer!

The second section of the box is the ported witch plays the higher end bass. As stated above with a ratio of 2:1 for ported section should be double the air space of the sealed section! Unlike normal ported boxes your not going to tune this low! In fact kind of high... The best (IMO) is between 45hrz-50hrz.

Now here is where it gets fun... Your subwoofer needs to come from the sealed section facing into the ported section. Since the woofer will be completely inside the box the more port area the better! That's where your sound will come from! I try to aim around 15sqin PER CUBIC FOOT! I say it nice and big because I myself forget sometimes and have to go back and redesign lol! From what I have read The shorter the port the better but that's not a big issue since tuning high normally means a short port.

Since your subwoofer needs to be inside the enclosure the best thing is to include a removable front panel in your design. I did mine with the face of the box. Another thing you may want to do is face the woofer into the sealed section so the motor and basket are in the ported section so you can smell if the coils burning! I trust my self not to fry anything so I leave it facing the ported section plus IMO its easier to calculate the displacement in the sealed and it keeps the ported section a little smaller!

Hope I covered everything and it helps the new comers!



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Edited by donellk, 01 December 2011 - 12:26 AM.

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#2
Sheena

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very nice writeup.

i have never read anything about 4th orders other than build logs and i have to say, this is a very easy to understand writeup.

thank you sir
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#3
baaudio

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Wow this is great. Definitely seems like a good baseline to start a design. I'm waiting for some of the more experienced guys to chime in on this.

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#4
Punkeyboozter

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Been reading up on this myself in recent times and what you say seems to be inline with the rest of the interwebz.

Question

When you tune the ported section do you tune it using normal methods for a standard ported box?

They say compound loading reduce the size in half (any experience with this?)

Somehow I fail to see how lets say a 12 will fit into a .5sealed and a 1cbf ported in this configuration...suppose its more relevant with huge enclosures.

Maybe also good to add the low F's QTS whatever guideline for woofer selection, although im going to try it with something that on paper should not work.

Another thing is that some have mentioned that a flat top graph design does not react favorable in a car environment.

It is also recommended not to do more then 3DB gain to prevent woofer damage.

Now after being a smart-ass and never actually build a BP Ill have to go and try it.

Thanks for the post.
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#5
DudeWith4x4

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I'm trying to design a band pass right now for a epic 8 and it's been confusin me quite a bit. Thank you for the write up!

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#6
One Way

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I agree good read
Gotta start someway
Nice ez explanation

#7
jokerklown

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Good info to have

#8
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Been reading up on this myself in recent times and what you say seems to be inline with the rest of the interwebz.

Question

When you tune the ported section do you tune it using normal methods for a standard ported box?

They say compound loading reduce the size in half (any experience with this?)

Somehow I fail to see how lets say a 12 will fit into a .5sealed and a 1cbf ported in this configuration...suppose its more relevant with huge enclosures.

Maybe also good to add the low F's QTS whatever guideline for woofer selection, although im going to try it with something that on paper should not work.

Another thing is that some have mentioned that a flat top graph design does not react favorable in a car environment.

It is also recommended not to do more then 3DB gain to prevent woofer damage.

Now after being a smart-ass and never actually build a BP Ill have to go and try it.

Thanks for the post.

good write up donell! and this is some more pretty good info too!
to answer your questions,
yes the ported is tuned with the exact same methods as a sealed box.
from what ive read isobaric or compound loading will allow you to cut the size of the enclosure in half but i also heard it was only good for low xmax subs.
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#9
Tarball

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Been reading up on this myself in recent times and what you say seems to be inline with the rest of the interwebz.

Question

When you tune the ported section do you tune it using normal methods for a standard ported box?

They say compound loading reduce the size in half (any experience with this?)

Somehow I fail to see how lets say a 12 will fit into a .5sealed and a 1cbf ported in this configuration...suppose its more relevant with huge enclosures.

Maybe also good to add the low F's QTS whatever guideline for woofer selection, although im going to try it with something that on paper should not work.

Another thing is that some have mentioned that a flat top graph design does not react favorable in a car environment.

It is also recommended not to do more then 3DB gain to prevent woofer damage.

Now after being a smart-ass and never actually build a BP Ill have to go and try it.

Thanks for the post.

good write up donell! and this is some more pretty good info too!
to answer your questions,
yes the ported is tuned with the exact same methods as a sealed box.
from what ive read isobaric or compound loading will allow you to cut the size of the enclosure in half but i also heard it was only good for low xmax subs.



I have not heard of the xmax issue with compound loading. Although, you would need to space the woofers out accordingly in a face to face compound. However, compound loading is normally used in a very limited space installation (due to the 1/2 size box), there is more potential to "get loud" if you don't compound load because you are sacrificing cone area. Hope that makes sense.

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I guess you didn't read the rules of no bare boxes for SOTM. 

 

I thought bare box ment that it didn't have the subs loaded into it...lol oohh well

 


#10
Sanitarium

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i really REALLY wish people would stop hugging up to the whole ratio thing.
dumbasses squeezing down the sealed side in search of a higher ratio/more peak.

Seriously. build the sealed side to spec/desired F3. then build ported side to accomodate taste. peaky or flat is up to you.
Throw the whole notion of ratios out the damn door.

#11
Punkeyboozter

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Im going to build one, screw it if i blow the woofer.
2x6" aeros enough for a 12?
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#12
Rockbrook

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i really REALLY wish people would stop hugging up to the whole ratio thing.
dumbasses squeezing down the sealed side in search of a higher ratio/more peak.

Seriously. build the sealed side to spec/desired F3. then build ported side to accomodate taste. peaky or flat is up to you.
Throw the whole notion of ratios out the damn door.

So what you are saying is just build the Ported side to your liking?

My 4 12's are in a sealed box right now but i have two seperate chambers for each pair of subs, could i turn this box in to a 4th order?

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#13
412 CVX

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this is a great start to get over that first hump of how to design bandpass. i have a small understanding myself, but would like to know more.

i'm curious as to what the tuning of the ported side does to the box. with regular ported 30-40hz tuning is kind of the norm.

but since you say tuning higher is pretty standard, what are kind of the norms, and what does that mean for sound?

also i think you could go into more depth on what passband is.

my understanding is that the lower the ratio (i.e. 1:1) the larger the passband and the more musical it is over a broad spectrum of frequencies.

the higher the ratio (i.e. 1:3 and up) the smaller the passband. which translates to a peaky, but louder box.

would love to see some experts chime in here and fill in some gaps. great reading!

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#14
tonytone4143

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Nice write up

#15
6berry

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this is a great start to get over that first hump of how to design bandpass. i have a small understanding myself, but would like to know more.

i'm curious as to what the tuning of the ported side does to the box. with regular ported 30-40hz tuning is kind of the norm.

but since you say tuning higher is pretty standard, what are kind of the norms, and what does that mean for sound?

also i think you could go into more depth on what passband is.

my understanding is that the lower the ratio (i.e. 1:1) the larger the passband and the more musical it is over a broad spectrum of frequencies.

the higher the ratio (i.e. 1:3 and up) the smaller the passband. which translates to a peaky, but louder box.

would love to see some experts chime in here and fill in some gaps. great reading!

I'm not really an expert but I can try fill in some gaps. The reason you can tune higher with a 4th order is because the sealed chambers control the low frequency cutoff where as in a ported box, the port controls the low frequency cutoff. Normal tuning is between 45-55 bit people go higher and lower than that.
The passband, or bandwidth is the space between the upper and lower f3 points. With a smaller ratio (1:1) you'll have a wider bandwidth but lower output. With a higher ratio (1:3) you'll have less bandwidth but more output.
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#16
scooter99

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WOW this is awesome! This gives me a greater insight of what I just did. I'm going to go back to the drawing board and see where i'm at. Thanks for this! :drinks:

#17
donellk

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Thanks everyone! If u have any question I'll try to answer them the best I can like I said I'm not a expert!

As for the ratio I wouldn't sacerfice a lower f3 to have a dead on ratio but I prefer a flatter response for daily listening! But that's another thing that would be up to the builder!

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basics of a 4th order


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#18
baaudio

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my biggest concern is how do you know if a woofer will work well in a bandpass enclosure? please explain this detail for me and others.

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#19
6berry

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my biggest concern is how do you know if a woofer will work well in a bandpass enclosure? please explain this detail for me and others.

I normally go by having a low Fs and having a qts of over .4
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#20
scooter99

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Sorry to be the idiot of the group, but what are the Fs and Qts?



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