First 4th order attempt, enclosure experts welcome.

[Wasteland_Audio]

So i am about to build my first 4th order box and i was wondering if some of the more knowledgeable box builders on here could weigh in a bit. Here are the specs:

2 DC lv6m3 18"

2 Audiopipe APK4500

Sealed ft3: 7.68cuft

Ported ft3: 16.32cuft Before displacement

tuned to 45hz

subs will most likely fire into the sealed side due to size constraints and to keep displacement at 2:1

[Triticum Agricolam]

What kind of usable bandwidth are you hoping to achieve? Whats your motivation for using a bandpass box vs. a regular ported box?

[Wasteland_Audio]

Well the last box i built was super peaky and i really just didn't do a very good job on it as it was my first ever box i built. Then a buddy of mine gave me a demo of his blazer with 6 xl 18"s in a 4th and i loved how powerful the bass was throughout the entire bandwidth. really i just want to have a smoother curve and be louder throughout a wider range.

[Triticum Agricolam]

What were the specs on your ported box?

[Wasteland_Audio]

11.5 cuft (after displacement) tuned to 32hz, subs up port back in the burban

[frogcase2002]

Usually the only good reason for a 4th order bandpass is if your doing some kind of blowthrough or you cant fit the subs and port area you need on the front of a flat wall .

[Wasteland_Audio]

Usually the only good reason for a 4th order bandpass is if your doing some kind of blowthrough or you cant fit the subs and port area you need on the front of a flat wall .

Well the reason i designed it the way i did was to keep it close to the same size as my current box and keep it below the window line. I want to do a wall eventually but since i only have 2 subs i don't see much point in it yet.

[Triticum Agricolam]

If you want to experiment with a bandpass box just to see the performance difference, by all means proceed. However if you are hoping to get smoother frequency response, you probably are going to be disappointed.

Here is the simulated frequency response of the bandpass box and your current ported box, both with approximated cabin gain added in:

The green line is the ported box and the red line is the BP4. Your ported box does look pretty peaky but the bandpass box is even more so. The big difference is it peaks at 45 Hz instead of 32 Hz. If you listen to a lot of material with content in the 40 - 50 Hz range, the bandpass box will be a lot louder, but it won't have any wider bandwidth.

[Wasteland_Audio]

Huh.. well that kind of screws up my whole understanding of 4th orders lol. i was under the impression that the sealed section would boost the low end and the ported side would extend the bandwidth. why would the opposite be true then?

[Triticum Agricolam]

Well the opposite isn't really true, its just a lot more complex than that.

The ported section boosts everything around its tuning frequency. The larger you make it, the more boost you get, but the more peaky it is. There are a lot of problems with bandpass box ratios, one of which is they assume every sub is going to perform the same in a given ratio, which is REALLY not the case. Your biggest problem is your subs have very stiff suspensions and this is going to make them peaky pretty much no matter what you do. Putting them in a ported section that is so big only makes this worse. A smaller ported section would smooth things out a bit, but its only going to help so much and its going to come at the cost of output.

Once you get much below the tuning frequency of the ported side, the box pretty much perform just like a regular sealed box, the port unloads and its like the ported section isn't even there. Making the sealed side as large as you can will boost low frequency performance, just like with a sealed box, but you have to balance how big your make it or you can run into cone excursion limitations. Also it takes a pretty big increase in sealed chamber size to make much difference in output, so you can quickly end up with a huge box.