HDS312s Fourth Order

[tayodell41]

Hey everyone, I'm working on designing a fourth order (with a 2:1 or 2.5:1 ratio) for two HDS312s to go in the trunk of my '01 Camry. Don't have an amp yet, plan on running an SQ 2200 or as a fallback maybe a Sundown 2500. I've done a lot of research on fourth orders and I'm looking to design it my self (not asking for any hand outs) but I'm having a hard time figuring out what to make my sealed side and how much port area I will need. Any help is appreciated.

[Triticum Agricolam]

So here is my philosophy on 4th order bandpass box design.

The purpose of the rear chamber is to provide output and control cone excursion below the tuning frequency of the front chamber. Making the rear chamber larger will reduce its sealed resonance frequency and lower the QTC, these are both good things and will help to increase output. Making the rear chamber larger also increase cone excursion, which becomes a problem if it gets to be too much. The trick is finding a balance between getting as much low end output as possible without cone excursion getting out of control. Modeling software (WinISD, BBP, Hornresp, etc) can help you find that balance. If you don't want to go to that effort, I'd suggest going with about 1 cube per sub of sealed volume for your HDS312s.

Forget everything you read about bandpass box ratios, they are based on a bunch of assumptions that are often false and lead to people doing silly things like making their front chamber bigger just because they made the rear chamber bigger and want to keep their "ratio" the same. Seriously, ratios are worthless. Sizing and tuning your front chamber comes down to how you want your bandpass box to sound. You want to tune your front chamber to close to what your rear chamber resonance is. Ideally for car audio use it should be somewhere in between 40-50 Hz. When it comes to front chamber sizing, much like with ported boxes, the larger you make the front chamber the more output you will get, but only around your tuning frequency. If you make it too big you will end up with a frequency response that looks like the Eiffel tower, a one-note-wonder. That's great for a burp box but its going to sound like hell on music. With bandpass boxes you have have really high efficiency (one-note-wonder) or you can have a wide bandwidth (that sounds good on music) or you can have something somewhere in between, but you can't max out efficiency and bandwidth at the same time, so choose carefully. Again, modeling software can help you make an more informed decision.

As far as port sizing goes, all your output is coming through that port so you don't want to cheat yourself on port area. I try to keep port velocities below 22 m/sec with bandpass boxes and even lower velocity is better. The size of your front chamber, the tuning frequency, the particular subs you are using, and the amount of power you are running are all going to affect how much port area you need. 1.5 - 2.0X as much port area as what you would use for a ported box is a reasonable starting point. Once you decide how big of a front chamber you are going to use I can get you a more specific recommendation for your application.

[tayodell41]

Okay, so I'd like to put up some decent numbers with the subs but when it comes down to it I want to sound great when playing music. If ratios don't matter, I'm unsure how to calculate the front chamber size.

[Triticum Agricolam]

For a starting point, you can use the same volume for the front chamber as you would use if this was going to be a regular ported box. So in your case you are looking at 1.5 - 2.5 cubes per sub, again this is only a starting point and you can go outside those numbers as the situation dictates.

As you make the front chamber larger, you will get more output, but give up bandwidth, the opposite happens as you make it smaller. You said you want decent numbers, but still want it to sound good. Which of those two goals is more important to you?

[tayodell41]

I don't know if I'll compete but I'll definitely listen every day, so sounding good is more important.

[Triticum Agricolam]

I really hate to give suggestions for the front chamber size, it really depends on just how you like your music to sound and how much you can tolerate a peaky box. However, in your case I'd probably go with 3 - 4 cubes ported at 45 Hz.

With 3 cubes your high and low f3 points will be at about 50 and 30 hz, so above and below that your output is going to drop pretty noticeably. If you increase your front chamber size to 4 cubes you will gain a little over 1 db of output, but your f3 points change to 50 and 33 Hz. At 5 cubes you gain another db and loose a couple more Hz on the low end F3.

Please note that the numbers I listed above are with approximated cabin gain added in. Cabin gain is notoriously unpredictable so what you actually end up with could be very different. That's just the nature of the beast. What I can tell you with more certainty is if you increase your front chamber size it will increase your output, but you will loose bandwidth.

For port size I'd suggest 80 sq in with a 3 cube front chamber, 100 sq in for a 4 cube chamber, and 120 sq in for 5 cube chamber. Also, I strongly recommend you make your port removeable or otherwise adjustable so you can tweek your front chamber tuning after you get a chance to listen to it. Bandpass boxes are very sensitive to chamber tuning changes and you may want to adjust it up or down a bit.