Bandpass Box Design - Reopened!

[Chase Wallace]

Hi everyone, I'm looking for suggestions on how to design bandpass boxes in general but I'll focus around my system and the recommended box specs given to me. The subwoofer's that I have are two PSI Platform 2 15's HP upgraded 1800rms. I have a Twisted Sounds 3.5k (dyno'ed around 4850rms) to power them. The box will be used in a blow-through application in an extended cab Chevy 1500 series.

These were the suggestions by the manufacture "You could run them in 2.0 ft.³ per subwoofer sealed and 3.0 ft.³ per subwoofer ported, tuned at 42 Hz. They should sound excellent and play very loud and that size of an enclosure. The port for a bandpass box needs to be bigger because all of the output comes from the port. You generally need to make the port as large as you can without taking up too much room in the enclosure."

We spoke back and forth in a few more emails and he later said to the box needed to be tuned closer to 45 Hz not 42 Hz as he stated earlier. I had questioned him some about their recommendation of 12.5 square inches per cubic foot and if that was based off using a slot port or aero port. This lead to me asking him if a 10" aero port would work for the bandpass box because I first misunderstood that he was originally giving me their suggestion for ported boxes and I had applied that suggestion to the ported section of the bandpass box. A 10" aero port would be just over 12.5 square inches per cubic foot in a 6 cu ft box. I would think that this would be far too small for a bandpass box but he insisted that a bandpass box with those specs would be fine with a 10" port. Now he was suggesting that this would work fine and I'm not trying to say that it wouldn't get loud with a 10" port BUT by his own suggestion of generally making the port as large as you can, a single 10" port for a bandpass box seems very small. It would be right at their suggested spec's for a ported box but from everything I've read this would be extremely small for a bandpass box and I don't want an anemic port after all the money and time that has and will go into this project.

WinISD modeling: I've been playing around with the suggested box specs that were given to me and I can't help but to notice that this has a huge nasty dip right in the 30-40 Hz area. It has a nice peak centered around 50 Hz but that peak has a steep drop off on either sides and doesn't seem like it will really do much below 40 Hz with the spec's given and would basically just play 40-65 Hz with a large peak at 50 Hz. Now I'm not used to seeing these kinds of peaks but mainly I'm bothered by it being centered around 50 Hz as I like the lowest low's...  So I started playing around with the spec's given to see if I could at least do something about this huge dip right at 40 Hz and basically my conclusion is that adding 2 cu ft to the ported side of the box would help's a LOT in many different ways, it takes out the crater in the dip below 40 Hz smoothing out the response falls off to a slop without a big dent between 30-40 Hz. It also brought the peak down by a tiny amount while making it taller/louder. So 8 cu ft ported 4 cu ft sealed seemed to be better in every way over 6 cf ported 4 cf sealed, every way except in that the box size is physically larger but I think I can manage it. I've heard some people giving ratio's as guidelines and 2:1 and 1.5:1 seem to be commonly given as suggestions. I'm wondering if I wasn't given the 6 cu ft ported and 4 cu ft sealed suggestion just as a starting guideline that would give me something that would work without taking up too much space. Does anyone think that there is some other reason why or why these spec's shouldn't be modified?

Also, back to the topic of port area and the actual port. I've read suggestions of 50-100% of the total cone area or SD should be used for a starting point. The total SD value converted to inches for my sub's is 257. The subwoofer cone area charts that everyone likes to argue over gives 353. It seems to me that if I went with two 10" aeroports it would put me at roughly 178 sq inches of cone area, roughly 0.69% of the total SD and, roughly 75% of the total cone area. 45 Hz with two 10" ports would require about 18.5" length if I remember right. This is for the modified larger ported section of 2:1 ratio that I came up with in WinISD and not the smaller recommended size above.

Could anyone teach me something? Anything to add or critique? Am I wrong somewhere, everywhere, totally still not getting it? Please let me know, thanks! 

[Joe X]

WinISD is free modeling software that support 4th bandpass boxes, it needs a little mod to roughly show in-car performance, you can test their suggestion on the software and make your mind, and compare with ported. You do get right one thing, the cutout would be much smaller than ported, worth looking into a 4th bandpass in your case.

[Chase Wallace]

Joe X, I sent them a reply and from the reply that I got back they were telling me that 12.5 in.³ is what they suggested for ported boxes no matter if it was a slot port or aero port which is something that I had asked about. Also in his reply he suggested that bigger is better in a bandpass box because all of the sound comes from the port. He didn't give me a size. He just suggested that I use WinISD like you. Now I've been using WinISD for many years and I haven't seen the option to model a bandpass box. He suggested it, you suggested it, so I just checked again and...there it is in my face, options for both a 4th order and 6th order! I suppose my question's are answered by "go as big as you can on the port and used WinISD to find the length to tune it".

[Joe X]

You need to apply a fix to see how it will play inside a car or else you will get ridiculous frequency response plots, if interested I can tell you how, you killed your thread a little early I think.

[Chase Wallace]

Joe X, of course I'm still interested! Please do let me know what fix you're talking about and why it needs to be applied? I mean I never thought that WinISD was going to be able to model what the box would actually do in each individual car with so many different factor's like the airspace in the vehicle, resonate frequency of the vehicle, layout ect. I just though of it as a general tool. If you know of something that will help make it more accurate and could read over the new version of my post that would be great.

I closed the topic because I didn't want to misquote someone especially not dealing with a respected manufacture. Plus I thought that a lot of the pieces were starting to come together better inside my head as far as my understanding of things. I sure hope it is because it generally doesn't seem like many people usually want to help on most forums. I'm sure everything I just wrote seems like a book to most people...

[Joe X]

WinISD wil plot in-car performance more accurately if applying the following filter:

 

 

Notice that since no actual filter exists in your car the other plots will be inaccurate while this filter is active, this is just for frequency response estimation.

This fix will prevent winISD to output a ridiculous and useless plot of in-car response but it's not meant to be accurate,  the only way to have an accurate cabin gain frequency response curve for a given vehicle, power level, listening location, etc is to measure it any apply that measurement to the response predicted by modeling software which corresponds to an "outdoors" listening environment. WinISD can't perform that operation so you will have to use a different software to do that.

 

[Chase Wallace]

Joe X, thanks, but I can't find where to enter in this information or a filter that looks like this. I always assumed it was guessing for outdoors just because the number's it predicts are so low lol. Since we can't accurately model everything inside of the vehicles, wouldn't it be the same to just compare apples to apples so to speak by overlaying multiple graphs? For example I have the graph for a ported box for these 15's and a graph for a ported box for the 18" model all overlayed with the graph for the bandpass box I described above to compare them. I know this isn't going to tell me how they behave in my vehicle but it seems like it would let me accurately compare them.

Maybe I'm missing the point because I don't understand exactually what this filter does?

[Joe X]

I entered the standard version of your sub in some software that supports cabin gain modeling so that you can see what it does to frequency response, box specs are for a single sub, 3 net selaed, 6 net @45Hz ported with a port area of 12.5 sqin per cube or 75 square inches of port area:

Even if it is not very accurate modeling cabin gain let's you make sense of what you are doing, the red plot, is the outdoors performance, as you can see it would be useless to build that box for an outdoors application and the plot provides little information on what you would get inside a car: 

You see here that the port airspeed peaks at about 18 m/s which would be fine in this case:

 

On winISD the filter in this application just imitates what cabin gain theoretically does to frequency response which is a 12 dB per octave boost downwards in frequency starting at f0 which defaults at 50Hz, on WinISD v0.709 you just select filter, add, and then select linkwitz transform, you then check to enable it and you are done for the default settings:

The frequency response plot will update to simulate cabin gain, remember the other plots are invalid until you disable the filter. 

[Chase Wallace]

Joe X, ok, so we are just using different version's of the software. I'm using WinISD beta because I've never been able to get WinISD to work correctly without giving me error messages when it loads. Your box spec's are way above what I was told or what I stated that I've tried. You went 1 cu ft larger sealed and 3 cu ft larger ported. From what I've noticed doing what you did has a diminishing return of flattening and smoothing the low end while making a higher peak. I'm just wondering if that was done on purpose or on error or chosen randomly?

Also, I understand that 12.5 sq in per cf of port area shouldn't cause a problem in a 6 cubic foot box "75 total" They were only making that suggestion for their normal ported boxes and saying that it would work for a bandpass box but bigger is better as in basically as big of a port as you can fit is best for a bandpass box as a general rule. So I wasn't questioning if there would be an issue with port noise. I was more questioning how much output would be lost or could be gained by going bigger.

If I understand what this filter does correctly it's going to boost the output by 12 dB centered around 50 Hz and sloping down to the F0 point. In essence it would boost what was shown by 12 dB at 50 Hz and drop that off by 12db per octive down to the F0 point. This sounds much like adding a 50 Hz 12 dB bass boost on a 12db Octive slope. I get why this would help you get an idea of what you might see with cabin gain but at the same time this is in no way accurate and seems like something that you could just picture in your mind the same way that I've always pictured there being a boost due to cabin gain that wasn't being shown. It also seems like it would give you an inaccurate picture of what different subwoofer's and enclosure's were actually doing when comparing between them.

I hope that you don't see this as critiquing you're help. I just want make sure I'm understanding and why things are like that. I've only ever been able to get WinISD beta to work correctly so I'm guessing I can't use the filter. I actually recognize the name of it so I'm thinking that I've seen it mentioned before. As long as I understand what it's doing then I don't really understand why I need it other than as a smudge factor to get the db scale to show up more closely to account for cabin gain.

[Joe X]

In the case of a 4th order bandpass the box will act mostly as a sealed box at low frequencies until it gets close to the tuning of the front chamber port, at that point the port will boost the output of the box creating that peak you saw modeled, if you were not to use any cabin gain fix the only thing that will happen is that WinISD will lead you to believe that you will have a poor bottom end performance, like no less than 40Hz as you stated in your OP but that will not be the case in the actual system mainly because cabin gain and the slower roll-off of sealed enclosures, with bandpass boxes the harder issue is to blend in with the front stage, as you have seen above the port tuning frequency output rolls-off fast so a mid-bass competent front stage is necessary.