Seeking design info beyond 'box calculators'

[loudNlow87]

So I feel I've built enough using these calculators (multiple different calcs and literally dozens of boxes) that I'm now interested in theories beyond what these things can tell me. I've read the living hell out of the web in an effort to understand more, yet still find myself unclear on some things.

Port area - I see all too often the "general rule" of X"-Y" per cube for Z-type of port. Or "1/3" or "1/2 cone".. This should be more so determined by box type, Sd, Fb and Xmax, no? I do understand velocity and the effects of it. How does power being applied come into play here? What about it dictates what kind of area you're looking at?

Same with Vb.. I get that larger boxes tend to be more efficient and help low end extension, but can also put you into harms way mechanically. Again, how does power directly relate to the determination of Vb?

Fb - This bugs the hell outta me. After playing with tunings on pervious boxes anywhere from upper 20's to sub 40's, I've learned quite a bit about response, roll-off's and peaks. What I don't get is when I see people saying things like "I'm tuned to 38 and play flat down to 25hz" How? I know the acoustics of the vehicle are a whole new bag of factors, but.. It's still not making sense to me after having personally tested these 'theories'. The last 6 boxes I've had in my car haven't done a damn thing much under Fb, minus the SP4 I'm currently using. At 28hz Fb, I can still dig down lower and it will play with some sort of authority. But at 36 Hz Fb, I was NOWHERE close to playing down into the 20's like some are quoting they can. Only reason for the low tuning is because decaf has become somewhat of a cocaine for me. So what am I missing here? What's the magic I'm missing that would allow me to bring tuning up a bit so I can get that mid 40's 'choke' back, yet still retain playability down to upper 20's at least?

I'm finally ditching the rear seats so I can accompany 4 15's comfortably. So I'm hoping to add any tips/tricks from some of you veterans to this box. Maybe shed some light on my confusions. Thanks for any and all HELPFUL input. Not really looking for "My friends, cousins, aunts plumber told me.. " type of stuff. More in search of factual and/or personal experience. Thanks again!

[loudNlow87]

Oh.. And how people get away with stuffing multiple large woofers in a tiny box with such little port and yield decent results..?

[Soccerballzs]

Oh.. And how people get away with stuffing multiple large woofers in a tiny box with such little port and yield decent results..?

Go Jeep!!

[RooTxBeeR]

In for info too. I know what your talking about when people say they are tuned high and can play low. I see a lot of that when peoplle are talking about the DD9500 series. They say tuned to 36-38 and still slam high 20's. I don't get it at all either. I want to because I plan on getting a pair of those some time in the future and would like to be another one of those guys doing that.

[Guest]

In for info too. I know what your talking about when people say they are tuned high and can play low. I see a lot of that when peoplle are talking about the DD9500 series. They say tuned to 36-38 and still slam high 20's. I don't get it at all either. I want to because I plan on getting a pair of those some time in the future and would like to be another one of those guys doing that.

stiff suspensions brah. helps with cone control

[RooTxBeeR]

In for info too. I know what your talking about when people say they are tuned high and can play low. I see a lot of that when peoplle are talking about the DD9500 series. They say tuned to 36-38 and still slam high 20's. I don't get it at all either. I want to because I plan on getting a pair of those some time in the future and would like to be another one of those guys doing that.

stiff suspensions brah. helps with cone control

What happens though after you have been beating on it for over a year. Wouldn't that start to diminish and propbaly start bottoming out a lot earlier than it used too?

[n8ball2013]

Dd stays stiff. Even the cones I pulled out to recone were stiff.

[Triticum Agricolam]

Another member PM'ed me a while back asking about this. Here is what I told them:

To understand how a ported box is going to sound you first need to understand how a ported box works. I'm not the best at explaining things, but I'll give it my best shot. Also this is all based off my understanding of ported box theory and it may not all be 100% correct, but I think I'm at least pretty close.

If you were to take a ported box and graph just the output from the driver alone, without the output from the port, it would look pretty much like a sealed box with a gradual ~12 db/octave decreasing response starting from pretty high up (100 hz or so, depends a lot on the individual driver and the enclosure).

Now if you were to graph just the output from the port the graph would look like a hump centered on the tuning frequency and tapering off at ~12 db/octave above and below that point.

Since the output from the driver is rolling off at 12 db/octave as you go down and the output from the port is rising at 12 db/octave as you approach the tuning frequency, when these two outputs are combined you get a nice flat response down to around the tuning frequency of the port. Then it rolls off at 24 db/octave since the output from the port and the driver are both decreasing at 12 db/octave.

Now that you (hopefully) understand how ported boxes work, lets look at what effect different parameters of the box has. First lets look at tuning frequency. The higher the tuning frequency the more gain you get from the port and the higher up the hump will be centered. If you look at the graph below there are three examples. The green line is tuned to 31.5 Hz, the red to 40 Hz, and the yellow to 20 Hz. All three use the same size enclosure.

As you can see the red line give too much gain at too high of a frequency, resulting in a hump in the combined response. The yellow line doesn't give enough gain and is at too low of a frequency, so we get a falling response curve instead of the flatter one we want.

If keep the tuning frequency the same and vary the enclosure size this is what we get:

The green line is 1.5 cu ft, the red line is 4 cu ft, and the yellow is 1 cu ft. As you can see the larger the enclosure, the more gain you get from the port. If you have too much, you get a hump, and if you have to little, you get a falling response.

So knowing this we know we need to have the right tuning frequency and the right internal volume to get a nice flat response curve down to as low as is practical. Looking at the graphs about the 1.5 cu ft enclosure tuned to ~31 Hz seems to be what we want right? Unfortunately it isn't that easy, there is a big part of the picture we are leaving out and that is cabin/room gain. If this subwoofer was going in your house the 1.5 cu ft enclosure tuned to 31 Hz would probably be pretty good. This is because right about where the output starts falling off is where the room gain is going to start kicking in, thus extending your flat response down another half an octave or more. In a car however, the cabin gain is going to kick in at a much higher frequency, giving you a response curve more like this:

The red lines are a ported box with and w/o cabin gain and the blue lines are a sealed box. Unfotunetly modeling cabin is gain is not an exact science. Every vehicle is different and the actual gain is not going to be a nice even curve. The best we can do is an approximation, but its better than nothing. Cabin gain does make a significant difference in how a subwoofer will sound in the vehicle so its important to factor it in.

Looking at the above graph it looks like a sealed box would be the way to go since it has an almost perfectly flat response down to well below 30 Hz. This is the reason a lot of folks like sealed boxes, especially in vehicles with smaller cabins (trucks). The thing is most people (myself included) don't prefer a flat response curve, at least not in their vehicles. Most folks like a rising response, to some varying degree. Personally I like a mildly rising response, something around 3-6 db of gentle rise between 80 and 30 Hz. To get this we go back to ported enclosures. Below is the enclosures from above that were all tuned to 31.5 Hz but with different volumes:

Here you can see the red line is the 4 cu ft enclosure. It has a huge peak at tuning, this would give you a very boomy sound, some folks like this, I am not one of them. The yellow line is the smallest box, it has a gradual increase, but I'd probably want a but more. The green line is the 1.5 cube box. I think I lot of folks would be happy with this, though I'd want something between it and the yellow line.

When you start looking at a lot of enclosures people use with the cabin gain added you will see that most have a huge peak in the response. This comes from the box having too much volume and from tuning too high. Its why I try to design my boxes to be tuned low and with volumes on the smaller side of things. I guess if boomy bass is all you have every heard you might like it but once you hear a box with a more even response I don't think you will want to go back. I would encourage you to learn to use WinISD. Its a powerful tool, and while not perfect, it will let you at least get an idea of how an enclosure will perform.

[Joe X]

OP every car (and it's install) is different and everyone's listening and personal taste also different, if something worked for someone, it doesn't mean it will work for you.. at all.

Also you frequently find that someone has a 150W front stage that will die off at 90Hz and a 3K sub stage with a peaky alignment and he is complaining that it doesn't play flat.

So applying common sense is important, gain the ability to do measurements is also important, estimating your cabin gain is a step towards understanding what's going on.

Again measurements are most important, you can try some modeling (with software made for car audio) to save you some time and anticipate potential damage to your drivers. Lastly do some reading of trustworthy materials before listening to rumors, claims, rules of thumb and such.

[alaskanzx5]

Living the info in this thread and waiting for more