Gummie

I finally downloaded winISD and started playing around with it. Of the four woofers I tried sealed, none of them got low very well at all.

I've discovered a few things thus far: increasing box volume for a given woofer will help low end while lowering output elsewhere (60 Hz and up). It seems to be a game of trades.

What kind of curve is considered good for a subwoofer? What would be the best for listening to rock, etc? Do I need to be able to 'get low'?

Gummie

Here are the same three subs again, this time ported. I discovered that greater box volume yields a much higher peek at the port frequency. I tried to blend box volume and tuning to get a fairly smooth curve while remaining within the volume limits of the box I will be building.

What would you consider to be optimal in this case? Obviously low end output can increase with the sacrifice of higher end output and/or a flat curve, but this is what I consider ‘ideal’ to the best of my knowledge. How much higher should I bump the tuning frequency? I’m currently between 24 and 27 depending on sub. How much effect would cabin gain have?

Gummie

For comparison's sake here are the two graphs overlaid on each other.

What's good? What's bad? Any and all advice/whatever is more than welcome as I'm trying to learn something new.

Jim85IROC

iTrader: (2)

You asked this stuff at a good time. I'm about halfway done with my "How to buy a subwoofer" article, and all of this info will tie into it very nicely, so I'm going to do a copy & paste from here into that article.

Generally, you should aim for a sealed box Qtc of about .707, at least to start off. This will give the lowest F3 (-3dB point in the rolloff), and will give a smooth 12dB/octave rolloff, which can compliment the car's cabin gain nicely. As you've found, as you go larger with the box, the low frequency rolloff is not as steep, but your F3 moves up. When you go smaller with the box, the rolloff is steeper, the F3 moves up, and you start to get a nasty bump in response right below the dropoff. Unfortunately, with the small box craze the way it is, all too often this is what you get when you use a manufacturer's "suggested" enclosure. Kicker is notorious for suggesting unrealistically small boxes, but the boomer crowd loves it because they get that big boost in output, even if it is in a narrow range.

Attached is a graph with the same subwoofer plotted with a Qtc of .707, .5, and 1.0.

The yellow plot has the Qtc of .707, and you can see that this one has the lowest F3, and has a nice smooth 12dB/octave rolloff, which will help compliment the car's cabin gain nicely.

The orange plot illustrates a Qtc of 1.0, which means that the box is too small. You can see that your F3 moved way up, and that the overall response really isn't very good. You get a bump in output, followed by a sharp dropoff. This is ok for the boomer crowd, provided the bump in output isn't at too high of a frequency.

The red plot shows the Qtc of .50, which is what's called "critically damped". You can see that the F3 moved up, but the rolloff is more gradual. When combined with a car's cabin gain, this will likely sound a bit lean in the upper bass, but will have the strongest deep bass of any sealed alignment.

One thing that's interresting to see here is that your frequency response really doesn't change all that drastically even with tripling the box size, however, cutting the box size in half really has a profound effect. What you can't see here is how the box size effects cone excursion. A smaller box will have a much larger resistive force on the cone motion, which will help increase power handling (physical power handling, NOT THERMAL). However, this increased power handling is at the expense of low frequency sensitivity, so you're still not getting additional output, just requiring more power to achieve it.

Ported info in the next reply.

Jim85IROC

iTrader: (2)

Here's a ported plot. I've only got 2 alignments here, the "standard" alignment shown in green , and the "EBS" alignment in blue.

The EBS (extended bass shelf) alignment allows you to get a lower F3, but it's at the expense of a smooth response. You can clearly see that it shelves off for a while, then falls below the port tuning frequency. This type of arrangement really isn't used all that often in a car environment, though it can be useful if you've got the space, because it helps push the port tuning frequency lower. Cone excursion below the tuning frequency in a ported enclosure is very, very high, and can easily cause damage to the sub if not controlled. For that reason, some people may intentionally tune the system low, and compensate with equalization, although generally once you factor in a car's cabin gain, you need heavy equalization to get rid of the excess bass, not to add to it. Since very little music has much content below around 35-40hz, it's usually a bad idea to tune your port above that area unless you run a sub amp that's got a subsonic filter that you can set just slightly below the tuning frequency.

The green plot shows the standard type of ported tuning in which a flat, smooth frequency response is desired. As with the EBS alignment, cone excursion is very high below the tuning frequency. This standard alignment is what WinISD will default to when you load a ported setup, but it can easily get confused if the t/s parameters are outside of a certain range. Once the Qts of a woofer gets too large, it becomes impossible to get a reasonable "standard" alignment without a massive box volume, and if the Qts gets even higher, it will be impossible to get a "standard" alignment regardless of box size. Maybe I'll make a separate graph to illustrate that situation.

The next reply will show small ported enclosures and how port tuning effects them.

Jim85IROC

iTrader: (2)

Here's what happens when you port a box that's too small.

First off, the yellow plot is the optimally damped (.707) sealed enclosure, just for comparison. Notice that the volume is about 1 cubic foot, just like the ported configuration I chose for this example.

The green plot is the optimally damped ported enclosure, used as another comparison.

The orange blue plots represent ported enclosures, each 1 cubic foot in volume. Their difference is the port tuning frequency.

For the orange alignment, the tuning frequency is about 23hz, just like the optimized green plot. This gives you a very good idea of what happens when you change just the box volume but not the tuning frequency. Overall the plot really doesn't look bad, in fact, it's a very nice comprimise between a conventional sealed alignment and an optimized ported one. The problem with this alignment isn't immediately noticable, and that's port length. When using a 3" diameter port, the length would need to be 34 inches. Good luck fitting that into anything reasonably sized. The smaller your box is, the longer your port needs to be to achieve the same tuning frequency. Re-read that, it's very important. The same tuning frequency in the 1.8 cubic foot plot (green) is achieved with a port only 16" long! If you build a complicated slot-port, or even if you decide to use a million elbows to fit the port into the enclosure, since the port's volume is NOT considered part of the enclosure volume, you'd have to make the box large enough to fit all of that port volume in addition to the 1 cubic foot of airspace. The result is a box that's bigger than if you just built the "standard" ported box to begin with.

So, we move on to the orange plot. This one shows the same 1 cubic foot, but with a more managable tuning frequency. This port length is only 13" long because it's tuned to 35hz instead of 23hz like the other two. But, you can see the disaster of a frequency response. This is a nice setup if you're a boomer who just wants the most output possible from your setup, but if you want any kind of sound quality, forget about it.