# Effects of over or undersizing a sub enclosures



## Gregor (Dec 16, 2006)

I'm trying to get a little more familiar with subs and looking for a bit if info on what effects oversizing or undersizing the enclosure would have on the sound. This would be for ported and non-ported designs. The scale would be between half the rated size to double rated size for a given driver.

From what I've read so far, oversizing provides better low end extension but may reduce power handling in drivers that are depending on air behind the driver to dampen thier excursion. For IB or long excursion drivers, this is not an issue. 

Undersizing reduces the low end extension as well as reduces the efficiency if the driver as the cone excursion is so heaviely damped by the limited amount of air behind the driver. 

I'm not sure what happens for ported designs except that in general they need to be larger, so its easier to undersize and end up with a very difficult to tune port design.

Am I headed in the right direction? Other thoughts and comments?


----------



## Xander (Mar 20, 2007)

I'm no expert, but you should download WinISD. Just play around with it, you can see the changes in frequency response while you play with box size and tuning, it's pretty cool. And you can choose a sub in the program or put in your own, as long as you have the T/S parameters


----------



## npdang (Jul 29, 2005)

Good suggestion... play with Winisd and find out 

For sealed enclosures really you're just changing the low end sensitivity... in other words how much power you'll need at a given frequency to achieve a given output. Increasing the sensitivity at low frequencies does lower mechanical powerhandling, but I've also seen where too small of a box can decrease thermal powerhandling where you have poor heat dissipation.

Another issue is dampening... an enclosure too small is what's referred to as an underdampened alignment... which audibly can sound like a "blurry" effect to bass notes... much like how your car panels store energy.

For vented enclosures generally you will get more port gain with a larger box, but also require a correspondingly larger port size for any given tuning frequency.


----------



## saMxp (Jun 22, 2007)

One thing to consider is that with ported designs, enclosure volume accuracy is more critical. If you tune for a certain enclosure volume and port length, and miscalculate, the effect will be more noticeable than with a sealed enclosure.

Measure twice, cut once.


----------



## Gregor (Dec 16, 2006)

Thanks for all the replies. I didn't realize WinISD was so easy to use. Lots drivers already in the database. Unfortunately not the two I was toying with, but they were plenty easy to add. 

Just sharing some results here. This was for the Polk MM 2104 for three different enclosure sizes. 

The green graph is a size of 0.4 (cubic feet). I choose this size to show how an undersized box causes a big spike in the frequency ranges between 60 and 100hz. This comes at the cost of the a fair bit of low end extension. Since the sub won’t even be playing much above 60, all this box does is reduce overall output.

Next is the at 0.66 in yellow. This Polk’s recommended size for the driver. You can see that the response is fairly flat out to about 90hz then gently tapers off. As is obvious, this seems like a good compromise.

Last is the blue which is 0.90. As you can see this enclosure has far better extension, 2db more than the .66 box at the lower frequencies. The cost is some sensitivity around 60hz to 100hz. As its only about 1db difference, I think I’d rather have 2db more around 40hz, than losing 1db above 60hz. Of course that assumes the driver can handle the excursion without mechical noise, or slapping the back of magnet. I also ran the numbers for 1.2 and 2.0. The same trend continues but not as dramatically. 

Thanks again for the tips!








[/QUOTE]


----------



## Scott Dodge (Nov 6, 2006)

Throughout my experience, adding a couple tenths of a cubic foot never really hurt anything. I've tried most of my subs in different boxes, and for the most part, adding .2-.4 to the recommended sealed size has added a noticable difference in low frequency extension, without sacrificing "accuracy".


----------



## npdang (Jul 29, 2005)

You may want to read the thread in the tutorial section on winisd also


----------



## Gregor (Dec 16, 2006)

npdang said:


> You may want to read the thread in the tutorial section on winisd also


Will do. Next question, what is the effect of polyfill in a sub enclosure? WinISD doesn't appear to model it.


----------



## 89grand (Nov 23, 2006)

Gregor said:


> Will do. Next question, what is the effect of polyfill in a sub enclosure? WinISD doesn't appear to model it.


I'd say it's pretty subtle. It certainly is no substitute for a properly sized enclosure.

I believe the rule is something like 1 lb. of polyfill buys you like 10-15% of the enclosure volume for roughly every 1 ft3. That's not very much really. 

In other words, if you wanted 1 ft3 but ended up with .9 ft3, 1 lb. polyfill would create an enclosure that would behaved like it was 1 ft3.


----------



## Oliver (Jun 25, 2007)

Tom Nousaine wrote an article once...http://web.archive.org/web/20020808224043/integra.cyberglobe.net/caraudio/resources/fiberfill/


----------



## 89grand (Nov 23, 2006)

Hic said:


> Tom Nousaine wrote an article once...http://web.archive.org/web/20020808224043/integra.cyberglobe.net/caraudio/resources/fiberfill/


I just glanced over that article, and it seems the gains are more than I thought they were.

One thing that I don't understand though, is with a ported enclosure, the 1.4ft3 enclosure behaved as though it was a 2 ft3 enclosure with 1.75 pounds of polyfill. Would the idea be to design the port for 2 ft3 box even though it was actually 1.4? Obviously the tuning frrquency would change if you started using polyfill.


----------

