# Why are small ported boxes bad?



## T3mpest (Dec 25, 2005)

I'm trying to figure out why no one seems to reccomend small ported boxes for car use. Just playing with WINISD and the box I think I like goes against most conventional wisdom so I wanted feedback. I'm modeling two AE AV15h's in 4 cubes tuned to 28hz. Now that's normally about the box you'd use for one of them, but is it really that bad in a car? The transfer function looks pretty close to my ascendant audio avalanche in 2.2cubes sealed, the main difference being I'd need 3 avalanches in 6.6 cubes to get the slighly less output. The avalanche sealed had a very nice sound to it, just not quite loud enough. Also smaller ported boxes like this have relatively low group delay vs a larger ported box. Anyway my car is a 99 eldorado so even though I do get cabin gain, I need alot of SPL just to get it into the cabin. Anyway what's so wrong with a ported box like this? You end up with a box that models like a slightly bottom heavy sealed box in a relatively small space. I guess in short, whats wrong with making the ported box smaller than even a sealed one and then using a low tuned port to make up the difference?


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## 6262ms3 (Feb 27, 2008)

I'm no expert on ported boxes, but if you watch your port specs in winisd while playing with box size you'll notice that to maintain the same tuning freq in a smaller box the port length will increase. Then you either have to increase the box size to compensate for the increased port volume (the space the port is taking up inside the box) or run an external port (port runs outside of the box). If you can find room for the longer port then I'd say give it a shot, I don't know what effect this has on sound but I doubt you'll notice it if the subs are in a trunk. Maybe some members with external porting experience can chime in?


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## Brian Steele (Jun 1, 2007)

T3mpest said:


> I guess in short, whats wrong with making the ported box smaller than even a sealed one and then using a low tuned port to make up the difference?


Nothing's wrong with it theoretically. In fact, one of my previous sub designs used an approach quite similar to what you're suggesting. The purpose of the vent was just to improve power handling in the passband, not extend frequency response in any significant way. 

Of course, power-handling below Fb went to hell.

Actually implementing the tuning with a suitably-sized vent might be difficult though.


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## mvw2 (Oct 2, 2005)

Download some box modeling software. Model up a sub and see what happens.

The design choice will show obvious results.

Usability will still depend on how well you blend it with your midrange.


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## 14642 (May 19, 2008)

THe problem with really small ported boxes is that it's often very difficult to tune them with a port that has a large enough diameter to pass enough air. For a pair of 15" woofers, you ought to use at least a pair of 6" ports.


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## 89grand (Nov 23, 2006)

Even if you manage to get the port diameter and length to tune a small box to the desired frequency, the response of the system is still going to be peaky and not even remotely flat.

Use something like WinISD and model a 10" sub tuned to 30hz in 1.5ft3 and then model it in .75ft3 tuned to 30hz also, and the two enclosures will have drastically different responses.


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## T3mpest (Dec 25, 2005)

89grand said:


> Even if you manage to get the port diameter and length to tune a small box to the desired frequency, the response of the system is still going to be peaky and not even remotely flat.
> 
> Use something like WinISD and model a 10" sub tuned to 30hz in 1.5ft3 and then model it in .75ft3 tuned to 30hz also, and the two enclosures will have drastically different responses.


A small ported box tuned low is peaky in the same way a sealed box is peaky. The only difference is that it rolls off a bit slower above tuning, and faster below, but I'm tuning near a point with no musical content. Using WINISD to actual model is what got me thinking about this in the first place. Sealed boxes dont have enough cabin gain in my car to be give me flat response and my cabin eats up enough bass that output it a issue unless I use pretty big subs. It seems like what I need is still a falling response from sixty down or so, but with a more shallow slope than sealed provides. To get around the long ports I may use passive radiators, but IDK yet as thats expensive.


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## chad (Jun 30, 2005)

89grand said:


> Use something like WinISD and model a 10" sub tuned to 30hz in 1.5ft3 and then model it in .75ft3 tuned to 30hz also, and the two enclosures will have drastically different responses.


That's grabbing as little deep, some drivers don't like small enclosures be it vented or sealed, it's driver dependent.


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