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treedog 13th January 2005 12:11 AM

4th order Bandpass- Effects of extending port outside of enclosure?
When building a 4th order bandpass enclosure let's say for example I am using a 3 inch diameter port and my calculations say the port should be 7 inches long....can I have 7 inches of port inside the box and say 3 inches or so sticking out of the box? Meaning the port would be 10 inches long in total...whould that change the tuning/effectiveness of the enclosure?

If the calculations call for a 7 inch port......If I wanted to have 3 inches of port sticking out of the enclosure would I just insert 4 inches into the enclosure and leave 3 inches out? OR would I use a 10 inch long port and leave 3 inches out?

Can this be done without negatively affecting the performance of the enclosure?

The reason I ask is because I am building a bandpass enclosure for my vehicle and I would like to have the enclosure in the trunk and port it into the passenger compartment.


simon5 13th January 2005 12:23 AM

I think you can do this, but you will need to measure the T/S parameters perfectly, 4th order bandpass is harder to design correctly.

treedog 13th January 2005 06:56 AM

Thanks Simon.

Can anyone else confirm this. My concern is how I would calculate this. Should I add an extra 3 inches to the port length or should I just not push the port all the way into the box?

I am using this program

Shavano 4th Order BandPass

to get me into the ballpark, then I will tune the box once built. I need a VERY small box that plays VERY loud and I need to inject the sound into the passenger compartment somehow. That is why I plan on going bandpass. The box is going into a 65 Shelby Cobra (no roof). I plan to put the box in my trunk and port it in between the pass & driver.


simon5 13th January 2005 06:30 PM

Just use WinISD Pro, the box volume they will give you doesn't include the port volume, amplifier volume or driver volume. So if the port is 100% external, don't add volume to the box, if it's only 3 inches outside, then you calculate the volume of the port sitting in the box and increase the size of the box that amount. Don't forget to add volume for the amplifier, bracing and driver to the box, in a 4th order box, it's very important.

BassAwdyO 14th January 2005 10:40 PM

The port length should be the same no matter if the port is inside or outside the box for the same tuning. Now box volume doesnt include the volume in the port, or the volume displaced by the driver itself. Keep these factors in mind for your bandpass.

paulspencer 17th January 2005 12:54 PM

This is quite acceptable. Be careful that the alignment you have in mind models well in the car when you consider cabin gain, which is often a 2nd order boost below 60-80 Hz. Hence to get a flat response in a car you need an F3 in that range for a sealed box. If you have any vented or other alignment, then you want to aim for rolloff to match that cabin gain. Don't design it as if it's a home theatre sub, it will boom.

Also watch how the vent terminates. It's best to have a flared radius on the outside end of the vent, not just have a non flared vent into free space. This relates to turbulence, although probably not an issue with a car sub.

treedog 17th January 2005 06:14 PM

Thank you very much guys.

So if my design calls for a 10" long port and I want to have 3 inches outside of the enclosure I will simply get a 10" port and leave 3 inches sticking out. Then only calculate port volume for the 7" part of the port that is in the box. Sounds simple enough.


troystg 17th January 2005 06:27 PM

Hey treedog-

Several installers including myself have been doing just such a thing for years.

It works especially well for preventing the all too familiar "trunk rattle" bass.

I have put the port in the rear dash and covered it with a non-discript grill so to the casual observer it was part of the car's venting..

Of course fancy lighting and paint usually hint at good audio and draw theives.. :xeye:

Another thing to consider is reversing the drivers mounting so the magnet is in the ported section to aid in VC cooling. If you are going to drive the woofer hard the extra cooling really helps.

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