port length?

Ok so im about to start building my box exclusively for spl. The resonant frequency of my car is 45hz. So im going to tune my box at 40hz. My ports are going to be on the outside of the box. So how would i figure the length of the port?

To figure port length you need to know a couple of things. First you need to know the internal volume of the enclosure. Next you need to know the cross sectional area of the port. Lastly, you need to know the tuning frequency, which in your case is 40 Hz.

When figuring out what size to make the port you need to take into account the air velocity through the port. You do not want it too high or you will have port noise. You can use a program like WinISD to figure out what size port to use without getting too high of an air velocity. That will also tell you the length of the port.

You can also use the calculator.
https://www.the12volt.com/caraudio/boxcalcs.asp#por

That calculator is for round ports, btw. You would need something else for vents.

whiterob wrote:

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That calculator is for round ports, btw. You would need something else for vents.

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Huh?  What are you talking about?  Vents?  All ports are vents.

It is a simple bit of 8th grade geometry to convert a round port calculation to a rectangular port.  Use the calculator to determine the port size required for your application.  Then just calculate the area of the circular opening (area of a circle = pi * radius squared, so a 4" port is 12.56 square inches.)  The create a rectangle or square that has the same (or close to the same) area.  12.56 sqin could be created with a 1.5" wide * 8.38" tall port, for example.  The length given by the calculator does not change.

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So does the port calculator subtract the port volume from the box volume ?

theterminator wrote:

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So does the port calculator subtract the port volume from the box volume ?

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No, none of them do.  You have to take that into consideration yourself as the calculators assume you are entering the airspace volume for the woofer, although in most cases even if you ignore the port volume you are usually still within 10% of the desired tuning frequency, which is close enough for every day use.

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o will then my problem is solved. Since my ports or going to on the outside of the box that port calculator will be perfect just for the fact that i dont need to subtract port volume from the box volume.