# Length/trimming of Precision Ports...

Discussion in 'Home Theater Projects' started by BrianJ>Y, Apr 20, 2003.

1. ### BrianJ>Y Stunt Coordinator

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Hi all,

After following the instructions for determining the lenth, my Port length comes out to 14.6 inches. Does that mean I have to cut 2.4 inches (17-14.6) off of the center tube?? I don't see any other way, but I'd like to make sure.

Thanks

2. ### Brian Bunge Producer

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Brian,

That's exactly what you do. Just cut the appropriate amount off of the center tube. IIRC, the overall port is 18" with the center tube at full length.

3. ### BrianJ>Y Stunt Coordinator

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Thanks for the help Brian, but don't you mean 17" for full length due to the flared ends?

Also, when calculating the port length using the manufacturer's equation, do I take into account the volume of the driver and the port for my vb(volume of box)? Or just the driver's volume?

My box is 3.86cu-ft and my driver is an av12...what box volume should I use for my equation?

4. ### Brian Bunge Producer

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All I was saying is that the port is a full 18" in length before you cut the center tube.

The best way to figure up things is to come up with a specific volume you want to use for the enclosure. Then determine your necessary port length. Then figure up the volume taken up by the driver, port, bracing, etc. and add that amount to the overall enclosure volume.

5. ### Mike Keith Second Unit

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I just went through this, and it was brought to my attention that the best way was to make the port a few inches longer than suggested then play test tones to determine the fine-tuning. I tried this and it worked very well, I'm very confident that my enclosures are exactly tuned to 20Hz, it's very obvious with the test tone method, even 1 Hz difference and there is substantially more or less cone movement.

Also I noticed that the port had to be longer by a few inches to get the required Tuning, as opposed to the WinISD Modeling, which was wrong by a couple of Hz.

Good Luck.

6. ### David Lorenzo Stunt Coordinator

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WinISD was wrong by 2 whole Hz? Man that's bad. I can see if it was accurate within +/- .5Hz due to climate changes, but 2 Hz?

How can these programs vary so much when calculating port length. They should all be based on the same mathematical formula right?

7. ### Sven_DP Agent

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Try Unibox once. I find it to be much more flexible and it gives me more information. And it takes into account the amount of stuffing.

Just my idea...

8. ### Mike Keith Second Unit

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It,s not the program as such, it's the un-calculateable parameters such as the exact net volume of the box, try changing the volume a few liters and see what happens to the Port Length, which leads to the real problem the port volume it's self. If you deduct the port volume which changes the net volume (as you should) then it changes the port length, which again changes the net volume, and if you account for the extra port length then it changes the volume again, and so on and so on..

I spoke with many different people who regularly use many different box modeling programs and they confirmed this, when I asked them what you do about it, they said to cut it long and fine tune with Test Tones.

I tried it and it worked beautifully, it is however a pain in the a\$\$.

First you need to Burn a CD with Sine Wave Test Tones (just use WinISD’s built in Tone Generator) starting at about 15Hz thru 30Hz in 1 Hz steps, each freq on a separate track @10 seconds.

Then play the CD with the sub connected and the port installed, and start a few Hz below the target frequency and lightly put your finger on the cone of the driver while the tone is playing (at about 85db). With the remote in your hand step up (by changing tracks) the frequency while feeling the driver. You will notice a major change when you hit the resonant frequency (the cone will almost be still compared to just 1 Hz above or below) then just cut a inch at a time to bring the Tuning up to the target frequency.

Remember cut the port about 3-4 inches longer that modeled, then cut approx 1 in per Hz to dial it in.

I was amazed at how well this works, and none of the modeling programs gave me the final length I used.

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