Port Math - Home Theater Forum and Systems - HomeTheaterShack.com

Old 01-19-12, 03:16 PM Thread Starter

Join Date: Jan 2012
Posts: 40
My System
Port Math

When calculating the internal volume of a slot port I used the center line distance (including effective length correction factor of h/2) x cross sectional area. That measurement seemed to make sense. However, when calculating the volume displaced by the port my result doesn't match re audio box calculator. I used the width plus wood thickness x height x length (not including effective length correction factor of h/2).

What is the proper way to calculate the port displacement?

In SketchUp I drew two rectangles over my slot port then pulled them to the proper height - one over the internal port (including error correction), the other over the port including the wood used to shape the port (without error correction). I've included a screen shot with both volumes dimensioned, the larger one on the left includes the wood and has the volume calculated by SketchUp listed, the internal volume on the right.

Edit - btw, the internal port volume listed by SketchUp is 1222.3125 (which matches re audio box calculator)
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Last edited by skrap853; 01-19-12 at 03:23 PM.
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Old 01-19-12, 03:46 PM
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Jim

Join Date: Apr 2009
Location: Ohio
Posts: 2,116
Re: Port Math

I'm confused. From looking at your drawing, which is the port?

I calculated the volume for the drawing on the left and got:
22.125x20.5x2.25 = 1020.515625
13.875x20.5x2.25= 639.984375
1020.51+639.98= 1660.5

So, approx 1660.5 CuIn.

Normally I've just calculated volumes and not relied on Sketchup to do so...

I get +- 1147 cuin for the model on the right.
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Old 01-19-12, 03:50 PM
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Jim

Join Date: Apr 2009
Location: Ohio
Posts: 2,116
Re: Port Math

If you haven't seen EVC, I recommend it. I find it easier to use than most calculators:
http://www.hometheatershack.com/foru...me-calculator/
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Old 01-19-12, 04:04 PM Thread Starter

Join Date: Jan 2012
Posts: 40
My System
Re: Port Math

Quote:
Zeitgeist wrote: View Post
I'm confused. From looking at your drawing, which is the port?

I calculated the volume for the drawing on the left and got:
22.125x20.5x2.25 = 1020.515625
13.875x20.5x2.25= 639.984375
1020.51+639.98= 1660.5

So, approx 1660.5 CuIn.

Normally I've just calculated volumes and not relied on Sketchup to do so...

I get +- 1147 cuin for the model on the right.
On the left in the drawing is the port including the wood to build it. The right is the internal volume of the port.

why did you use 13.875 instead of 16.125 in your math listed above?
skrap853 is offline
Old 01-19-12, 04:28 PM Thread Starter

Join Date: Jan 2012
Posts: 40
My System
Re: Port Math

Quote:
Zeitgeist wrote: View Post
If you haven't seen EVC, I recommend it. I find it easier to use than most calculators:
http://www.hometheatershack.com/foru...me-calculator/
I plugged the following numbers into the EVC

W - 20.5
H - 2.25
D - 38.25

= 1764.281

Maybe the re audio calculator is wrong. I was just trying to write the formulas so wanted to make sure I'm doing it correctly.

Also I was reading that a correction factor of half the height divided by two should be added to determine the effective port length since the enclosure walls are being used as part of the port. I wasn't sure if that should be added to the calculation when determining port volume displacement.
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Old 01-19-12, 05:26 PM
Elite Shackster

Jim

Join Date: Apr 2009
Location: Ohio
Posts: 2,116
Re: Port Math

Quote:
skrap853 wrote: View Post
On the left in the drawing is the port including the wood to build it. The right is the internal volume of the port.

why did you use 13.875 instead of 16.125 in your math listed above?
I did 16.125-2.25, which I realize that's wrong now - from looking at the dimensions.
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Old 01-19-12, 05:33 PM Thread Starter

Join Date: Jan 2012
Posts: 40
My System
Re: Port Math

Quote:
Zeitgeist wrote: View Post
I did 16.125-2.25, which I realize that's wrong now - from looking at the dimensions.
Just making sure I wasn't doing something wrong.

Only thing I'm not sure about now is what happens when the port face and corners are rounded and the correction factor.

Thanks for helping.
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