converting round port to slotted port

Area of circle = pi * R^2. Once you know the area of the circle, any L * W = A is what you require for the port.  A 4" round port should = 12.56 in ^ 2 IIRC.

Leave the length of the port the same.

Why doesn't the length change?  Isn't the goal to maintain the same port volume?  If so, doesn't the length of the port decrease as the width and height increase?

Jason

haemphyst wrote:

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No, because the AREA remains the same... 12.56 square inches of area, whether 12X1 or 4 inches across, is still 12.56 square inches of area.

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I see!  So the area of the port will remain the same...so the length of the port is more of a tuning device?

Yes.  As long as the port opening area (in square inches) remains the same, you can vary the tuning by changing the port length.  A longer port length results in a lower tuning frequency.

It is not volume of air in a port that counts...it is a calculation of port opening area and port length.  It's fairly complicated as it considers the speed of sound in the calculation.  For a given desired tuning frequency, a larger port opening requires a longer port.  That is the opposite of the idea that maintaining the same port air volume is what is necessary.  That's why when doubling the number of ports, the length of each one is doubled as well.  It's because the port opening space has been doubled.

The port opening area is measured as diameter if it is a tube, or it is length X width if it is a slotted port.  If it is a tube, use the pi formula Rob posted.

Here is one of my posts of long ago on the subject:

pi r^2 to find the area of a circle

pi is 3.14
r is radius, or half the diameter of a circle
^2 is squared, or the number times itself
pi r^2 is 3.14 X the result of radius squared
pi r^2 gives you the square inches of a circle

You want to change the shape of the opening but keep it the same square inches.  The length of the port is not a factor and you will leave it exactly the length it already is.

Find the square inches of your circle using pi r^2 (ex. a 6 inch diameter circle yields 28.26 square inches)
Decide on the longest side you want your square or slotted port opening to be
Divide the total square inches of the circle by that long side, in inches (ex. 28.26 / 11.5 = 2.46)
The result is the measurement of the shorter side of the port opening

In the example, the 6" round port equals a slotted port 11.5 X 2.46 inches.  Round measurements to the nearest Imperial or metric value.  2.46 would be  2 7/16 to 2 1/2.

You should keep the ratio of length to width of the slotted opening at no greater than 8:1.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

That is an absolute fact.

I read over what I posted and it sounds like a math professor.  Just for the sake of this thread I'll break that down a little more clearly:

• pi is always 3.14.  It actually has a lot more digits after the decimal point (3.14159265) but 3.14 is what is commonly used.
• When you see a math symbol followed by a number or a calculation (sometimes in parentheses), it is known that the math is to be multiplied.  That's why "pie are squared" (pi r^2) doesn't show the multiplication symbol  X or * between the pi and the r squared.  You just know it is to be multiplied, but you have to figure out what to multiply first.. You would do the math in parentheses first but in this case the math is to find the square, so it doesn't need to be parenthesized.  The number is simply shown with the small 2 trailing it.
• On normal keyboards, when you don't want to look up commands to use special math symbols, the small 2 that means "squared" is shown as ^2.  (Cubing a number would be shown as ^3, and so forth.)
• To square a number, multiply the number times itself.  In box building you will use both squared and cubed...square inches of surface;  cubic inches (or feet) of volume.  To cube a number you first square it and then multipy by the number again, such as 12 X 12 X 12 = 1728...the cubic inches in a cubic foot.

• Do the r^2 math first.  Divide the diameter of your circle by two to get radius, then multiply radius times itself.  That gives you the r^2 part (radius squared).
• Now multiply that answer by pi, which is 3.14.  That's the square inches in the circle.

For my 6" circle example that I used above, first find half of 6" which is 3".  Multiply 3 times 3 which gives you 9.  That is radius squared.  Multiply 9 times 3.14 to get 28.26 square inches.