Use simple formulas to calculate your enclosure port size.
The Formula for Port Area
Port area in square inches = volume in cubic ft x 16
For example, 3.4 ft3 x 16 in2 = 54.5 in2 of port area.
A 16 in. port length tunes the box to about 40 Hz. Measure the port length down the center of the passage.
How to Make a Slot Port
The easiest and most effective way to build a port is to use a slot-shaped port. A slot port uses 3 sides of your enclosure as port walls. If the port is located at one end of the box, only one additional piece of wood is required to complete the port. The additional internal wall will have a space equal to the width of the port between its end and the back wall of the box.
The port area is in square inches, meaning it takes height times width to calculate total area. A port 4 inches high by 4 inches wide would calculate as 4 x 4= 16 in2.
In our example box, we have an inside height of 14.5 inches. To calculate the slot width, divide 54.5 sq. in. by the height of 14.5 in. The result is 3.76 inches. Round that figure to nearest ¼ inch, and you get an offset measurement of 3 ¾ inch wide. The slot port would be 3 ¾ inches wide by 14 ½ inches tall.
You don’t have to get too hung up on the box and port volume. Staying within 5-10% of calculated volumes will result in very little audible effect.
Port Length Limitations
Some box dimensions do not allow for straight 16-inch port depths. The end of the port can’t get any closer to the back wall than the width of the port. So back to our example, we have an internal box depth of 12.5 inches. We need a 16-inch deep port. We will have to bend the port along the back wall to get the full port length. To calculate where to make the bend, subtract the port width from the inside box depth (12.5″ – 3.75″ = 8.75″). We next have to bend the port at 8.75 inches, continuing the port wall parallel to the back wall for an additional 7.25″ (8.75″ + 7.25″ = 16″). You now have a 16″ deep L-shaped port.
Practice calculating port size using ready-made examples of ports, check out our DIY enclosure page.