Port Tuning Calculator
Already have a port? Find the tuning frequency of your box from volume, port area, length and number of ports.
Inputs
Physical length of the tube, not including end corrections.
How it works
This is the reverse of the ported-box calculator: you already have a port (or a pile of PVC tube) and want to know where the box will be tuned.
Fb = (c / 2π) · √( S / (V · (L + k·D)) ) c = 343 m/s S = total port area V = net volume L = port length D = equivalent diameter k = end correction
Air in the port acts as a moving mass; the air in the box is a spring. A longer or narrower port means more mass → lower tuning. A bigger box is a softer spring → lower tuning. Doubling the number of ports doubles the area, so each one must be about twice as long to hold the same tuning.
End correction: air just outside each end of the tube moves along with it, so the port behaves slightly longer than it is. A flared or flanged end adds ≈ 0.85 × radius; a free, open end adds ≈ 0.61 × radius. A slot port that shares walls with the box is treated as flanged at the inside end.
For a port to be effective, its area should not be too small for the driver, see the air-speed check on the ported-box page. Also keep the open end of the port at least one port-diameter away from walls or the cone.