Port ends: why the same tube tunes differently

A port is a plug of air doing the work of a mass. The air does not stop neatly at the ends of the tube: a little of it beyond each opening is dragged along too, so the port behaves as though it were longer than it measures.

That extra is the end correction, and how much you get depends on what each end opens into.

The three cases

| Ends | Correction | Where you see it | | --- | --- | --- | | Both free | 1.22 | A tube floating clear of any surface | | One flanged, one free | 1.46 | The usual case: flush in the baffle, open inside the box | | Both flanged | 1.70 | Flush at the baffle and terminating against a panel inside |

"Flanged" means the opening sits in the middle of a surface, so the air spreads out over a flat plane rather than escaping in every direction. A port cut flush into the front baffle is flanged at that end. The other end is usually just an open tube in the middle of the box, so it is free.

Why it matters more than it looks

The effective length is the physical length plus the correction times the port radius, so the correction is not a small adjustment: on a short, wide port it can be a third of the total.

Picking 1.70 when the truth is 1.22 makes the port acoustically longer than you calculated, and the box tunes lower than you intended. Cut a port from a formula that assumed the wrong ends and the finished box does not measure where the paper said it would. This is one of the commonest reasons a build disagrees with its simulation.

What to choose

One flanged is right for almost every ordinary build: a round port flush in the baffle, opening into free space inside the cabinet.

Both flanged if the inner end is close to a panel, within about a diameter of it. A port aimed at a nearby back wall is effectively flanged at that end, and it is also a good way to make port noise worse.

Both free if the tube stands clear at both ends, which in practice means an external port or a tube well away from any surface.

If you are unsure, leave it at one flanged and check the finished box: the impedance curve will tell you where it actually tuned.