Gross, net, and the four things that eat the difference
External volume is the box you can measure with a tape. Gross internal volume is what is left after the walls, which on a 16 by 15 by 14 inch box with three-quarter-inch panels is already 20 percent smaller than the external figure. Net internal volume is what the driver actually sees, after the driver body, the bracing, the terminal cup and the port tube have all taken their share.
The gap matters because every published alignment is quoted in net volume. Build a box to a net specification using external dimensions and you will end up somewhere between 15 and 30 percent under, which shifts the response audibly and, on a ported box, moves the tuning as well. Work in the direction this page does: external dimensions to internal cavity to gross, then subtract every solid object inside.
Wall thickness is the input people get wrong. Nominal three-quarter-inch sheet material is usually a little under, often around 0.70 to 0.72 inches, and the error is doubled on every axis. Measure the sheet you have rather than typing 0.75 because that is what the label says.
Driver displacement is a published number
The volume a driver occupies inside the box is not the volume of a cylinder the size of the cutout. It is the magnet assembly, the basket, the cone and the part of the frame that sits below the baffle, and the shape is complicated enough that manufacturers just publish the figure. Find it in the driver specification, usually in litres or cubic inches.
If it is genuinely unavailable, the practical measurement is displacement: seal the driver in a bag, submerge it to the mounting flange in a container of water and measure what it pushes out. That is more effort than emailing the manufacturer, which is the recommended route. Estimating it from the cone diameter is not reliable, because the magnet is where most of the volume is and magnet sizes vary enormously between drivers of the same nominal size.
Port length, and why the calculated figure is a starting point
A ported box is a Helmholtz resonator: the air in the port is a mass, the air in the box is a spring, and together they resonate at the tuning frequency. Rearranging the resonator equation for length gives length equals the speed of sound squared, times the total port area, divided by four pi squared times the tuning frequency squared times the net box volume, and then minus an end correction.
The end correction exists because the air that moves does not stop neatly at the ends of the tube. Some air outside each end moves with it, so the port behaves as though it were longer than it is. The correction used here is 1.463 times the port radius, which is the standard figure for one flanged end and one free end. Change the geometry and the correction changes: a flare at both ends, a mouth close to a wall, or a bend in the tube all shift it, and none of them shift it by an amount you can predict from the dimensions alone.
Three practical consequences. Port length rises very steeply as tuning falls, because the frequency term is squared, which is why deep tunings need absurd lengths. Doubling the port area roughly doubles the required length, so two ports are not a way of making the port shorter. And the port itself takes up box volume, which lowers the net volume, which lengthens the port again, which is why this page settles the two by iteration rather than a single pass.
| Change | Effect on port length |
|---|---|
| Tuning down from 40 Hz to 30 Hz | Roughly 1.8 times longer |
| Port diameter from 3 in to 4 in | Roughly 1.8 times longer |
| Two ports instead of one, same size | Roughly twice as long each |
| Box volume doubled | Roughly half as long |
Why a bigger port is not simply better
A small port makes air move fast through it, and past a certain speed the flow becomes turbulent and audible as a chuffing or huffing noise on bass notes. Enlarging the port fixes that by slowing the air down, which is why big drivers get big ports. The cost is that a bigger port needs to be much longer for the same tuning, and there comes a point where the tube will not fit in the box at all.
That is the trade every ported design negotiates: enough port area to avoid noise, short enough to fit, at a tuning that suits the driver. The escape routes are slot ports along an internal wall, which use the box geometry as part of the tube, L-shaped or folded ports, and flared ends that tolerate higher velocity before turning turbulent. All of them change the effective length in ways the plain formula does not capture, so all of them end at the same place: build it long, measure, trim.
Measuring the tuning you actually got
The real tuning of a finished ported box is visible in its impedance curve. A ported enclosure shows two impedance peaks with a minimum between them, and that minimum sits at the box tuning frequency. Sweeping the impedance with a small test rig will tell you in a few minutes whether you landed on the number, and if you did not, whether to lengthen or shorten.
Everything on this page is a starting point to be measured, not a specification. What governs the design is the driver manufacturer parameters and the alignment those parameters call for. A box volume that is correct for one driver is wrong for another of the same size, and no amount of careful joinery compensates for the wrong volume.
Where this connects
Once the box exists, where it goes in the room matters at least as much as what is in it, which is the subject of the speaker placement calculator and the acoustic treatment calculator. For the frequencies and wavelengths involved, the note frequency calculator converts pitches to hertz and metres. If the build is happening in a shop, the cut list optimizer handles the panel layout and the plywood sheet calculator the sheet count. Power and heat for whatever drives it are on the AV rack power and heat calculator.
Questions people ask
Do I use internal or external dimensions for enclosure volume?
Internal, and then you subtract from there. Published box volumes are net internal, meaning the air the driver actually sees. Start from external dimensions, take off twice the wall thickness on each axis to get the internal cavity, multiply for gross internal volume, then subtract the driver displacement, the bracing, the terminal cup and, on a ported box, the port tube. On a small box those subtractions can be a fifth of the gross figure, which is more than enough to change how the finished box sounds.
What is the port length formula?
Length equals c squared times the total port area, divided by 4 pi squared times the tuning frequency squared times the net box volume, minus 1.463 times the port radius. Speed of sound c is taken as 343 metres per second, and the whole calculation is done in consistent SI units before converting back to inches. The last term is the end correction, which accounts for air outside the tube moving with the air inside it. It assumes one flush-mounted end and one free end, so flares, bends and a mouth near a wall all change it. Treat the answer as a length to cut long and trim after measuring.
Does stuffing the box with damping material change the volume?
Acoustically, yes, in a sealed box. Fibrous material makes the compression and expansion of the air closer to isothermal, which makes the box behave as though it were larger, commonly by a few percent and sometimes up to around ten. It also absorbs internal reflections, which is worth doing on its own account. In a ported box the effect on tuning is smaller and the usual practice is to line the walls rather than fill the cavity, because heavy stuffing damps the port resonance you built the box to get. Either way this calculator reports the physical net volume, so expect the acoustic behaviour of a stuffed sealed box to sit slightly above it.
Should I use two small ports or one big one?
It depends on what is constraining you. Two ports of a given diameter give twice the area of one, which halves the air velocity and reduces the chance of audible port noise, but each of them then needs to be about twice as long for the same tuning. If depth is the constraint, one larger port is often no help either, since area drives length the same way. The usual solutions when the tube will not fit are a slot port formed by internal panels, which uses the box geometry as part of the length, or a folded port. Both work; both need measuring afterwards because the effective length is no longer the geometric one.
How do I know if my finished box is tuned where I intended?
Measure the impedance. A ported enclosure has two impedance peaks with a dip between them, and the frequency of that dip is the box tuning. A simple test setup with a signal generator and a series resistor is enough to find it, and there is inexpensive software that will sweep it for you. If the measured tuning is above your target, the port is too short, so lengthen it. If it is below, shorten it. This is why the advice is always to build the port long: adding length to a fitted port is awkward, while cutting some off takes a minute.