Playground · research instrument
DSP · AudioBox volume, port diameter, target tuning — get the length to cut, with the end correction already taken out. Includes the port velocity that decides whether it will chuff.
Independent research instrument — not claimed as MakerPortal shipped product code. Methods, equations, assumptions, and limitations are disclosed so you can inspect what the page does and does not establish.
8.5 cm
12.2 cm
3.66 cm
19.6 cm²
1.7
0.17 L
343.2 m/s
Port air velocity, not port length, is what makes a vent audible. Enter the driver's effective cone area and the peak one-way excursion, and this gives the port air speed implied by that much cone displacement. It is the cone's full-excursion volume velocity pushed through the port area: a first estimate, not a bound. At fb the cone barely moves, but the box resonance multiplies whatever it does move: the port's volume velocity there is about QL times the cone's (QL, the box's loss Q, is often 5 to 15), so a small cone motion can drive a fast port. Check the port speed at full power in a box simulation, or listen for it on the built cabinet.
16.9 m/s
A vented box is a Helmholtz resonator: the cabinet is the cavity, the port is the neck. Solving the resonance condition for length rather than frequency gives the acoustic length the port must present.
S is port area, Vbnet box volume, fb the tuning frequency you want. The first term is the acoustic length required; the second subtracts what the port ends supply for free. The cut length is Lv; the acoustic length is Lv + 1.4633√(S/π). Each end of the port carries air beyond it, and each adds its own term: 0.85·r for the outer end, flush with the baffle (a flanged opening, Rayleigh's 8/3π rounded), and 0.6133·r for the inner end, open inside the box (an unflanged pipe end, Levine and Schwinger). Together that is 1.4633·r, the 0.732·D of most port formulas. An inner end close to a wall or the back panel acts more like a flanged end, adds more length, and tunes lower: keep it at least one diameter clear.
Run the other way, a built port of cut length Lv tunes the box to:
The two other figures on the page, the smallest port that reaches the tuning (where the cut length is zero) and the peak port air speed for a driver of cone area Sd and peak excursion xmax:
Required length scales with port area, so it scales with the square of diameter. Going from a 5 cm port to a 10 cm port at the same tuning does not double the length — it roughly quadruples it. That single relationship is why cabinet design keeps running into ports that will not fit, and why flared and slot ports exist: they buy velocity headroom without buying area.
Meanwhile the end correction is fixed by the port radius, so it grows only linearly. For small ports it is a trim; for large ones it is a rounding error against a very long tube. The negative-length case sits exactly where those two curves cross.
Both are solved when the page is built, by the same function the calculator above runs, and checked in the site's test suite against the Helmholtz resonance run backwards from the printed length. Both assume a straight round port with its outer end flush with the baffle and its inner end free, an end correction of 1.4633 × the radius; the box volume is the net volume after the driver, bracing and port are subtracted. The same function answers /api/v1/port-length, so either example can be embedded in your own page or called as JSON.
A · A 50 L subwoofer box tuned to 32 Hz
50 L box tuned to 32 Hz, 10 cm round port, air at 20 °C (343.2 m/s). Driver: 520 cm² cone, 12 mm peak excursion.
The port is 3.8 times its own diameter and holds 3.02 L of air, which comes out of the net box volume. The smallest diameter that can reach this tuning at all is 1.6 cm.
B · A 25 L box tuned to 40 Hz
25 L box tuned to 40 Hz, 7.5 cm round port, air at 20 °C (343.2 m/s). Driver: 220 cm² cone, 6 mm peak excursion.
Halving the box and raising the tuning shortens the port, but the narrower port runs faster for the same excursion. At 7.51 m/s the peak air speed is 2.2% of the speed of sound. The smallest diameter that can reach this tuning at all is 1.2 cm.
Add about 10 percent and install it. A port that is too long tunes below target and can be trimmed; a port that is too short cannot be un-cut. PVC pipe held with a hose clamp is forgiving while you converge.
A sealed box has one impedance peak. A vented box has two, and the minimum between them is the actual tuning frequency. This is the most reliable measurement in loudspeaker building — it needs only a signal generator, a resistor and any interface input.
Expect the first build to tune a few percent low: bracing, driver displacement and any stuffing all reduce the net volume below the figure you entered. Shorten the port, measure again, and stop when the minimum sits where you want it.
An interface with a clean input finds the impedance minimum; a measurement microphone confirms the near-field response actually did what the alignment promised.
$229.00Audio interfaceAudio interface used building Biquadia — 8-preamp USB I/O for real-time DSP testing.
$49.99MicrophoneUSB-C calibrated mic for iPhone/iPad — take to listening room and verify room-mode eigenfrequencies against calculator's mode list up to 300Hz.
$38.38BookBible of RT60, Sabine, and absorption coefficients — the exact formulas these acoustic calculators implement for reverb, room modes, and critical distance.
$149.95MicrophoneCalibrated USB mic with individual cal file — measure your room's RT60 and modal peaks to validate the room-mode eigenfrequencies this calculator predicts.
Prices shown were retrieved from the Amazon Product Advertising API on 8 October 2026 and are indicative only — the price and availability on Amazon at the time of purchase apply.
Where the box volume and tuning frequency come from — Thiele-Small alignments for the driver you actually have.
The same resonance solved the other way: fix the port and read the frequency it produces.
Once the box is right, the room is the next thing standing between you and the response you designed.
You need the net internal box volume, the port diameter and the tuning frequency. The port must present an acoustic length S / (k²V) with k = 2πf/c, and the cut length is that minus the end corrections of both ends: 1.4633 × the port radius (0.85 for the end flush with the baffle plus 0.6133 for the free end inside the box, the 0.732 × diameter of most port formulas). For a 32 Hz tuning in a 50 L box with a 10 cm port, the acoustic length is 45.8 cm, the end correction is 7.32 cm, and the port to cut is 38.5 cm (both worked examples above are solved by the same function this calculator runs). Enter the box volume with the driver, bracing and port already subtracted.
Pick the diameter by port air speed, then check the length fits. The calculator prints the peak port air speed implied by a driver cone area and peak excursion: 7.51 m/s (2.2% of the speed of sound) for a 220 cm² cone moving 6 mm into a 7.5 cm port at 40 Hz, and 16 m/s for a 520 cm² cone moving 12 mm into a 10 cm port at 32 Hz. A narrow port is short but fast; once the flow separates from the wall you hear chuffing and the bass compresses at high level. A wide port keeps the air speed down but the required length scales with port area, so it gets long fast: the 38.5 cm port in the first example is 3.8 times its own diameter. Take the largest diameter whose length still fits the cabinet, or use two smaller ports.
It solves the vented-box resonance for the length of a straight, round port of constant diameter: Helmholtz resonance with the cabinet as the cavity and the port as the neck, using an end correction for each end of the port: 0.85 times the port radius for the outer end, flush with the baffle, and 0.6133 times the radius for the inner end, open inside the box (1.4633 r in all, the 0.732 × diameter of most port formulas), and the speed of sound at the air temperature you enter. Run the other way, the tuning frequency of a built port is f = (c / 2π) √(S / (V L)), where L is the cut length plus the end correction; the unit tests check each worked example by that formula. It does not model a port flare, port wall thickness, a driver or bracing displacing the box volume, stuffing, or the response of the box. The tuning frequency itself comes from the driver and the alignment you choose, not from here.
Approximately. The calculator takes a round diameter and uses only the cross-section area, with the end correction taken from the radius of the round port of equal area, √(S/π). For a rectangular port, enter the diameter of the round port with the same area: 2√(width × height / π). A 5 cm × 5 cm square port has the area of a 5.642 cm round port, so enter 5.64. That equal-area substitution is an approximation: the real end correction depends on the outline, and it is less reliable for a tall narrow slot than for a square. Cut the port long, measure the impedance minimum, and trim.
No. This page computes the length of a straight port of constant diameter and has no flare input. Flared ports (sold under names such as aero or precision ports) widen toward one or both ends mainly to lower the air speed at the port opening, which reduces turbulence and chuffing; this page does not compute how a given flare changes the tuning or the length. For a flared port, the straight-port length from this calculator is a first estimate only: use the maker’s stated length for that specific port if one is given, or cut long, measure the impedance minimum between the two peaks, and trim. The port air speed figure on this page is for the straight port you entered.
Because the port is too narrow to reach that tuning at all. The required acoustic length scales with port area, but the end correction — the air dragged along outside each opening — scales only with port radius. Shrink the port and the correction shrinks more slowly than the requirement, until the free length from the two ends alone overshoots the target before you have added any physical tube. It is a real answer, not a failure. The fix is to go wider: increase the port diameter, and the calculator prints the minimum diameter that works for your box and tuning. Reducing the box volume or tuning lower also helps, since the minimum diameter scales with box volume and with the square of the tuning frequency.
Yes, and it is often the right answer when a single port of adequate area would be too long to fit. Two ports of the same diameter have twice the area, so each one needs about the same acoustic length as a single port of the doubled area — meaning each of two ports is longer than one port of the same diameter, not shorter. Enter the combined area as the equivalent single diameter: for two 5 cm ports, that is 5 × √2 ≈ 7.07 cm. The calculator’s end correction then uses that larger radius, so add 1.5 cm to the result for each real port (1.4633 × the difference in radius).
Close enough to build from, not close enough to skip measuring. The formula assumes a rigid box of the volume you entered, and real cabinets are not rigid, the driver and bracing displace some volume, and stuffing changes the effective compliance of the air. Expect the built tuning to land a few percent below the prediction. That is why the standard practice is to build the port long, measure the impedance minimum between the two peaks, and trim.
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