Sealed vs ported subwoofer box: which to build
Updated · Fernhart Labs
Short answer
A sealed box is smaller, rolls off gently below its resonance (12 dB per octave) and keeps the cone under control at every frequency: tight, accurate bass that fits in less space. A ported (vented) box is bigger and, with the same driver, plays deeper and louder near its tuning frequency, because the port adds output there; below tuning it rolls off faster (24 dB per octave) and the cone loses control, so it needs a subsonic filter.
The driver matters as much as taste. As a rule of thumb, a driver whose fs/Qes (EBP) is under about 50 suits sealed, over about 100 suits ported, and in between it can do either.
The differences at a glance
| Sealed | Ported (vented) | |
|---|---|---|
| Box size for the same driver | Smaller | Larger (2.3 times the flat sealed box in the example below) |
| Low-frequency rolloff | 12 dB per octave below the box resonance | 24 dB per octave below the port tuning |
| Output near the bottom of the range | Less | More, around the tuning frequency |
| Cone control below the cutoff | Kept: the air in the box acts as a spring | Lost below tuning: the cone unloads and its excursion rises fast |
| Subsonic filter | Helpful, rarely essential | Strongly recommended, set below the tuning |
| Building it | Simple: the volume is what matters | Volume plus a port of the right area and length |
| Port noise | None | Possible if the port is too small for the power |
How a sealed box works
The air inside a sealed box adds stiffness to the driver's suspension, which raises its resonance and its Q. Richard Small's closed-box analysis gives both from the driver's Thiele-Small parameters and the box's net volume Vb:
Qtc = Qts × √(1 + Vas/Vb) and fc = fs × √(1 + Vas/Vb).
Qtc sets the shape of the low end. Qtc 0.707 gives the flattest response before the rolloff (a Butterworth alignment). A lower Qtc, such as 0.577, needs a bigger box and gives a gentler rolloff and the tightest transient response. A higher Qtc, such as 0.9, gives a smaller box with a little more mid-bass punch and a slight peak before the rolloff. Below fc, the response falls at 12 dB per octave.
How a ported box works
A ported box adds a tube or slot whose air mass resonates with the box at the tuning frequency (fb). Near fb the port does much of the radiating and the cone barely moves, so the system plays deeper for its size than a sealed box with the same driver. Below fb the port's output and the cone's cancel out, the response falls at 24 dB per octave, and the cone moves freely, which is why a subsonic filter matters.
A widely used starting point for a flat ported alignment, attributed to D. B. Keele Jr., is Vb ≈ 15 × Qts^2.87 × Vas, fb ≈ 0.42 × fs × Qts^−0.9, with the −3 dB point at f3 ≈ 0.26 × fs × Qts^−1.4. Treat it as a starting point and check it with a simulation of your exact driver.
Port air speed matters too: a common rule of thumb is to keep the peak air speed in the port below about 17 m/s at full power, or the port may chuff. More port area lowers the speed but needs a longer port for the same tuning.
A worked example
Take an example driver (not a real model) with fs = 30 Hz, Qts = 0.40, Qes = 0.45 and Vas = 50 litres. Its EBP is 30 ÷ 0.45 ≈ 67, so it can go either way:
| Design | Net volume | Box resonance or tuning | −3 dB point |
|---|---|---|---|
| Sealed, Qtc 0.9 (compact) | 12.3 L | fc 67.5 Hz | about 56 Hz |
| Sealed, Qtc 0.707 (flat) | 23.5 L | fc 53.0 Hz | 53.0 Hz |
| Sealed, Qtc 0.577 (gentle rolloff) | 46.3 L | fc 43.3 Hz | about 55 Hz |
| Ported, Keele starting point | 54.1 L | fb 28.7 Hz | about 28 Hz |
The ported box is about 2.3 times the volume of the flat sealed one and reaches about an octave lower at −3 dB. The 0.577 sealed box does not have a lower −3 dB point than the 0.707 one: its benefit is the gentler slope and better transient behaviour, not a deeper cutoff. In a car, the cabin itself lifts the lowest frequencies (cabin gain); Boxsmith can add it to the simulation.
How to choose
- Check the driver first. The manufacturer often recommends a box type and volume. EBP (fs ÷ Qes) under about 50 points to sealed, over about 100 to ported.
- Space. If the box has to fit behind a seat or in a small trunk, sealed usually wins.
- Sound. If you want tight, controlled bass, sealed. If you want the deepest, loudest bass from one driver and have the space, ported.
- Power and safety. A ported box needs a subsonic filter below its tuning, and a port big enough not to chuff at your amplifier's power.
- Simulate before you cut. Box volume is the net volume after the driver, port and bracing are taken out.
Questions
Is sealed or ported better for a car?
Neither always. Sealed boxes are smaller and keep the cone controlled at every frequency; ported boxes give more output in the lowest octave from the same driver but need more space and a subsonic filter. Check what your driver's parameters and its maker recommend.
Which is louder, sealed or ported?
With the same driver and power, a ported box is louder around its tuning frequency, because the port adds output there. Above that region the two are similar.
Which goes deeper, sealed or ported?
Ported, for the same driver: in the example above the ported box reaches about 28 Hz at −3 dB against about 53 Hz for a flat sealed box, but in more than twice the volume.
Can I put a ported-box sub in a sealed box?
Often yes, if its Qts is high enough for a reasonable Qtc in a box you can fit; the response and box size will differ from the ported design. Run the sealed formulas or a simulation with your driver's parameters.
Why does my ported box make a chuffing noise?
The air in the port is moving too fast. Keep the peak port air speed under about 17 m/s by using more port area (with the longer port that needs for the same tuning) or less power at the lowest frequencies.
Do I need a subsonic filter?
For a ported box, yes: below the tuning frequency the cone is no longer controlled by the box and can exceed its excursion limit. Set the filter a little below the tuning.
Boxsmith: Subwoofer Box Design
Subwoofer box designer with real simulation, port sizing and cut sheets.
Boxsmith simulates your exact driver in a sealed, ported or bandpass box, with response, maximum output, cone excursion against Xmax, port air speed and impedance graphs. Its "Suggest a box" finds flat, deeper and compact designs, it sizes the port so it stays under the air-speed limit, and it turns the design into a cut list and sheet layout.
Sources
- R. H. Small, "Closed-Box Loudspeaker Systems, Part I: Analysis", Journal of the Audio Engineering Society 20 (1972)
- A. N. Thiele, "Loudspeakers in Vented Boxes", Journal of the Audio Engineering Society 19 (1971)
- R. H. Small, "Vented-Box Loudspeaker Systems", Journal of the Audio Engineering Society 21 (1973)
- Keele's closed-form vented-box approximation and the EBP and 17 m/s port-speed rules of thumb, as used in Boxsmith's design tools Boxsmith