A sealed box is smaller, tighter and more forgiving, with a gentle roll-off that pairs well with car cabin gain. A ported box is bigger and needs more care to design, but it plays 3-5 dB louder around its tuning frequency and digs deeper for the same amplifier power.
How each box works
In a sealed (acoustic suspension) box the air trapped behind the cone acts as a spring. That spring adds stiffness to the driver, so the box controls the cone at every frequency. The result is a smooth 12 dB/octave roll-off below the system's resonance (Fc) and very predictable behaviour.
A ported (bass reflex) box adds a vent: a tube or slot whose air mass resonates with the box's air spring at the tuning frequency, Fb. Near Fb the port does most of the radiating while the cone barely moves. Above Fb the cone and port add up; below Fb they cancel and the response drops at about 24 dB/octave.
Sealed vs ported at a glance
| Factor | Sealed | Ported |
|---|---|---|
| Box size | Small (often 0.5-1.5 ft³ / 15-40 L for a 12") | Larger (often 1.5-3 ft³ / 40-85 L for a 12"), plus port volume |
| Output near tuning | Baseline | About 3-5 dB more |
| Roll-off | 12 dB/octave, gradual | 24 dB/octave below Fb |
| Transient feel | Tight, accurate | Fuller, more "boom" around Fb |
| Cone control below tuning | Always controlled | Unloads, needs a subsonic filter |
| Power needed for a given SPL | More | Less (around Fb) |
| Design tolerance | Forgiving of volume errors | Needs correct volume, port area and length |
| Build effort | Simple | Extra port, more panels |
| Typical use | Sound quality, small trunks, rock, jazz, metal | Hip-hop, EDM, SPL, home theater |
Start with the driver, not the preference
The subwoofer itself narrows the choice. Its Thiele/Small parameters tell you which box it is designed for. A quick check is the Efficiency Bandwidth Product:
EBP = Fs / Qeswhere Fs is the free-air resonance (Hz) and Qes the electrical Q. As a rule of thumb: below 50 points to sealed, 50-100 works either way, above 100 points to ported.
Total Q (Qts) matters too. Drivers with Qts around 0.5 or higher tend to sound boomy in a ported box and are happier sealed. Low-Qts drivers (0.3-0.4) usually come alive in a vented box. If you are not sure what these numbers mean, read our Thiele/Small parameters guide.
Worked example: the same 12" both ways
Take a 12" driver with Fs = 30 Hz, Qts = 0.40, Qes = 0.44 and Vas = 45 L (1.59 ft³). Its EBP is 30 / 0.44 = 68, so it can go either way.
Sealed
For a flat Butterworth alignment (Qtc = 0.707) the box volume is:
Vb = Vas / ((Qtc / Qts)² − 1) = 45 / ((0.707 / 0.40)² − 1) = 21.2 LThat is 0.75 ft³ net. The system resonance is Fc = Fs·sqrt(1 + Vas/Vb) = 30 × sqrt(1 + 45/21.2) = 53 Hz.
An Fc of 53 Hz sounds high, but in a car that is roughly where cabin gain starts lifting the low end at about 12 dB/octave, which fills in the sealed box's roll-off almost perfectly. If you want deeper, flatter bass at home, go bigger: Qtc = 0.6 needs 36 L (1.27 ft³) and drops Fc to 45 Hz.
Ported
A QB3-style starting point gives:
Vb ≈ 15·Vas·Qts^2.87 = 48.7 L Fb ≈ 0.42·Fs·Qts^-0.9 = 28.7 HzThat is 1.72 ft³ net, before the port's own volume is added.
So the ported version needs more than twice the internal air, plus a port, but it reaches about 29 Hz with real authority. For a car, most builders would tune this a little higher (32-35 Hz) to get more punch. See how to choose a tuning frequency.
Model the whole build: response, cone travel, port speed, 3D and cut list
Open the calculatorSealed or ported box for a car?
A car changes the comparison in three ways:
- Cabin gain. A closed cabin boosts low frequencies below its transition frequency (roughly 40-70 Hz depending on size). A sealed box's gradual roll-off plus cabin gain often gives a flat, deep response. A ported box gets the same boost, so a low tune can sound overwhelming. More in our car cabin gain guide.
- Space. A 0.75 ft³ sealed box fits behind a pickup seat. A 2 ft³ ported box plus port may eat half a hatchback's trunk.
- Power. Car amplifiers are often the limit. If your amp is modest, a ported box gets more volume from every watt.
Sealed or ported for home?
Rooms have little cabin gain, so deep extension has to come from the box. Ported designs tuned around 18-25 Hz are popular for home theater because they deliver movie-effect output efficiently. Sealed boxes are popular for music rooms and for multi-sub setups where room equalisation can fill in the low end and their smooth phase behaviour helps integration.
Common mistakes to avoid
- Ignoring net volume. The driver, the port and the bracing all take up space. Design to net volume and add those to get the gross volume. Our box volume guide shows how.
- Undersized ports. A port that is too small chuffs and compresses at high power. As a rule of thumb aim for 12-16 in² of port area per ft³ of box (about 2.7-3.6 cm² per litre) and check peak air speed.
- No subsonic filter on a ported box. Below Fb the cone unloads and excursion rises quickly. Set a high-pass a few Hz below tuning.
- Copying someone else's box. A box designed for one driver can be wrong for another, even at the same size.
Bottom line
Neither design is better in general. Sealed trades some output for smaller size, simpler construction and full cone control. Ported trades size and complexity for efficiency and extension. Enter your driver's parameters in the BassDesign calculator and compare the predicted response of both alignments side by side before you cut any wood. It is free, needs no sign-up and runs in your browser.
Try it with your driver: box volume, response and cone travel
Open the calculator