Short answer:
- Sealed: smallest box for the job, gentle roll-off, very forgiving to build. Needs more cone movement and more power down low, and loves a bit of EQ.
- Ported: more output and a deeper -3 dB point from the same driver, but a bigger box, a steep roll-off, possible port noise, and the driver unloads below the tuning frequency.
- Passive radiator: ported-style behaviour without a long port, for when the port won't fit. Costs more and takes more working out.
Which one is "best" depends on the driver, the space you've got, and whether you'll use DSP.
How each one works, without the maths
Sealed: the box is airtight. The air inside acts like a spring behind the cone. The bass rolls off gently (about 12 dB per octave) below the box's resonance, and that gentle slope blends nicely with the natural bass boost you get in a room.
Ported (bass reflex): a tube or slot lets the air inside resonate at a chosen frequency (the tuning). Around that frequency the port does a lot of the work and the cone barely moves, so you get more output down low. Below the tuning the port stops helping, the bass falls away quickly (about 24 dB per octave), and the cone is effectively unloaded, so it can flap about if you feed it deep bass.
Passive radiator (PR): swap the port for a cone with no motor. The air in the box drives it, and its mass sets the tuning, much like a port. You get the ported behaviour without needing a port long enough to reach the tuning you want.
Side by side
| Sealed | Ported | Passive radiator | |
|---|---|---|---|
| Box size for a given driver | Smallest | Larger | Similar to ported, sometimes smaller |
| Roll-off below the box's limit | Gentle (~12 dB/oct) | Steep (~24 dB/oct) | Steep, with a notch just below tuning |
| Output near the bottom | Lower | Higher | Higher |
| Cone movement at deep bass | High, and rises as frequency falls | Lowest near tuning, very high below it | Similar to ported |
| Needs a high-pass filter? | Helpful | Strongly recommended (protects below tuning) | Strongly recommended |
| Building it | Easiest | Easy, but port size matters | Fiddly: PR choice and mass tuning |
| Things that go wrong | Not enough excursion | Port noise ("chuffing"), port too long to fit | PR runs out of travel |
| Suits drivers with... | Higher Qts, EBP under ~50 | Low Qts, EBP over ~100 | Same as ported |
Not sure where your driver sits? How to read driver specs covers Qts and EBP.
Sealed: Qtc, and why DSP changes everything
A sealed box's character is summed up by its Qtc:
- About 0.5: critically damped: very tight, with an early, gentle roll-off. Needs a big box.
- About 0.707: the classic "flattest response" alignment.
- About 1.0 and up: a small box, a little bump above the roll-off, punchier and less deep.
Here's where it gets interesting. With an amp that has EQ or DSP, you can take a small sealed box and boost the bottom end to get deeper extension, as long as the driver has the excursion and the amp has the power. That's the whole idea behind our Sealed 18SW500 build: an 18" driver in a 90 L net box. In the guide that lands at a Qtc of 1.06, and a couple of EQ filters extend it in the sim. A big driver in a small sealed box with DSP is a common way to get a lot of bass from a modest box.
The catch: EQ costs power and excursion. Boosting by 6 dB needs four times the amplifier power at that frequency. Check excursion at full power in your modelling software before you commit.
Ported: getting the port right
- Port area matters. Too small and air rushes through fast enough to chuff and whistle at volume. Flared ports help, and bigger is usually better, within reason.
- Bigger port means longer port. For a given tuning, increasing the area makes the port longer. In a small box, a big low-tuned port often simply won't fit. That's where passive radiators come in.
- Keep the inside end clear. Leave at least a port diameter between the inner end and the back or side wall.
- Use a high-pass filter. Below tuning the driver is unloaded. A subsonic filter just below the tuning frequency protects it.
We stock round ports from small to large if you're not building a slot port.
Passive radiators: when they make sense
Reach for a PR when:
- the port you need won't fit in the box (common with small, low-tuned designs);
- you can't stand port noise; or
- you want a tidy front with no hole.
Two things to get right:
- Displacement. The PR needs to move more air than the driver: a common guide is at least 1.5 to 2 times the driver's Vd (cone area × Xmax). In practice that means a bigger PR than the driver, or two of them.
- Tuning by mass. You set the tuning by adding mass to the PR. Many have add-on weights, like SB Acoustics' 50 g passive radiator weights. Model it (WinISD and VituixCAD both handle PRs) rather than guessing.
There's a full range of passive radiators from 1" to 15".
Room gain: the free bass
In a normal room, low bass gets a boost from the room itself. A sealed sub's gentle roll-off lines up well with that boost, which is part of why sealed subs often sound deeper in a room than their anechoic graph suggests. Every room is different, so treat it as a bonus, not something to design around.
So which should you pick?
- Small box, you've got DSP or a plate amp with EQ: sealed.
- Want the most output per watt from a low-Qts driver, and have the space: ported.
- Small box, low tuning, port won't fit: passive radiator.
- Building a published design: whatever the designer chose. The crossover and bass alignment were worked out together.
Quick questions
Is sealed "tighter" than ported? Sealed has less group delay and a gentler roll-off, which many people hear as tighter. A well-designed ported box with sensible tuning sounds great too. Badly tuned ports give ported boxes their "boomy" reputation.
Can I just block the port on a ported speaker? You can, and it'll roll off earlier and more gently. It'll also change the bass balance the designer intended. A port bung is a reasonable fix for a speaker that's too boomy up against a wall.
Do passive radiators sound better than ports? Not better, just different trade-offs: no port noise and no port length problem, at more cost and complexity.
What's a Linkwitz transform? An EQ that reshapes a sealed box's response to act like a different box: usually a deeper, flatter bottom end. It's what most "small sealed sub with DSP" designs are doing, one way or another.