Bass reflex vs sealed speakers: what the port does

How a port turns the rear wave into bass

Every time a woofer pushes forward, it also pushes air backward into the cabinet. A sealed box traps that rear wave: the enclosed air acts as a spring on the cone, and the energy behind it is lost inside the box. A bass reflex box, also called ported or vented, gives that air a way out.

The air in the port has mass, the air in the box behaves like a spring, and together they form a Helmholtz resonator, the effect that makes a bottle hum when you blow across its neck. The designer picks the box volume and the port's length and area to tune that resonance, usually labeled Fb, near the bottom of the speaker's range. At Fb and just above it, the port's output arrives in step with the front of the cone and adds to it.

At the tuning frequency itself the cone barely moves and the port does most of the radiating. Less cone travel there means less distortion from the woofer and more level before it runs out of excursion. Further up, the port's share fades and the cone carries the bass again.

What the port changes in the bass you hear

The gain is depth. The same woofer reaches lower in a ported box than in a sealed one of the same size, or reaches the same depth from a smaller cabinet. Five of the six speakers reviewed on this site use a port, from the active Taga Harmony TAV-500B v.2 at €249/pair to the Triangle Australe EZ at $4,999/pair.

The first cost is the slope. Below Fb a ported speaker's output drops at about 24 dB per octave, twice the 12 dB of a sealed box, so the bass stops more abruptly. Down there the cone is no longer held by the air in the box, and on content below the tuning it can travel a long way while adding little sound.

The second is time. A resonator stores energy, so near the tuning frequency a ported speaker delays the lowest notes slightly more than a sealed one and lets them hang on a little after the note has stopped. Some listeners hear that as looser bass, and how audible it is at normal tunings is argued over. The enclosure alone doesn't settle it: the Triangle Australe EZ is ported, and its review calls the bass "clean and articulate", free of muddiness or bloat.

The third is noise. Play a small port hard and the air rushing through it can chuff. A larger port area slows the air down and flared ends ease its way in and out, which keeps that noise out of the music.

Sealed boxes and their gentler slope

A sealed, or closed-box, speaker keeps the rear wave inside. For the same woofer and box it starts to roll off higher up, but gently, and the air spring keeps the cone under control all the way down. That slow slope tends to blend with the lift in bass that a room and its nearby walls add, which is why sealed speakers have a reputation for being easy to place.

The Diora Acoustics Chors 5 is the sealed example in our reviews, a two-way floor-stander quoted at 44 Hz at -3 dB. Its review notes that the non-vented box rolls off slowly enough to leave usable output at around 35 Hz, and that without a port the speakers can sit somewhat close to the rear or side walls. It also warns that some listeners may hear them as bass-light at first, though its reviewer calls the tuning very balanced and hears the bass reaching deeper than the specs suggest. What any sealed box gives up is depth from a given cabinet, and output per watt in the lowest octave.

How to spot the tuning on a spec sheet

The speaker type usually says it. The spec table in our DIAMOND B-60 v.3 review reads "2-way bass-reflex bookshelf", and "ported" or "vented" mean the same thing.

The low end of the frequency response reads differently for each design. Because a ported speaker falls away fast below its limit, the quoted figure sits close to where its bass stops. A sealed speaker's -3 dB figure understates how far down it keeps producing some output, as the Chors 5 shows. What those figures mean for the lowest octave of music is covered on the sub-bass page.

If a maker or a review publishes an impedance curve, it tells you the tuning directly. A sealed box shows one peak in the bass. A ported box shows two, with a dip between them, and the bottom of that dip marks the tuning frequency.

Where the port points, and where the speaker goes

Any speaker gains bass as it nears a wall, because the wall sends its low frequencies back in step with the direct sound. A port doesn't change that, though its position changes the details.

A rear port close to a wall has its output thrown straight back, so makers of rear-ported speakers usually ask for space behind them, and many supply foam bungs that plug the port when there isn't room. The Taga Harmony DIAMOND B-60 v.3 vents from the back, and its review adds a detail spec sheets rarely give: the tweeter has no sealed chamber of its own, so it shares the cabinet volume the rear port vents.

A front port keeps the port itself away from the wall behind. It doesn't cancel the wall's effect on the woofer: the Triangle Borea BR09 has its port on the front, and its review still warns that the bass runs into trouble if the speaker sits too close to the rear or side walls. Below about 200 Hz the room's own resonances decide where the bass piles up, which the room modes page explains.

Ported or sealed for your room

A ported speaker suits a room where it can stand well out from the wall and where you want the lowest octave without a subwoofer. If your room forces the speakers close to the wall, a sealed design, or a ported one with its bung fitted, will usually give the more even bass.

Work out your room's modes before you move anything. The room mode calculator shows which bass frequencies your room will exaggerate, and the best speakers guide ranks every pair we've reviewed, ported and sealed, by score.