Bass Reflex vs Sealed Speakers: What the Port Does

What the port does

A woofer moving forward also pushes air backward into the cabinet. In a sealed box that rear wave is trapped and absorbed. In a bass reflex design the cabinet has an opening, a port, and the air inside the port and the air in the box form a resonator, the same effect that makes a bottle hum when you blow across it.

The designer tunes that resonance to a chosen frequency near the bottom of the speaker's range. Around it, the air in the port moves strongly and in step with the front of the cone, adding output exactly where a small woofer would otherwise be running out. At the tuning frequency itself, the port does most of the work and the woofer barely moves, which also lowers its distortion there.

What a ported speaker gains

Deeper bass from the same box. A port typically extends a speaker's usable bass meaningfully below what the same woofer manages sealed.

More level for the same power. Because the port adds output rather than wasting the rear wave, ported speakers are generally more sensitive, and they play louder before the woofer reaches its limits. Our Triangle Australe EZ review credits its front-firing port with both better bass extension and higher sensitivity.

What it costs

A steeper roll-off. Below the tuning frequency a ported speaker's output falls away quickly, at roughly 24 dB per octave against a sealed speaker's 12, and the woofer, no longer controlled by the air in the box, can flap freely on very low content.

Timing. A resonator stores energy and releases it a little late. Ported bass can sound slightly slower and less defined than sealed bass, particularly on fast, deep bass lines.

Port noise. At high levels the air through a small port moves fast enough to chuff or whistle. Flared ports and slots reduce it.

Where the port points

Rear ports fire into the wall behind the speaker, so the distance to that wall changes the bass a great deal. Too close and the bass turns boomy and thick; move the speakers out and it tightens. Foam bungs, often supplied, partly seal the port to tame bass in a small room.

Front ports fire into the room, which makes the speaker far easier to place near a wall.

Down-firing ports and slots use the floor or the plinth as the boundary and sit somewhere between.

In our catalogue the rear-ported speakers include the Taga Harmony DIAMOND B-60 v.3, which vents its tweeter into the same cabinet volume, and the front-ported ones include both Triangle floor-standers.

Sealed speakers

A sealed, or closed-box, speaker rolls off gradually, at around half the slope of a ported one, and that gentle slope happens to blend well with the natural bass boost a room adds, so sealed speakers often sound fuller in a room than their specification suggests. Their bass is tight and well timed, and they are indifferent to the wall behind them. The costs are lower sensitivity and less extension from a given box.

Our Diora Acoustics Chors 5 review is the sealed example in the catalogue, a closed-box floor-stander, and it makes the placement point directly: with no port, the speakers can sit somewhat close to the rear or side walls without the low end falling apart.

Choosing and placing

Neither design is better; they are different trades. A ported speaker suits a room where it can stand out from the wall and you want deep bass and headroom from a modest amplifier. A sealed speaker suits a room where the speakers must sit near a wall, or a listener who values tight, well-timed bass over the last few hertz. Either way the room's own resonances shape the bass more than the port does, and the site's room mode calculator shows where they fall.