The box and the brain: cabinets, ports, damping and crossovers

Why a driver needs a box at all

A bare driver makes almost no bass. As the cone pushes forward it makes a pressure wave at the front and an opposite one at the back; left in open air, the two wrap around and cancel each other, especially at low frequencies. The cabinet's first job is simply to separate the front wave from the back wave so the bass survives. Its second job is harder: to do that while adding nothing of its own.

The cabinet: fighting its own resonance

The enemy is coloration. When the drivers play, the cabinet walls want to flex and vibrate, adding a boxy 'sound of the box' on top of the music. The fixes are mass and stiffness: dense fibreboard (MDF/HDF) or plywood, thick walls, and shapes that resist resonance. Curved or non-parallel walls do double duty — they're harder to set ringing and they break up the internal standing waves that build between flat parallel surfaces.

Sonus Faber's lute-shaped cabinets are a well-known example of curved walls used for exactly this. Real-wood veneers, by contrast, are mostly cosmetic — the acoustic work is done by the material underneath.

Bracing and damping — the work you never see

Two internal techniques quiet the box further:

• Bracing — internal struts that tie opposite panels together so they can't flex and 'sing'. Bowers & Wilkins' Matrix is a famous honeycomb of internal bracing; Q Acoustics uses Point-to-Point (P2P) braces placed exactly where the panels move most.
• Damping — absorbent wadding or foam inside the box soaks up the internal standing waves and reflections behind the cones. Some makers go further into the walls themselves: Q Acoustics' Gelcore bonds layers of MDF with a non-hardening gel that turns panel vibration into a tiny amount of heat.

Sealed, ported, and the alternatives

How the box handles that rear wave is the single biggest decision in its design. There are four common approaches:

• Sealed (closed box): the rear wave is simply trapped. Bass is tight and accurate and rolls off gently, and the speaker is forgiving about placement — but it needs more power for the same depth. ATC is a longtime champion of the sealed design.
• Bass-reflex (ported): a tuned port lets the rear wave out in phase with the front, reinforcing the bass for more depth and efficiency. It's by far the most common design; the cost is a steeper roll-off below the tuning point and sensitivity to how close it sits to a wall.
• Passive radiator: an undriven cone does the port's job, giving deep bass from a compact box with no port noise. Amphion uses passive radiators across its Argon range.
• Transmission line: the rear wave travels down a long, folded, damped internal tunnel before exiting, for deep and very even bass in a large cabinet. It's the signature of PMC, whose ATL (Advanced Transmission Line) loading runs through its whole range.

A fifth approach rejects the box premise altogether: an open baffle mounts the drivers on a flat panel with no enclosure at all, so the rear wave radiates freely and the speaker works as a dipole (a figure-of-eight radiation pattern). With no box to add a sound of its own, fans prize its uncoloured, uncannily open midrange; the trade-offs are limited deep bass, low efficiency and a real need for space away from walls — PureAudioProject, the one open-baffle maker in the catalog, asks for a metre of clear air behind the panel. It stays a niche, enthusiast style, but it's worth knowing the box isn't mandatory.

We summarise these on every spec page too — see the enclosure section of How to read a speaker spec sheet.

Ports and your room

If a speaker is ported, where the port sits changes how you can place it. A front port is the most wall-friendly; a down-firing port (venting into the plinth) is also fairly placement-tolerant; a rear port is the main reason a speaker needs breathing room behind it, because firing bass straight at a nearby wall makes it boom.

Pushed hard, a small port can also 'chuff' — an audible gust of air noise — which is why many makers flare the port mouth. Some ship with foam bungs to plug the port and tame the bass when the speaker has to live close to a wall.

The crossover: the speaker's brain

The crossover is the network that splits the incoming signal by frequency and sends each band to the right driver — bass to the woofer, treble to the tweeter — so no driver is asked to play notes it can't. It's invisible, but its choices (where the drivers hand off, how steeply, how their levels and timing align) are exactly what gives a speaker its tonal balance. This is the single component where 'voicing' most lives.

There are two families:

• Passive: a network of capacitors, inductors and resistors sitting between the amplifier and the drivers, inside the speaker. It's simple and needs only one amplifier, but it filters the signal after it's been amplified, which wastes a little power and limits control. This is what almost every conventional passive speaker uses.
• Active: the signal is split before amplification, and each driver gets its own dedicated amplifier channel — often with DSP doing the filtering. It's more complex and only works in a powered speaker, but it gives the designer far more control over every driver. It's how active studio monitors and powered hi-fi speakers work.

The bottom line

The cabinet, its porting and damping, and the crossover are the least visible parts of a speaker and among the most decisive. They're the reason a spec sheet can never fully predict how something sounds, and the reason two speakers built from the same drivers can be worlds apart. Read the enclosure and driver details as clues to the designer's intent — then let your ears settle it. With the drivers covered in the tweeter and cone-material guides, you now have the whole picture of what's inside the box.