Aperiodic box: a sealed box that breathes

An aperiodic box looks like a sealed one, but it has a small opening filled with damping material. This opening is called a resistive vent. The best known is the Variovent from Dynaudio. Air leaks through it slowly, so the box is not tuned like a bass reflex, but it is not fully sealed either.

Section through an aperiodic box. The driver on the left, on the right in the rear panel the resistive vent filled with dense damping material. Air leaks through it slowly.

How it works

At the driver's resonance the cone moves the most, and that is when the most air flows through the vent. The damping material in it turns the motion into heat and so damps the resonance. The impedance peak drops and the cone is better controlled at the resonance. The bass rolls off as slowly as in a sealed box.

What the calculation shows

All charts in this series on enclosure types are calculated by Loudsmith for the same 6.5" woofer, the SB Acoustics SB17NRX2C35-8 (Fs 36.5 Hz, Qts 0.43, Vas 27 l), so they can be compared. The 0 dB level is the driver's level in the midrange. An aperiodic box with a net volume of 9.5 l.

TotalSealed box 13.6 l for comparison

Aperiodic box 9.5 l. The −3 dB point is at 70 Hz, similar to the sealed box, with a smaller volume.

The impedance peak is only 28 Ω at 69 Hz. In the sealed box it was 52 Ω. A flatter impedance is easier to split with a passive crossover.

Pros and cons

Pros: tighter, better controlled bass, a lower impedance peak, less influence of placement near a wall.
Cons: the bass does not go deeper than with a sealed box. The result depends on the density of the material in the vent, and that can only be set by trial and measurement. Ready-made vents cost more than a port.

Which driver and which room

It suits woofers with a higher Qts that would have a hump in a sealed box, and speakers that stand near a wall. Dynaudio recommends one Variovent up to 50 l, two for 50 to 80 l and three for larger boxes.

Well-known designs

Dynaudio used resistive vents in several of its speakers, and you will also find them in SEAS kits (for example the A26). We could not find complete data, meaning the volume, the vent and the resulting impedance, in any published design. That is why this type is not yet verified against finished speakers like the others.

In Loudsmith

The aperiodic box is one of the types Loudsmith designs. It chooses the vent resistance to make the bass as flat as possible. When building, start with a denser vent filling and remove material until the measured impedance matches the calculation.

Enclosure types series: sealed · bass reflex · passive radiator · aperiodic · transmission line · TQWT · ML-TL · open baffle · back-loaded horn. You will find the formulas in the article Speaker box calculations step by step.