Bass reflex: the most common ported loudspeaker

A bass reflex is a box with an opening that holds a tube, the so-called port. It is the most common type of loudspeaker in shops and among kits, because it gets deeper bass out of the same driver than a sealed box does.

Section through a bass reflex. The driver at the top left, the port below it, a tube running from the front panel into the box. The air in the port moves like a weight on a spring, and the spring is the air in the box.

How it works

The air in the port has mass and the air in the box acts as a spring. Together they form a resonator with its own frequency, the tuning. Around the tuning the port plays in phase with the cone and adds bass. Right at the tuning the cone almost stops and nearly all the sound comes from the port. That is why a bass reflex has the smallest cone excursion at the tuning.

Below the tuning this reverses: the port plays against the cone and the bass falls steeply, at 24 dB per octave. The cone is no longer held back there and its excursion grows quickly. That is why a filter that cuts the lowest frequencies is useful below the tuning.

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. A bass reflex of 29.1 l with a port of Ø 44 mm and 99 mm long, tuned to 33 Hz.

TotalConePortSealed box 13.6 l for comparison

Bass reflex 29.1 l tuned to 33 Hz. The −3 dB point is at 33 Hz, almost an octave lower than the sealed box with the same driver.

The chart shows how it works. The blue dashed cone curve has a deep dip at 33 Hz, where nearly all the sound comes from the port (orange). The sum is flat down to the tuning and falls sharply below it. You pay for the deeper bass with a bigger box: 29 l instead of 13.6 l.

The impedance of a bass reflex has two peaks, 36 Ω at 22 Hz and 45 Ω at 56 Hz. The minimum between them lies at the tuning. This is how you check the tuning of a finished box by measurement.

Pros and cons

Pros: deeper bass and higher bass output from the same driver, small cone excursion around the tuning, common and cheap ports.
Cons: a bigger box, a steep roll-off and a worse time response below the tuning. A narrow port makes noise with loud music, so the air speed in it should stay below 17 m/s. A long port has its own resonance in the midrange.

Which driver and which room

It suits woofers with a Qts of about 0.25 to 0.45. A rear port needs at least 20 to 30 cm from the wall, otherwise the bass gets boomy. Closer to the wall a front port or a lower tuning is better.

Well-known designs

Paul Carmody (Classix III, Amiga, Carrera and others): he gives the volume, port dimensions and tuning for every design. The Classix III, for example, has 15 l, a port of Ø 2" and 8" long, and a tuning of 40 Hz.
SEAS kits (Idunn, Mimir, Trym, Bragi and others): the Mimir, for example, has 14 l, a port of Ø 50 mm and 200 mm long, and a tuning of 42 Hz.
Troels Gravesen measures the tuning of every box from the impedance. On his page about port tuning he shows that damping in the box moves the tuning a few hertz lower.

In Loudsmith

Loudsmith picks a port from the shop's range so that the air in it does not exceed 17 m/s. In Dimensions you can enter your own diameter, length, flared ends and the box damping. We compared the tuning calculation with more than 70 published bass reflex designs. It matches the Scan-Speak example calculations within 2 % and Carmody's designs within 2 % on average. The default damping is lined walls, because finished boxes tune lower than the bare formula gives.

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.