Passive radiator: a bass reflex without a port

A passive radiator is a driver without a magnet and voice coil, just a cone with a suspension. It is fitted in place of a port and does the same job: in the bass it plays together with the driver. But it avoids the noise of moving air and the long tube that would not fit into a small box.

Section through a box with a passive radiator. The driver at the top, the passive radiator below it, a cone without a magnet. It is moved by the air pressure in the box, and the mass on it sets the tuning.

How it works

The mass of the radiator cone and the springiness of the air in the box form a resonator, as with a port. The mass can be increased with an added weight, which moves the tuning lower. Unlike a port, the radiator also has its own suspension, which acts as a spring. Below its resonance the radiator therefore not only stops supporting the bass but also takes some away, so the roll-off below the tuning is even steeper than with a port.

A radiator needs a lot of excursion. Its displacement volume, that is area times excursion, should be about twice that of the driver, otherwise it bottoms out in loud bass. That is why two radiators or one larger one are often used.

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 27.2 l box with two Purifi PTT8.0PR radiators, each with 17 g of added mass, tuned to 34 Hz.

TotalConeRadiatorsSealed box 13.6 l for comparison

Passive radiator 27.2 l, tuning 34 Hz. The −3 dB point is at 34 Hz, at 30 Hz it is 6 dB down. Below the tuning it falls more steeply than a bass reflex.

As with a bass reflex, the impedance has two peaks, 35 Ω at 23 Hz and 47 Ω at 57 Hz, with the minimum at the tuning.

Pros and cons

Pros: no port noise, no tube resonance in the midrange, a low tuning even in a small box. The tuning can be adjusted with added mass even after the build.
Cons: radiators cost more than a port and need space on a box panel and free excursion. Below the tuning the bass falls more steeply, and the radiator can bottom out loudly in strong bass.

Which driver and which room

It pays off most in small boxes, where a port would have to be longer than the box, and in subwoofers, where a port would make noise. For a box near a wall a radiator on the front or on the side is better.

Well-known designs

Purifi SPK16: a 6.5" woofer and two radiators in about 19 l. In the build description Purifi gives a tuning of 33.2 Hz from the impedance minimum. Purifi describes the design process in a technical article: the optimum is around 22 l, the radiators are loaded to nearly 70 g and the woofer path has about 1 Ω in total.
Audio Judgement measured a small box with two Dayton ND90-PR radiators. Without added mass it tunes to 95 Hz, with 10.7 g to 62.6 Hz. For the same box Loudsmith calculates 95.2 and 62.2 Hz.
My bookshelf speaker Tang Band and Peerless has 9.5 l and an SB15SFCR-00 radiator, because a port would not fit inside.

In Loudsmith

Loudsmith picks a radiator from the shop's range that has enough excursion, and calculates the added mass for the chosen tuning. For the tuning it uses the same volume as the simulation, including the box damping.

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.