Having expressed interest since my first sighting of these models more than a year ago, I was excited to learn that the time had finally come. With 8m of open space behind my desktop speakers, I opted for the bigger One18. Its filter hinge at 1'600Hz combines with 85dB sensitivity for a claimed bandwidth of 48-20'000kHz ±3dB. That's milked from a 380 x 191 x 315mm HxWxD box in plain black paint. The designer colours are reserved for the home models. Using Amphion's proven recipe of wave-guided 1" Titanium dome tweeter for more seamless radiation continuity with the mid/woofer, the ride nouveau—French for new wrinkle—is the ABR. And that's not French but geek talk for an auxiliary bass radiator. Facing back, it replaces the usual port. Pourquoi?

Passive radiators aren't new. But neither are they exactly common except in a small way in subwoofers. The Sonus faber Electa Amator and Extreme monitors had one. So did the Pawel/Ensemble PA1. Today Phil Jones' Pure Sound 7000i still does. And the Kaiser Kawero Chiara. And all of Amphion's Pro range. As I'd put it in my 2014 report, "Amphion are about practical solutions for the real world, not vanity projects for the 1-percenters."

What is a passive radiator? It's a driver without voice coil or magnet, hence it's not wired up. What controls it is its own mass. A basic recipe to calculate the latter is that the passive cone's weight should approximate the air mass of the port it replaces. Why go for an ABR? It eliminates potential port chuff and pipe resonances; reduces motion compression from high air velocity particularly for narrow ports; damps the main driver for flatter response and bigger head room; and adds cone surface for more air displacement. Over a bass-reflex port, it also slightly accelerates roll-off below the tuning frequency; and creates a notch at the ABR's free-air resonant frequency.


A properly selected passive radiator will be large enough relative to its box volume to move the notch below audibility. An ABR can shrink the size of the box over a ported version which must house a certain tube length, hence depth. Port length relates to ABR weight. For an extreme example, a sub with dual 15-inch 1.4kg passive radiators would need an 18" diameter port of 46-foot length to do the same job. Of course an 18" Ø port isn't feasible. Nor is one 15 meters long. Enter the rationale for a passive radiator. One special requirement of successful ABR use is a main woofer of lower Q than appropriate for sealed boxes. If properly chosen, a passive radiator creates lower bass from a smaller box than a ported alignment. That rosy theory and hard-nosed practice don't meet that often would seem indicated by the rarity whereby ABRs show up in hifi speakers. One expects that there's more to successful execution to prevent living up to its old nickname of drone cone. But the basic passive radiator promise really is more and better bass from a smaller box.

Seeing how the One18 was conceived as a professional monitor, it's cheap to predict that it will sound like one: accurate, uncoloured and linear. Whilst there are pro-arena speaker companies who've made inroads into home audio to involve jumping hurdles—ADAM, ATC and Genelec come to mind—Amphion have earned their spurs in our arena. They are a bona fide hifi company who've gone pro much later. Proposing their dedicated pro models for home use now is the very latest development. To my way of thinking, that made this assignment extra compelling.