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KIH #76 – Passive radiators

  • Passive radiance. Like their Danish colleagues at Børresen Acoustics, Sweden’s Mårten Design sport in their name a letter which for Brits and Americans isn’t on the keyboard. Like the Danes, it’s become a stylized part of their logo. Another mark of distinction goes to a long-standing collaboration with German driver supplier Thiele & Partners aka Accuton. Like Tidal in Germany and Albedo in Italy, Mårten champions their ceramic and diamond transducers over all others. So it’s no surprise that in their newly introduced middle range replacing the prior Heritage models, the three-deep Parker series goes all-ceramic again, even offers an upgrade path to a diamond tweeter. That’s posh!

    But there it doesn’t stop. Where the same catalogue’s equivalent Heritage Duke 2 monitor was still ported, the Parker Duo successor grew a 9″ aluminium passive radiator in the back. The 2.5-way Parker Trio adds another 7.5″ midrange on its front and a second radiator on the rear. The 2.5-way Parker Quintet adds two more of each beyond the Trio.

    Kaiser Chiara & Furioso Mini.

    Be it Fram, Sehring, Kaiser, Tidal/Vimberg or now Mårten, passive radiators are making quite the comeback by appearing also in very expensive speakers, no longer just ‘midfield’ models from Amphion, Burchardt, GoldenEar and LSA. In a passive radiator, the moving mass of a driver without motor or electrical connection—the weight of its diaphragm, surround and spider—replaces the air volume inside a port tube. That works on a similar principle. Yet it avoids the air turbulence colloquially known as port chuff or pumping and the time-domain errors of vent tubes. After all, the other term for a bass reflex port is Helmholtz resonator. It rings by design. Instead of pop, it goes pofff.

    Due to its tuning, the passive 9″ membrane of the Mårten Duo adds itself for just the bass to the active 7.5″ diaphragm of the mid/woofer. Now both combine into the surface area of an 11.7″ woofer. That explains the speaker’s claimed 36Hz bass reach at -2dB which fits into a box just 28x42x36cm WxHxD.

    Vimberg Amea with passive radiator at right.

    Vimberg’s ~€13’000/pr Amea plays the same tune including Accuton drivers and diamond upgrade. Kaiser’s Chiara has one rear-firing passive, their newer Furioso Mini two, one per cheek to ‘surround’ the front-firing 2-way in a point-source cluster. For years now Finland’s Amphion has been busy transferring their passive radiators from their pro line to their consumer models which previously ran ports. GoldenEar has gone passively radiant for a long while. This shows how a growing number of companies now view passive radiators as superior to ports.

    And there’s more to such parts. They can be made adjustable. Sehring of Berlin shows us how. Unscrew the phase plugs on their passive drivers, add or remove a weighted disc or two. Changing a passive cone’s mass alters its tuning and damping. With adaptive Sehring, you either get more extension with less weight or less extension with more weight. It’ll depend on your room and taste.

    Another way to think of passive radiators is as half ways. A classic 2.5-way speaker will have a tweeter and two equal mid/woofers. The upper woofer meets the tweeter on a low pass, then runs down as far as it can. The lower woofer sneaks in on a second lower pass that’s usually set to ~100Hz or below. That begins to gently add bass to the upper driver before its filter slope kicks it in fully to add power and reach to below where the upper driver would go alone. The advantage of doing that rather than configuring a 3-way is that the upper mid/woofer sees no high-pass filter. It runs wide open to place fewer reactive elements between it and your amplifier. The idea is that fewer filters are better.

    Sehring with adaptive passive & Fram with four passives.

    In a speaker with one or multiple passive radiator/s, they add that same halfway. Its bandwidth only overlaps the active driver at its bottom to add reach and output. Unlike the classic 2.5-way or 3.5-way, it simply manages without an electrical crossover. It’s done purely mechanical. And unlike a port, it adds actual cone surface. When you thus see specs for monitors with a rear-firing passive like Buchardt’s S400 which looks like a 6” two-way on the front but on the back adds an oval 5×8” passive, it actually becomes more of a 10” two-way where bass is concerned. That explains why Buchardt specifies a 33Hz –3dB in-room response. Having reviewed it, they’re selling no porkies.

    Burchardt S400.

    So the suggestion behind today’s soapbox is to become hip to the presence/absence of passive radiators, on speakers you might consider. The holes and plastic pipes of ports are cheaper to implement than passive radiators (they’re also called auxiliary bass radiators aka ABR) but the extra expense does have sonic advantages. Those include fewer room interactions, less overlay on the midband for better vocal clarity and better time-domain behaviour. If properly done, they also give you bass extension which you might not have thought possible from the size box they’re associated with.

    Perhaps you too will become passively radiant one fine day? Worse things could happen…

    Srajan Ebaen

    Written by Srajan Ebaen

    Srajan is the owner and publisher of 6moons. He used to play clarinet at the conservatory. Later he worked in audio retail, then marketing for three different hifi manufacturers. Writing about hifi and music came next, then launching his own mag. Today he lives with his wife Ivette and Nori and Chai the Bengal cats in a very small village on Ireland’s west coast, between the holy mountain Croagh Patrick and the Atlantic ocean of Clew Bay in County Mayo’s Westport area. Srajan derives his income from the ad revenues of 6moons but contributes to Darko.Audio pro bono.

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