MBL Radialstrahler 120 loudspeaker

You never know when an idea might hit you, maybe when brushing your teeth, standing in the shower, or stirring the stew.

Have you ever flexed a playing card (or a few) back and forth close to your ear? They generate a little sound. According to MBL company lore, that action and sound sparked the design idea for the original Radialstrahler omnidirectional driver.

Omnidirectional speakers are rare. I can think of only a few companies that make them: German Physiks. Morrison Audio. Ohm Acoustics. Duevel. MBL (footnote 1).

The MBL 120's design, like the design of most other omnidirectional speakers, is visually distinctive. More than one visiting friend who saw them in their piano-black finish (with grilles on) called them "the Darth Vader speakers." With the grilles off, or in piano white, the Darth Vader effect is less pronounced. Even so, they don't look like other speakers.

They sound different, too. MBL's omnidirectional drivers—which, as the adjective suggests, send energy out into the room equally in all directions—provide an open, 3D, relaxed sound, as in a concert hall, which of course presents lots of direct and reflected sound to your ears. As in life, the sound seems freed up, not boxed in.

If the MBL 120 were a book, it would be one that was hard to put down: I kept listening when I should have been going to bed—or finishing this review.


Design and drivers
According to some MBL history shared with me by Jeremy Bryan of MBL North America, Wolfgang Meletzky—the M in "MBL"—came up with the radial tweeter idea while playing a game of cards. He held a few cards near his ear and flexed them. He noticed the way the bending card "membrane" moved air against his ear. That action led to the patent on the radial tweeter concept, in 1979. The company's engineers, Bienecke and Lenhardt (the B and L in "MBL"), had trouble getting the concept to work reliably in the real world. Jürgen Reis (footnote 2), now MBL's chief engineer and technical designer, heard the prototype at a hi-fi show in Berlin soon after he had completed his electroacoustical engineering studies. The sound wasn't good, but he believed he could make the concept viable. He got a job at MBL. In 1984, he invented the radial drivers; nothing remains from the original design. They've evolved some since—for example, the tweeter now uses Kapton—but their basic design remains the same.


The MBL 120 ($24,900/pair in piano finish plus $1850/pair for the matching stands) is a three-way design. The tweeter and midrange are "Radialstrahler" omnidirectional "bending mode" drivers. They're mounted atop the 120's bass-reflex cabinet. These intricate, delicate diaphragms are comprised of thin carbon-fiber strips called lamellae—the playing card–inspired part—glued by hand into bowed arcs that contract and expand. The tweeter lamellae are unidirectional and single-layer. The midranges use carbon fiber that's woven like a flattened rope, two-layer, and bidirectional. Copper-wire voice-coils wind around the bottom of each driver, and on top of the array is an enclosed magnet structure. Bass comes from a pair of side-firing, opposed 6.5" woofers with aluminum-membrane cones mounted on heavy aluminum rods in the rear-ported bass reflex cabinet; Reis says that the edge of the cone "is capped for stability." This push-push configuration causes the woofers' back waves to cancel to reduce cabinet vibration, and delivers a more homogeneous radiation pattern that better matches the midrange's and tweeter's omnidirectional radiation, Reis said in an email.

Reis shared a few of the 120's tricks for extending bass in a smallish cabinet. The 120's crossover adds a passive second-order high-pass filter below the midbass region, which "pushes the lower bass and also helps to suppress subsonic bass in order to prevent too large of a woofer excursion," Reis explained by email. "This additional high-pass filter also allows me to tune the group delay of the woofers....This delay helps me to make the bass more 'full' because I can timewise modulate this behavior. I can also adjust the tilt of the bass to match the sound character of the midrange and tweeter."


The 120 has a nominal 4-ohm impedance. The impedance curve doesn't dip lower than 3.2 ohms, Reis said in an email, and it's "mostly between 4 ohms and about 10 ohms with an average around 7 ohms," he added. "All our speakers do have load-matching circuits built in to compensate for any bigger impedance and phase variations in the main audio area."

Detachable aluminum grilles on metal frames shield the delicate tweeter and midrange drivers and keep curious hands and paws away from the conductive membranes. If you have pets or kids, you might want to leave the grilles on, but I found the sound clearer and more open without them.

Arrival and setup
A pair of 120s ships in a single wooden crate; the stands ship in a separate box. Unpacking and assembly proved easy enough, but it's advisable to have a second pair of hands to assist. The 120s aren't very heavy, at 33lb each, but because of those delicate drivers, they require careful handling. I enlisted a friend's help, and Bryan talked us through the process on a video call.

I set the 120s up according to the manual's instructions then moved them about as I listened. Experimenting with placement, I found I wanted to sit closer to them than I do to most other speakers, as if I were drawn to them magnetically. My preferred setup wound up being close to an equilateral triangle, the speakers 6' 8" apart center to center and about an equal distance from my listening couch. At times, I moved the couch closer. The reason I wanted to get close, I suspect, was to get more direct sound from the speakers' blend of direct and reflected sound. MBL says the 120s must be run in for about 100 hours before they're at their best. During the pandemic, it didn't take me very many days to reach that number.


As the pandemic settled down and the vaccination rate approached 50%, Bryan visited to dial in the setup and observe how the speakers worked in my room. He'd been listening to some old vinyl favorites from his childhood including Neil Diamond's live album Hot August Night (Geffen Records). For fun, we put on several different versions of the album, streamed from Tidal and Qobuz via Roon, moving between a couple of familiar songs. Bryan made some slight adjustments by ear, and I moved around some of my room treatments. We found that removing the bass traps from the corners behind the speakers made the bass seem tuneful and prominent, especially on bass-heavy tracks. I liked the effect, but the measured room response was smoother with the traps in place, and at times I heard better integration that way. I went back and forth. With or without traps, the bass seemed to extend deeper than the specs indicate.


During his visit, Bryan said that the 120s "thrive in chaos." What he meant was that, more than with more conventional loudspeakers, the omnidirectional MBLs work well in an actual living space with furniture and stuff. Room treatment can improve the sound, but generally the 120s function well playing off whatever is in the room, except for some highly reflective materials like glass windows. (My apartment has lots of glass, but I do all my critical listening with the shades down.)

For my critical listening, I powered the 120s with one of three integrated amplifiers: the VAC Sigma 170i iQ tubed amplifier, the Soulution 330—both of those amps have an onboard phono stage—and my MBL Noble N51, which does not have the available phono option installed. I did most of my listening with the N51, which MBL says is an "atypical" class-D design. It's rated at 380 watts into 4 ohms. According to MBL's recommendations, the VAC amp was underpowered, but read on.

Loudspeakers with hybrid driver designs—here I'm referring to the MBL's blend of radial midrange and treble and bass-reflex bass—can be tricky to blend. And yet, to me the MBLs were strikingly coherent. They delivered seamless sound from top to bottom, which made the presentation seem more realistic. Music sounded "of a piece," seamlessly woven within the soundstage, not a patchwork of disparate pieces stitched together. These effects often enhanced artistic expression.

Footnote 1: The more you look, the more you find: There's also an Italian company called Alkèmia Audi—see alkemiavero.com—and a Dutch company, Veddan: veddan.com. There probably are others. Some of these designs are quite beautiful.—Jim Austin

Footnote 2: Reis also records music (including some Concerto Köln performances), plays in a rock band, and sings in a choir.

MBL Akustikgeräte GmbH & Co. KG
US distributor: MBL North America
217 North Seacrest Blvd. #276
Boynton Beach, FL 33425
(561) 735-9300

gbo328's picture

I spent some time at the MBL room in Munich (2019 High End), nice speakers. Omnidirectionals are not "that" rare, though. eg Ondacustica www.ondacustica.com and others, mostly Europeans I guess, in additions to the ones you mentioned.

cgh's picture

I've enjoyed the MBLs, mainly at shows, although I had one friend that had them. Seems like the design really hasn't changed much. It's interesting that the bending wave people didn't emerge more through the years (or that the term "bending wave" is avoided wrt MBL). I've heard numerous times that so-and-so was going to come back with some idea from the 80's based on....

Love the playlist. This Must Be the Place is my top 5 cheer up song. Always makes me happy too.

Julie Mullins's picture

Love the playlist. This Must Be the Place is my top 5 cheer up song. Always makes me happy too.

Glad you enjoyed that Talking Heads track—and the playlist in general. It's fun to mix things up!

remlab's picture

bending wave transducers has never ceased to amaze me. Nothing else in driver technology even comes close. Imagine what it would be like developing these things from scratch. The trial and error phase must have taken forever! And the tooling! Sheesh!

JoeE SP9's picture

There is a How It's Made episode that features the Radialstrahler speakers being constructed.

There is a lot of skilled hand labor in their construction.

Julie Mullins's picture

It is quite a painstaking process.

georgehifi's picture

"I love how the sound seems to float in the air rather than be fired at you."

I've found this with many good speakers, usually with esl, planer, or ribbon tweeters.

Cheers George

remlab's picture

short, but fun to watch..

windansea's picture

Fun review. I can kinda see how these omnis provide a meatier pushing of air, but can they keep up with planars or even domes for speed? What's the mass on these things?
And they look so delicate, can they last or do they break apart after a while?

The MBL tweeters made me think of the can tweeters on top of Anthony Gallo Nucleus Solo speakers. Those radiated 360 degrees.

volvic's picture

I have heard all iterations of the MBL speakers over the years, one day will own a pair. Some of the finest transducers I have ever heard.

RH's picture

I'd always lusted after the MBL 101D (or E) speakers, having heard them many times in some good conditions. I've never heard more realistic sound. But way out of my budget.

I managed at one point to get a pair of the MBL 121 stand mounted monitors - the slightly smaller precursor to the 120s in this review.

They sure had that MBL midrange and high end magic in spades! I favor speakers that do a "disappearing" act and image really well, and have owned many speakers that do that beautifully (e.g. various audio physic, Thiel, Waveform, many others). But nothing did the 3D thing quite like the MBLS. Also, I found the MBLS to have among the finest and most subtly realistic detail I've ever heard. I think Jonathan Valin in TAS was on to something, calling the MBL tweeter among the best in the world. For instance, when listening to a classical guitar piece it wasn't "detail" thrown in my face but more like I could simply listen down to the micro level of the finger padding on the strings, as I could with a real guitarist in front of me. And they did drum cymbals more believably than anything I've heard. Most speakers sound to me like drum cymbals are being "squeezed" through tiny tweeters, so the drum cymbals sound like smaller spots of brightness in the soundstage, where real drum cymbals sound BIG, like big vibrating discs of metal. The MBLs just sounded more real and more complete with drum cymbals.

Plus the 121s had some real solidity and kick in the upper bass.

Vocals could just be eerily real in they way they transported a performer like the Star Trek holodeck in to the room.

Further: At least in my room (very good acoustically and well treated) the MBLs did NOT do what many accuse them of doing: unrealistically stretched or diffuse imaging. The could be just about as focused as any of my box speakers, but without any sense of a speaker producing the sound.

The weird thing is that, as much as the MBLs mesmerized me, and I owned them for many years, I also owned other regular dynamic speakers and I found myself gravitating to "regular" speakers for lots of the music I like. It's hard to put my finger on why, because it's not like the MBLs were hopelessly ethereal sounding as they where quite dynamic, like electrostatics with balls. But there was just something richer, more comfortable, probably more familiar with the regular speakers that seemed more right and overall satisfying when throwing on, say, Rush or some Funk or Prog or many other genres. Ultimately I sold the MBLs, yet remain incredibly happy about having owned the MBL magic for many years.

I still have never heard subtle detail and instrumental timbre reproduced so naturally. And I still can't fully get the idea of the 101s out of my system.

robertbadcock's picture

That was the best five minutes I've spent on YouTube for a while. Thanks!

thatguy's picture

"I kept listening when I should have been going to bed—or finishing this review."

That has always been the sign for me that the sound was just right. When I'm listening despite having other things to do rather than because I don't have anything else to do.

Julie Mullins's picture

...this. It's a double-edged sword: the best kind of audiophile experience, but it makes it that much harder to pull yourself away. Not to mention the temptation to exceed one's budget!