MBL 120 Radialstrahler
Stereophile|November 2021
LOUDSPEAKER
JULIE MULLINS

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.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,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.

SPECIFICATIONS

Description Three-way standmount loudspeaker with “Radialstrahler” radial carbon-fiber omnidirectional tweeter and midrange drivers and two side-firing 6.5 woofers with aluminum membrane cones in a rear-ported enclosure. Sensitivity: 79dB/2.83V/m into 8 ohms, 80.5dB in-room. Crossover: fourth-order Linkwitz-Riley crossing over at 600Hz and 3.5kHz and with a second-order high-pass filter below the port-tuning frequency. Frequency response: 48Hz– 15.5kHz –6 dB in-room. Nominal impedance: 4 ohms. Recommended amplifier power: 200W into 4 ohms.

Dimensions 11.8 (300mm) W × 23.6 (600mm) H (27.4 w/grille; 47.7 on stand) × 15.3 (389mm) D. Weight: 33lb (17kg); 61.7lb (28kg) w/stand.

Finishes Piano Black, Piano White, Piano Silver with chrome or gold accents on any color cabinet.

Serial numbers of units reviewed 44940, 45040. Designed and built in Germany.

Price $21,400/pair; matching stands $1630/pair. Approximate number of US dealers: 10. Warranty: 5 years parts and labor.

Manufacturer MBL Akustikgeräte GmbH & Co. KG, Kurfürstendamm 182, D-10707 Berlin, Germany. Tel: +49 (0) 30 23005840. Web: MBL.de. US distributor: MBL North America, 217 North Seacrest Blvd. #276, Boynton Beach, FL 33425. Tel: (561) 735-9300. Web: MBL-northamerica.com.

The MBL 120 ($21,400/pair) 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.

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