In contrast to the many sound alike and look-alike mics that have saturated the market in recent years, the MBHO MBNM-622 is about as different as they come. This studio-quality stereo microphone doesn't just have an unusual appearance; it also breaks new ground in mic design. By combining two proven technologies — the Jecklin disk and the boundary-layer microphone — German mic manufacturer MBHO has come up with an innovative unit that truly stands out both sonically and visually.
The MBNM-622 looks like a cross between a sundial and a futuristic spacecraft. Its all-black construction only adds to its imposing presence. The base of the 622 is a metal disk nearly one foot in diameter. Because the disk is meant to be placed or secured on a flat surface, two mounting holes are provided on the bottom end, and a thin felt cushion covers the underside.
The disk is bisected by a finlike semicircular plastic piece covered on both sides with a ¼-inch layer of foam rubber. The 622's two boundary-layer mics, which resemble large-caliber bullets, are mounted directly on the metal disk on either side of the fin and aligned with an inner concentric circle that measures 6.5 inches in diameter. The fin functions as an acoustic baffle between the two mics, and the foam reduces sonic reflections. (Incidentally, MBHO also makes the MBNM-621 mono-boundary mic for single-channel uses.)
No carrying case is provided for the unit; however, it would probably fit nicely into a small tom-tom case.
As mentioned, the origin of the 622's curious design can be traced to two technologies: the Jecklin disk and the boundary-layer microphone. The former is a critical component of the Optimal Stereo Signal (OSS) recording system pioneered by Swiss engineer Juerg Jecklin. Jecklin's OSS setup specifies a pair of omnidirectional microphones positioned 6.5 inches apart and separated by a sound-absorbing Jecklin disk measuring 11.81 inches in diameter. The microphone capsules align with the disk's center.
Jecklin devised his recording system to simulate the positioning and pickup characteristics of human ears. It integrates the three cues our brains use to determine the location of sounds — sound intensity (level), time delays (phase relationships), and variations in the way frequency ranges are picked up — to produce lifelike stereo imaging, albeit with not-so-good mono compatibility. Indeed, the omni mics act much like our ears, picking up frequencies below 200 Hz equally. As the frequencies get higher, the disk — like a human head — blocks more sound, and the mics — like human ears — operate more directionally. (Additional information about OSS recording is available on the Web site of mic designer and theorist David Josephson at www.josephson.com/tn5.html.)
The other inspiration for the 622 is the boundary-layer mic. Boundary microphones have been around for decades, but their operation remains a mystery for many engineers. Still, it is based on a relatively simple principle: at a solid boundary (a wall or floor, for example), no air can move because the mass of the boundary prevents it. Sounds that reach a boundary manifest as changes in air pressure, and therefore, a pressure-type mic capsule located at the boundary can respond accurately to the pressure changes while picking up a minimum of reflected, out-of-phase signals.
Some boundary-mic designs use a recessed capsule flush with a plate (the boundary), whereas others use a capsule suspended a small distance above the plate and facing toward it. Crown's well-known Pressure Zone Mic (PZM) employs the latter design; the gap between the capsule and plate is called the pressure zone. As incoming sound waves reflect from the boundary, they reach the capsule at virtually the same instant as the direct sound waves. As long as the gap is no more than 0.1 inch from the boundary, the direct and reflected sound will remain in phase to 20 kHz.
In Crown's design, a hood over the mic further protects the element from errant sonic reflections in the room — sounds that can produce phase distortion and comb filtering in a recording. As a result, the microphone primarily picks up sound that takes a direct path to the capsule, along with phase-aligned reflections from the boundary plate. The waves combine and act as one reinforced signal at the capsule, boosting the microphone's sensitivity by about 6 dB.
With any boundary-layer microphone, increasing the size of the boundary panel enhances low-frequency pickup because the fundamental wavelengths of low-frequency vibrations can be longer than the boundary plate. Therefore, boundary-layer microphones work best when they are placed on a bare (noncarpeted) floor or wall. For convenience, they are often mounted on a flat, movable four-foot or larger square panel.
For the 622, which MBHO dubbed OSS-II, the Jecklin disk was essentially cut in half and attached to a round plate fitted with two boundary-layer mics, one on either side of the Jecklin “half-disk.”
TWO EARS IS HUMAN
The 622 somewhat resembles a binaural recording system in that a solid object separates the microphone pair, the distance between the microphones is roughly equivalent to the width of a human head, and each mic's pickup is omnidirectional. But that's about as far as the similarities go. I recorded composer Dan Plonsey's Portcullis Ensemble with the 622 (mounted on a wall 30 inches above the floor with the baffle oriented vertically), a binaural head, and a near-coincident pair of omni mics (Neumann KM 183s) and compared the results.
Not surprisingly, when auditioned on headphones, the 622 recording did not sound as stunning as the binaural recording. Nonetheless, it was impressive on several counts. The 622 captured discrete separation among the ensemble's left, right, and centered components, and each voice was clearly positioned in the stereo field. If anything, the positioning of the instruments appeared a bit wider than their placement in the room. In contrast, the Neumann pair yielded a more realistic and blended sound in the headphones but sounded much more roomy and lacked the focused stereo imaging of the 622 when auditioned on monitor speakers.
In addition, the reduced room reflections at the 622's capsules not only made for a drier, more controlled recording but also seemed to bring distant instruments closer to the listening position. In particular, the drums — a full rock set with a deadened kick — had a tightly defined sound with no hint of the mushiness that can plague more traditional stereo drum recordings. The 622 also bestowed a round enhanced low end to the kit. Its high-end response, however, was not quite as smooth or pleasing as that of the Neumann omnis.
On the rest of the ensemble, the 622 gently highlighted the midrange instruments — trumpet, cello, and electronic keyboards — whereas the Neumann pair brought out the airy, higher ranges of the stringed instruments, piccolo, and cymbals. Considering that a single Neumann KM 183 costs hundreds of dollars more than the 622, this stereo microphone is an excellent value.
Acting on a tip from drummer and EM contributor Karen Stackpole, I tried the 622 placed on the floor directly in front of the kick on a full drum kit. If wide, dramatically panned drum sounds are your thing, give that technique a listen. I was particularly intrigued by the tom sounds in the recording, which, despite the fact that the mic was positioned below the toms, had a clearly imaged attack and a natural, uncluttered resonance.
The kick sounded respectable too. With careful placement and a bit of reinforcement from overhead mics, the 622 could become a new favorite for capturing dramatic drum sounds.
I tested the 622's performance on acoustic guitar in three sonic environments and, not surprisingly for a stereo mic positioned at a distance, received three very different results. I was seated for each recording, playing the guitar with the microphone positioned on the floor four feet in front of my chair's base.
Inside the carpeted and acoustically dead Headless Buddha Mastering Lab, the guitar sounded controlled and even, although not sparkly or exciting. Interestingly, the width of the stereo recording was enhanced by subtle arrival-time differences between the right and left channels. Despite warnings against using boundary mics on carpeted surfaces, the guitar's low end sounded full and even a bit boomy on a low G note.
Next, I recorded the acoustic guitar in a narrow, linoleum-floored hallway using the same setup. It was easy to hear the midrange resonance and lively quality imparted by the close walls. Again, the stereo separation was amazing, with the pick sound clearly audible on one side and my sloppy, buzzing fretwork on the other. In that environment, however, the 622 picked up an overbearing amount of bass.
The third environment, a wider and more open section of the same hallway, produced relatively lean bass on the recording. That prompted me to return to the two hall spaces with the guitar and confirm that it was indeed the spaces' quirky qualities, not the 622, playing tricks on me.
Clearly, the 622 is by no means immune to room sound; it won't hide a space's sonic defects. However, it requires neither an acoustically perfect room or concert hall nor excessive fussing over placement to get usable results.
Incidentally, I left the tape running between takes while recording the aforementioned acoustic-guitar tracks, letting me hear the recorded handling noise as I repositioned the mic for each take. I was impressed by how little handling noise I heard. The mic's self-noise, as heard through the preamps in a Mackie 1402-VLZ mixer, was also acceptably low.
I used one of the 622's channels as an ambient drum-room mic for a CD project by guitarist Shelley Doty, with EM associate editor Brian Knave thumping the tubs. As the control in this experiment, I employed a Neumann M 147, a large-diaphragm tube condenser that excels at capturing huge kick-drum sounds at a distance. The M 147 also downplays unwanted high-end harshness, which is helpful in the reflective confines of the Guerrilla Recording drum room. Having experienced high-end problems with other omni and boundary mics in that environment, I was interested to see how the 622 measured up. I placed both mics about nine feet from Knave's drum kit, with the 622 on the floor and the M 147 six to eight inches above the floor.
The 622 delivered room sounds that were tighter and less reverberant than what I am used to. The thunderous low end that I rely on from the Neumann tube mic wasn't quite there, but the 622 sounds were still meaty and free from midrange boxiness. As I observed earlier while recording the larger ensemble, the mic sounded nice and full in the crucial 200 Hz “punch” area, and it also responded favorably to experiments with additive EQ in that range. To my surprise, I could boost signals in the 40 Hz to 100 Hz range (using the digital parametric EQ in a TC Electronic M2000) on the MBHO tracks without encountering uneven response or excessive muddiness.
In addition, EQ sweeps throughout the high end showed that the 622 wasn't harsh or exaggerated in the 2 to 3 kHz region, as many condensers mics can be in that application. However, I did note a gentle boost between 5 and 8 kHz, which is typical for many condenser mics. That made the hi-hat and upper frequencies of the snare drum a bit more brittle sounding than I prefer, especially as I boosted the 622 room track to increase the proportion of distant room sound in the mix.
ONE OF A KIND
The MBHO MBNM-622 is an unusual and innovative stereo microphone that successfully combines the characteristics of Jecklin-disk OSS recording and boundary-layer mics. It captures precise stereo imaging and powerful low end, yet its ability to simultaneously magnify detail while minimizing the washing-out effect of reflected room sound is truly unique.
The mic's highs can sound slightly brittle on some sources, but overall, I rate the 622 highly, both for imaging and sound quality. What it might lack in clinically flat frequency response is more than made up for in rock 'n' roll attitude. In addition, the 622 is affordable and easy to use.
It's also quite the inspirational tool. The more I recorded with the 622, the more ideas I had for applications and experiments I'd like to pursue using it. (I'm especially excited about using the 622 as the main mic on a “vintage” jazz or blues session.)
The favored use of the 622 is likely to be for dramatic stereo pickup of drum kits, physically expansive instruments (pianos, marimbas, vibes), and acoustically balanced ensembles such as string sections, choirs, and chamber groups. Its acceptance in the pop-recording world might be limited, especially among novice recordists. But advanced and creative engineers should investigate the wealth of possibilities offered by the 622's twin talents — OSS and boundary-layer miking. The MBNM-622 may not prove to be the most used implement in your audio toolbox, but it unquestionably dares to go where no microphone has gone before.
Myles Boisenis a guitarist, producer, composer, and head engineer and instructor at Guerrilla Recording and the Headless Buddha Mastering Lab in Oakland, California. You can reach him by e-mail email@example.com. Thanks to Dan Plonsey and Karen Stackpole for their assistance.
stereo boundary-layer microphone
FEATURES5.0AUDIO QUALITY4.0EASE OF USE5.0VALUE5.0
RATING PRODUCTS FROM 1 TO 5
PROS: Affordable. Easy to position. Full low-end response without muddiness. Precise stereo imaging.
CONS: No carrying case. Highs sound slightly brittle on some sources.
Type(2) stereo boundary-layer pressure-gradient transducers (electret), with modified Jecklin diskCapsules¼”-diameter aluminum-sputtered Mylar diaphragmsPolar PatternhemisphericalFrequency Response10 Hz-26 kHz (±1.5 dB)Dynamic Range130 dBSignal-to-Noise Ratio64 dB (@ 1 Pa A)Self-Noise30 dBMaximum SPL130 dB (for 0.5% THD @ 1 k•)Sensitivity5 mV/Pa (@ 1 kHz)Power Requirements16-48V phantomDimensions6" (H) × 12" (D)Weight1.13 lb.