Neumann M150 in a Decca Tree Configuration

Apr 1, 2002 12:00 PM, BY TY FORD

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At $5,300 (including suspension-mount, vintage-style power supply, multiconductor cable and aluminum flight case), the Neumann M150 small-diaphragm, transformerless, tube condenser microphone will probably not be found in many basement studios. Perhaps more relevant than its cost, the M150's fixed-omni pattern makes it a less than desirable choice for recording in small spaces with challenging acoustics — the mic is quite capable of “hearing” exactly how good or how bad a room sounds.

However, Neumann's operating instructions for the M150 note that the mic is especially suited for Decca Tree recording (see sidebar). Because that configuration requires three M150s, anyone whose microphone budget is less than $15,900 can stop reading here. But, if you're looking for a reliable method for producing a stable stereo image that will hold up throughout the application of Dolby and other surround sound matrix systems, the Decca Tree technique is worth examining.

I had heard about an upcoming orchestral session at Phase Recording, a new facility in College Park, Md., a suburb of Washington, D.C. Composer Charlie Barnett booked a 21-piece orchestra for a session intended to produce the stereo music track to accompany a 100×15-foot video wall at the Newseum in Arlington, Va. Jim Ebert had been called in to oversee recording and mix the session. I talked with studio owner Bruce Falkinburg and chief engineer Tony Eichler, and contacted Karl Winkler at Neumann; shortly afterward, three Neumann M150s arrived at my door. A Decca Tree T-Bar from Wes Dooley at Audio Engineering Associates was purchased for the session ($795 for the 2×1-meter T-bar; a 2.5×1.25-meter Super Tree version is available for $945).

Phase Recording is a new studio complex, built into an existing warehouse, just north of the University of Maryland, and offers a large, tunable studio that's comfortable and flexible. The main studio space is fairly large by today's standards, 35×45 feet with 18-foot ceilings, and features a splayed portion of one of its longest walls that produces a live, but smooth sound.

AIR TO SPARE

The night before the session, engineer Tony Eichler and I suspended the AEA T-Bar from a very large and heavy-duty rolling mic stand, attached the Neumann EA 170 suspension mounts and hung the mics. I had some difficulty working with the EA 170 suspensions and the T-Bar, because the upper extremities of the suspension mounts got in the way, keeping me from angling the mics easily — it would have been nice to have a set of 4-inch extension stubs to drop the suspension mounts out of the T-Bar's range.

The size, liveliness of the room, height of the rig above the musicians, distance from the musicians, distance between the mics themselves, and rotational and vertical angles are all important considerations to get the correct stereo image and one that is mono-compatible. The challenge would be placing the mics to get just enough air to create the big image that composer Barnett wanted without getting sloppy. We started with the mic mounts pushed almost all the way out to the ends of each rail and set the height for 10.5 feet.

We used three channels of GML mic pre's for the M150s, and ran line-level back to the control room, where the main console was a Trident 80B supplemented with a 16-channel API sidecar. UREI monitors were mounted in the control room walls, but for this date, we monitored on powered Mackies.

With 21 musicians to cover, and because this was the first time a Decca Tree array had been used in this space, 20 spot and section mics were also used as backup. All mics were routed to the main recorder (a 2-inch, 24-track Studer running BASF 900), and the three M150s were also multed to one of the new RADAR24 HD recorders from iZ, recording at 24/96.

TREE TRIMMING

The morning of the session, Charlie Barnett gave us about 10 minutes to play with the Decca Tree placement while the orchestra rehearsed. With our original position, the sound was too diffuse and the stereo image was too exaggerated. Based on cues from Ebert and Eichler in the control room, I lowered the Decca Tree to about seven feet off the floor and pushed it slightly farther out toward the musicians. That was too close and the sound was too direct, especially from the front mic. Pulling the mic back a foot helped.

The stereo image was still very exaggerated. I moved each side mic in about six to eight inches, re-angled them to cover the left and right edges of the sound stage, and aimed them down at about the middle of each section. Ebert and Eichler said that brought the stereo image together and helped mono-compatibility. I re-angled the front mic slightly to hit the middle of the sections in front of it and we began.

After all of the cues were recorded, Ebert set up a stereo mix to DA-88. Because of the lack of percussion, which we had set up in a different room, the Decca Tree mix without the additional mics sounded extremely nice — spacious and clear. Ebert commented that the audio from the three M150s in the Decca Tree, with percussion added, could have been used by itself for TV work.

The added spot and section mics brought in more intimacy and thickened the mix with minimal phasing problems. For his final mix, Ebert used 50% of the Decca Tree and added spot and section mics. Most of the spot mics (first cello, clarinet, French horn, bass and harp) were positioned in the center to reinforce mono-compatibility. String section mics were placed within the same perspectives as the Decca Tree image.

Ebert used very little EQ while recording, mainly on the percussion instruments. Although the studio has a fairly live sound, Ebert added some plate to sweeten the mix. The final mix sounded large, rich, detailed and full.

IN CONCLUSION

The Neumann M150s did an excellent job of capturing the performance. The tube mics were extremely clear and “untube-y” in the traditional sense. The gentle rising frequency response above 1 kHz brought sounds forward, without being too in-your-face. I had a quick chance to put one up in my studio and confirm that, while the M150 doesn't have the proximity effect of a pressure-gradient cardioid, it could very easily be used for V/O or vocals, acoustic guitar or just about anything, given the right acoustical environment.

Perhaps the most interesting discovery was that during comparisons between the Decca Tree tracks of the Studer and the new iZ RADAR24, no one was able to consistently tell which was which. The analog head bump gave the Studer away, but all of the “analog heads,” including Barnett, were very surprised by how smooth the RADAR24 sounded.

Neumann USA, 1 Enterprise Dr., Old Lyme, CT 06371; 860/434-5220; fax 860/434-3148; www.neumannusa.com.

Audio Engineering Associates, 1029 North Allen Ave., Pasadena, CA 91104; 626/798-9128; fax 626/798-2378; www.wesdooley.com.


Reach Ty Ford at www.tyford.com.

THE DECCA TREE

The stereo microphone array commonly referred to as the Decca Tree was originally conceived by recording engineers at Decca Records in London. The array consists of three omnidirectional microphones situated at the ends of a large T-shaped fixture, with spacing between left and right microphones approximately two meters, and the central microphone placed in front of these by about 1.5 meters. The complete array is typically positioned a few feet behind and about eight to 10 feet above the conductor's head. Traditionally, the microphones used were “classic” Neumann M 50 large-diaphragm tube condenser mics, which provided a characteristically warm and enveloping sound. The three microphones are usually panned left, center and right, respectively, across a pair of recording tracks.
Text courtesy AEA

M150 TECHNICAL SPECIFICATIONS

The M150 capsule is an omni at most mid- and lower frequencies, but, according to the polar plot, between 8 and 16 kHz, it changes from an omni to a hypercardioid pattern. Get more than 45° off-axis and the HF fades, a characteristic I confirmed with a simple “test, test, test” while rotating the mic. The polar diagram accurately shows the 16kHz response line down 3 dB at just over 45° off the front axis and down 9 dB at 90° off-axis. The polar diagram shows a return of 16kHz response (-5 dB) in a fairly narrow lobe at 180°, indicating that the M150's pattern is not a pure omni.

The capsule is a pressure, not a pressure-gradient, transducer, and has a very flat low end, down well below 20 Hz. A low-frequency roll-off switch on the body of the mic engages a 6dB/octave slope that starts at about 75 Hz and is down 10 dB by 25 Hz. The manual also points out that, in the “LIN” (flat) position, a limit frequency circuit rolls off at 16 Hz to protect the console inputs from the effects of sub-audio noise. A 10dB pad is also available.

On the high end, the frequency response begins a gentle upward slope at 1 kHz to a +4dB peak at 3 kHz. The curve drops 2 dB at 5 kHz and rises again to +4 dB at 8 kHz. From there, the top end gently rolls off, crossing 0 dB at 15 kHz and down 4 dB by 20 kHz.

Sensitivity is 20 mV/Pa at 1 kHz and 1 kohm (1 Pa = 94dB SPL). Nominal impedance is 50 ohms. Nominal load impedance is 1 kohm. Self-noise is 15 dBA. Maximum SPL for .5% THD is 99 dB, and for 5%, 134 dB. Peak output voltage is ±10 volts at 20 mA.

The head grille is similar in shape and size to that of the M 149, with coarse outer and finer inner metal meshes as was used in the original M50. The diaphragm is slightly over ½-inch in diameter, and is mounted on the surface of a round plastic globe, 1.5 inches in diameter. The diaphragm is oriented so that it faces forward, sort of like an eyeball on a stick. The eyeball, stick and base are directly connected, with no shock-mounting. Two small wires exit the back of the globe and carry the signal through two thin contact pins in the flat plastic base to the JAN6111 vacuum tube and circuitry in the microphone's base.

The KT 8, a 30-foot multiconductor cable, connects the mic to the N 149 V (vintage) power supply, which is available to U.S. customers only.

The N 149 V power supply looks like a vintage Neumann power supply, a rugged metal case with a padded carrying handle, simple on/off toggle switch, light, XLR audio output connector and multipin connector. The on/off toggle only controls the secondary voltage, so power remains to the input of the power supply as long as it is plugged into the wall. An EA 170 suspension-mount, typical of most Neumann mounts, is also included, as is an aluminum flight case that houses, mic, mount, cables and power supply.

Neumann suggests that the multipin cable can be extended to 100 meters without noticeable losses. Cable losses of up to 4 VDC (those approximated at 100-meter length) are detected and compensated for by an integrated sensor line. The audio cable from the power supply output to a microphone preamp input can be extended to 300 meters without signal degradation.
Ty Ford






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