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Surround Sound Miking and Microphones: An Experimental Recording

by David Moulton

As alert readers will recall, for the last several months I've been discussing the emergence of surround sound as a major playback medium for both video and music production. To date, I've confined my discussion to questions of playback format and possible production approaches.

Now, I'd like to discuss the issue of surround recording microphone usage. Although much surround production, particularly for video, will actually get assembled in post-production from mono and stereo tracks, the temptation to actually record live multichannel surround information is quite strong, and no doubt we are all going to try it at least several times! Such recording presents some interesting problems, and they are worth considering briefly. All this comes to mind because I just finished working on an experimental orchestral recording of the Boston Classical Orchestra at a Faneuil Hall
concert, where we recorded the orchestra with three different surround setups, as part of a combined Boston AES and Acoustical Society project. More about that adventure later.

Two Schools of Thought About Surround Miking
In a nutshell, there are two ways to think about surround miking. On the one hand, we can treat the microphones as a spaced array so that during playback we sort of "listen within the space" recorded by the microphones. On the other hand, we can treat the microphones as a coincident array, like a dummy head, so that during playback we listen to what sounded AT A PARTICULAR POINT IN SPACE, or "listen inside our head." Both views have some merit, and both views present some problems. Fortunately, the views aren't mutually exclusive, and it is possible to make very successful surround recordings including elements of both techniques.

When we use spaced techniques, in very general terms, the playback space becomes sort of a miniature hall and as we move about in it, our perspective shifts from increasingly dry to increasingly reverberant, just like in a real concert hall as we move toward and away from the stage. Meanwhile, localization is a little vague, and we get little sense of any sounds originating from WITHIN the array of playback speakers. When we record with a coincident array, we get a more specific, highly defined illusion that is primarily detected from a very specific point in space (the "sweet spot"). That illusion includes comparatively sharply delineated acoustic sources and ambiences, as well as a clear sense of being at a particular vantage point in the hall.

Both illusions are fun. Both can be extremely satisfying musically. But, the sensations they provide are quite different. For video and film production, I'd probably use both techniques, each for specific points of view: where I wanted the audience to view a scene as static observers, I'd probably use a spaced-array approach; where I wanted the audience to share the point-of-view of an actor, I'd probably use a coincident approach. However, keep in mind that we still have much to learn, which leads us to the recording in question.

The Boston Classical Orchestra at Faneuil Hall
In the case of the Boston Classical Orchestra recording, we recorded the orchestra with two different coincident arrays and a quad (actually quint) spaced array. These were recorded on 16 channels of digital multitrack with no intervening signal processing. The quad/quint spaced array consisted of four Earthworks omni microphones hung in a trapezoidal pattern about 12 feet up in the air, with the front pair approximately six feet apart and just above the first row of the audience, while the rear pair was about fifteen feet further back and 12 feet apart. A fifth microphone, a cardioid, was aimed at the ceiling from above and behind the rear array.

One coincident array was a purely experimental microphone array built by acoustician Dick Campbell. It consisted of four cardioid capsules provided by Audio-Technica placed at 90° to each other. This mic was oriented in a classic XY configuration, with capsules facing 45° Left and Right and 45° Left-rear and Right-rear. This mic was suspended
approximately six feet behind the podium and nine feet up.

A New 6.1 Channel Surround Microphone Designed for Film Work
The final mic we used is a prototype developed by Michael Godfrey of Toronto, working with George Wong of Canada's National Research Council. This microphone is designed for location film work, complete with battery power and seven channels of wireless transmission from the microphone to the recorder position. Godfrey intends it for use on location sets and for recording surround SFX and Foley. It is actually suitable for a broader range of applications.

Six Sennheiser capsules are mounted on the "dummy head" (actually, made out of hard black glossy plastic, it looks more like an alien head from a late 40s sci-fi movie than the typical dummy head we've come to know and love) to pick up Left, Center, Right, Left Rear, Right Rear and overhead. In addition, a seventh capsule inside picks up low frequencies omnidirectionally.

This mic was hung about 10 feet behind the conductor and seven feet up. With these mics in place, we just recorded directly to tape, warts 'n all, recording Steven Lipsitt conducting a program of Mozart's Marriage of Figaro Overture, a work for string orchestra by Alfred Schnittke with some interesting stage movements by the players, Haydn's Trumpet Concerto (Stephen Burns, soloist) and his London Symphony. The house was full, to a point where the sound was just slightly dry. The playing was generally excellent, in terms of ensemble, intonation and time. No major calamities occurred, just the usual coughs, rustles and urban grunge.

How Did It Sound? To Be Continued . . .
Due to busy work schedules, we haven't had a chance to play back these recordings for any purposes, let alone serious study. We are planning a listening session at an upcoming Boston AES meeting, hopefully in Boston Acoustics new big surround playback theater. In the meantime, I will check out the recordings and report back to you next month how they sound.

The opportunity to audition three such techniques in the same hall on the same (good) orchestral performance is just invaluable, and I'm looking forward to sharing the resultsand their implications with TV Technology readers.

Thanks for listening. See you next month!
Dave Moulton is back at work. You can complain to him about anything at dmoulton@ma.ultranet.com.