Adventures in 10.2

Facing a 10.2 channel recording and mix can be a daunting task. My colleagues and I recently made a recording of the New World Symphony in Miami playing Aaron Copland’s symphony that includes the Fanfare for the Common Man.1 After editing and mixing it, the recording was played for an audience of about 550 people attending an Internet2 conference at USC to wide acclaim. I realized about halfway through mixing that we were relying on the developments of many others, and had made some of our own, to make this recording work as well as it did, and that I should document these, because many of them can be used every day in both classical and pop recording for multichannel, whether it’s 5.1 or 10.2.

Main Mics
The first of these is selection of the basic microphone array. Cardioid microphones spaced at about ear distance of 7 inches and splayed out at 110 degrees form the main pair, panned just left and right of center. This “ORTF” array has been shown in double-blind listening tests to beat either spaced omnis or pure coincident techniques like crossed Figure 8’s. But there is a problem: the microphones don’t face the center of the soundfield, so capturing the center instruments is off-axis, and the high frequencies will normally be attenuated somewhat more than the midrange on most microphone models.

There is a way around this: two-way microphones. The first that I was aware of was the AKG dynamic mic, the D224E, which I owned some years ago. By making the high-frequency pickup smaller than the low, it could maintain the polar pattern more uniformly with frequency than single-diaphragm types. However, it’s not in the catalog anymore — guess it didn’t sell. There is a more modern version, however. John Eargle made us aware of the Sanken two-way electrostatic cardioid, the CU-41. Although heavy, this type permits good on- and off-axis response needed for the optimum ORTF array. The “wide” mics, and numerous spot mics, were mostly Schoeps Mk2 omnis. More about these below.

Surround Mics
The main surround was a Schoeps surround sphere mic with bidirectional outriggers and an MS-style decoder box. The basic sphere was developed by Günther Theile of the IRT, and Jerry Bruck extended it to surround with the bidirectional pair and decoder. Additional surrounds were a pair of Neumann TLM-103 cardioids, chosen for their low noise level, since they were positioned in the balcony and facing away from the orchestra to let room sound predominate. Mic noise can dominate if the microphone used this way is not extra quiet.

Noise Mic
The final microphone is really unusual: a Digi-Key P9925-ND electret microphone cartridge ($2.83) wired up with three resistors, a capacitor, and a battery and wrapped in a condom. This “condom” microphone was placed in the supply air duct of the air-handling system to pick up the sound that eventually generates the background noise in the room, after complicated transformations by the air path and room acoustics. The purpose of this is to do research on whether such a microphone can be used with an advance on LMS techniques to subtract the noise from the other microphones, but the problem is quite hairy and is subject to ongoing research. It ain’t just flippin’ the phase by far, but that’s a related, although simpler, idea.

Altogether 24 microphone tracks were recorded:

2……..Main L/R ORTF cardioid pair Sanken CU-41
2……..Main L/R omni outriggers Schoeps CK2
2……..Harp spot cardioids Schoeps CK4
2……..Tympani spot omnis Schoeps CK2
1……..Bass drum spot omni Schoeps CK2
2……..Other percussion spot omnis Schoeps CK2
2……..Woodwind spot omnis Schoeps CK2
2……..Double bass spot omnis Schoeps CK2
2……..Proscenium height omnis Schoeps CK2
4……..Surround sphere (special) Schoeps KFM360
2……..Surround cardioids Neumann TLM103
1……..Condom mic (see text) Digi-Key P9925

Dynamic Range: Pads and Calculations
For most recording, it is possible to rehearse and set a level using the input trim control of the mic preamp for the best tradeoff between headroom and noise. What if you don’t get a rehearsal? Maybe then you use your experience in a given situation (instrument(s), room, microphone, gain structure). But if it’s a “cold” recording, what then? Well then you can calculate the required preamp gain from the expected sound pressure level, the microphone sensitivity, and the input sensitivity of the device being fed, such as a recorder.

The first thing to consider is whether the expected sound pressure level at the microphone might clip the microphone’s own electronics in the case of electrostatic microphones, those most often used for this type of recording.

The Schoeps electronics operating on 48V phantom clip at just over 130 dB, but they don’t reach 138. So we need pads, ones between the capsule and the electronics. We used 10 dB pads on the five percussion spot mics as these were the ones that were close enough to instruments loud enough to cause potential problems, and the signal-to-noise ratio was not harmed as the instruments were so loud that their spot mic contribution was well down in the mix.

Then, to calculate the gain, we use the microphone sensitivity, the pad value, and the input sensitivity of the recorder for Full Scale to determine the unknown in the overall equation — the gain setting of the microphone preamplifier. The mic sensitivity is 15 mV/Pa, the pad is 10 dB, and the input sensitivity for Full Scale is 18 dB over +4 dBu, namely +22 dBu. Let’s take 138 dB SPLpk as our level. Then rms level is 135 dB, and the rest of our calculation can be in rms. With 15 mV at 1 Pascal or 94 dB SPL, 135 dB SPL is 41 dB hotter. The 10 dB pad takes this down to 31 dB hotter. Thirty-one dB up from 15 mV is 530 mV (calculated by dividing 31 by 20, then raising 10 to the power of the remainder: 10^(31/20)=35.5 times. 15 mV x 35.5 = 530 mVrms. +22 dBu = 9.76 Vrms. (Get from 10^(22/20) x 0.775 (the reference level for 0 dBu) = 9.76 Vrms.) Now 20 log (9.76/0.53) = 25 dB. So the maximum gain we can use is 25 dB, and, to leave a little headroom beyond 135 dB SPL in case this drum is louder, let’s make it 20 dB. We did. It worked. The maximum recorded level hit about –3 dBFSpk, and, with 24-bit recording, we had a huge dynamic range captured.

This can be done with a $10 Radio Shack scientific calculator in five minutes. Or it’s more fun to do it in your head and amaze your friends. The trick is to know how to do dB in your head by learning a few numbers: 1 dB = 1.1, 3 dB = 1.4, 6 dB = 2, 10 dB = 3.1, 20 dB = 10. From this you can decompose any number quickly. Take 87 dB. Eighty dB is a factor of 10,000 (from four 20s). Seven dB more is a factor of two times 1.1 (6+1 dB). So 87 dB is 22,000, in round numbers.

On the other end of the dynamic range, with 24-bit recording, it is acoustic noise that usually dominates, at least at low frequencies. I’ve already described the condom mic for the air handler, but what about other acoustic noises? Here is where the Miami Beach Police Department got into the act. They closed off the streets around the Lincoln Theatre, home to the New World Symphony, for two hours while we got this recording! They were under the impression we were a movie shoot (we were shooting HD video as well as the sound), and they knew how to handle that from all the film and television work done there. And it worked: there was no intrusive noise.

So, on the day of the event, we recorded 24 channels of 24-bit/48 kHz audio to tape, and merrily came away with two run throughs of the piece to serve as editing elements. The room acoustics of the Lincoln Theatre leave something to be desired. While efforts were obviously made to add diffusion and keep reverberation under control in the orchestra’s re-outfitting of the space from its original purpose of movie theater, this left them with a fairly small hall, and they’ve got a fairly large band. So if the available drapery for absorption is left open, it’s too loud, and if closed, it’s too dead. What we did to overcome this obstacle, and other tricks, will be discussed next time.

Surround Professional Magazine