May 31, 2023 at 9:10 am #5814
One HOA Microphone, Four Virtual Microphones, Unlimited Flexibility
We recently recorded Angelica Women’s Chamber Choir at Christ and St. Stephen’s Church in New York City.
Here’s a track from that performance – you can listen online or download it:
Here’s how it was made:
Angelica is a twelve voice treble choir. For performance, they arrange themselves in a semi-circle. (See the photo below.)
For this performance, they were accompanied on some songs by a Viola da Gamba, Recorder and Zills (finger cymbals), all played by members of the choir at their places. For part of the program, they were accompanied by an accordion, positioned within the semi-circle at the right. And for others parts of the performance they were accompanied by three string players (a cello and two violins), positioned at the extreme right of the stage, beyond and forward (towards the audience) of the choir’s semi-circle.
To record, we used a single Core Sound OctoMic higher-order ambisonic (HOA) microphone, positioned slightly to the left of the center of the semi-circle, and forward from the ends of the semi-circle by about 15-feet. (The positioning was dictated by the fire regulations to not block the central aisle.)
OctoMic is not a microphone in the way we usually think about microphones. It’s really an acoustic sensor that captures everything going on acoustically at its point in space.
The OctoMic was used to capture the complete performance, including precise and accurate venue acoustics. As a result, all microphone decisions – how many to use, their polar patterns and pointing angles – were deferred to post-production.
In post-production, the first thing we did was convert (“encode”) the A-format recording to standard second-order B-format. For spatial precision and accuracy, that process used the individual calibration file for that specific OctoMic.
Next, we rotated the spatial image 15 degrees clockwise to compensate for the off-center position of the OctoMic relative to the choir.
Then we started decoding the recording specifically for stereo playback. (We could have just as easily decoded the same recordings for 5.1, 7.1, ATMOS, full immersive for VR headsets, binaural – both fixed head and headtracked – and many others – all from the same original recording.)
We created and listened to many different virtual microphone configurations, including Blumlein, XY, XCY and XCY with auxiliary microphones.
We settled on XCY, with the X and Y microphones positioned at plus and minus 55 degrees in azimuth/0 degrees elevation, and the Center microphone at 0 degrees azimuth/0 degrees elevation.
The Center mic served as a fill, to correct for the increased distance to the singers at the back of the semi-circle. It was fed to both Left and Right channels at – 9 dB relative to the X and Y.
For the string section, we decoded a fourth virtual microphone, and rotated it to 135 clockwise in azimuth and down 15 degrees in elevation.
All of the virtual microphones were set to be first-order hypercardioids. (You can see a polar pattern graph for OctoMic’s first-order hypercardioid below.)
All of the virtual microphones are truly coincident, in both azimuth and elevation. When combining microphone feeds there is no phase cancellation.
After decoding to two channels, we applied a high pass filter at 125 Hz to reduce the annoying drone of the HVAC system. Then, because the room was quite dead, we applied a medium room reverb.
And that completed the post-recording process.
Thanks to Marie Caruso, Artistic Director: Angelica Women’s Chamber Choir, for permission to use this recording of Angelica performing Monteverdi’s “Pur ti moro”.
Angelica Women’s Chamber Choir: https://angelicawomenschamberchoir.org/
Other equipment used for this recording:
Millennia Media HV-316
JoeCo BBR-64 Dante
June 1, 2023 at 11:10 pm #5828Phil KoenigParticipant
Len – Actually, I didn’t know that, thanks for enlightening me. The first time I heard of Ambisonics was when I saw a post several years ago in the Reaper DAW forum announcing a suite of Ambisonic plugins downloadable for free. Very interesting toys in that box! I played with them for a while but my studio is set up for 2.0 stereo. I have the gear to go to 6.0 surround, but my clientele and market is geared for 2.0 (or 2.1) stereo, so while it would be a fun exercise it would consume time & effort that would be better spent on making product.
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