Thanks all!
I believe you're hearing HVAC rumble that I could not isolate. We recorded in a chapel music practice room at Goucher College near Baltimore; it was an acoustically sensitive, but not a true studio space. Next time I'll cut the ventilation system to the building...
This was recorded live with fifteen voices, no audio baffles. It has a very slight touch of iZotope reverb on the final mix. Most of the cuts on this album (there will be nine all told) have a subtle natural room reverb and delay from mic placement.
The singers formed a circle around a mic tree with an SP C3 set as omni, suspended from a large choir boom in the center, two cardioid LDCs underneath and two PZM mics providing four compass points of source. I recorded from one to five tracks; this involved three with the OH and the LDCs.
Signal was run through subtractive eq; one track had predominantly lower voices and I applied a filter and so forth with each track, intending to accentuate its strongest frequencies. Each track then ran through a light compressor before mixing. Everything had a low cut from 100 hz down because of the HVAC rumble.
I was also able to pick out a band of rumble using Adobe Audition's spectral toys and pulled those frequencies down. I've learned to keep a long header on these tunes and can get a room noise signature from that, and so using that signature ran the tune through consecutive light passes of AA's noise reduction. Final mix was then compressed, reverb added and I ran it all through a brick wall limiter.
