[jason]

I recently recorded a 24-voice acapella group performing in a great-
sounding church.

When I make a recording like this, I set up early and record a minute or
two of "the room" before anybody's there. I've used that a bit to
eliminate low-freq rumbles from HVAC and the like, but this recent
performance didn't have any of that. There -was- a low-freq "room sound"
(sorry I cannot characterize that better). It wasn't hvac or ac hum; it
sounded like stepping into a spacious, empty room. I've experienced that
ambient background many places where I've made recordings.

For fun, I used an Audition plugin to convert the X-Y recording to M-S
because I wanted to see what the effect of "more S and less M" would do to
spread out the image. The result was very pleasing if I didn't push it
very hard. But I noticed something I can't explain: the M-S rendition had
much less of "the room" ambience than the X-Y recording. The vocals didn't
sound different except for being spread out some. I actually liked the
fact that silences during the performance were really silent compared to
X-Y.

What accounts for this or am I hearing things? (or not)