Reference the acoustic treatment of Riverside Church (and incidentally,
St. Thomas Church NYC a few years ago).
 
The Gustavino tiles (which are also found in most NYC subway stations)
were a cheap and dirty way to plug up ceilings.  They also absorbed
noise.
 
At Riverside, the Gustavino tiles were not really the big problem -- it
was the porous stone used in the walls.  The vaulting was Gustavino,
however.  All were coated with several layers of sealant.
 
St. Barthomew's has a similar stone inside, but the dome is not
Gustavino tile.  Nonetheless, there are thoughts about coating the
whole works in a sealant at some future point.  It remains a fairly
dead church, reverberation-wise.
 
More reports today from those attending Fred Swann's recital at
Riverside concur:  WHAT A DIFFERENCE!  Of course, we're blessed in NYC
by having churches which never had -- or probably never will have --
carpeting.  The AGOers are amazed at the good-to-great acoustics in our
un-air conditioned churches.
 
Steve Lawson in NYC