"Scott Dorsey" wrote...
jschoep wrote:
I just want to add some bad experiences I had to make with modern
African rondavels (cement walls about 1.5m high and grass roofs)...
really awful reflections...

It's the cement walls that do it. And because the place is round, ALL
of the room modes are on the same frequencies. And all of the high
frequency reflections all go right into the center of the room. This
is pretty much the worst possible case for acoustics.

You could have parallel concrete floor and ceiling! :-)