Back in the 80's I heard about tectum... the second t was silent, or I didn't
quite get it.
Anyway, the thing I remember is that it reflected higher frequencies and
dampend the lower ones, (or maybe mids & lower mids).
At least that's what I was told, never saw any specs on it.
That could be one advantage tectum would have in balancing the
frequency responce of your room ambience.