natural slap echo

[Haible Juergen]

	>This sounds similar to the well-known "whispering galleries".
Possibly the
	>radius of the bowl is at the floor, so that upward-going waves are
	>re-focused back to the source. It can't be louder than the source,
of
	>course, but you could get that illusion since you expect echos to
be rather
	>weaker.
	>
	>  Ian

I think the most effective whisper gallery has an elliptic shape. 
In that case, when both the speaker and the listener are ideally
located, the volume should actually be amplified. Why's that ?
Normally the listener will only get a fraction of the acoustic power
(the part that is directed to him), while in the ideal whisper chamber
a great portion of the power that was transmitted in *any* direction
is focussed on the listener.

In my home town, Nuernberg, there was an opening of a new shopping 
center a few months ago. When you go up the escalator to the highest
story you can hear what people are talking on the other escalator that's 
going down, even though it's so far away that you hardly can see their
faces.
It's a striking experience, even when you're familiar with the simple
theory behind it.

JH.