Thanks Baheera,

I stand corrected. <img src="images/graemlins/smile.gif" alt="" /> That's why I keep on reading these posts, you never stop learning here.

After posting something indeed did cross my mind: 'primitive' tribes in Africa used to pass messages through tam tam's (not sure what they're called in English). Some sort of drum (like a small conga?) they place on the ground with legs like a stool that conduct the beats into the soil. And yes, the didgeridoo carries far too.

It makes sense. The problem is - I think - to construct a compact device that produces a low frequency noise with enough power to be usefull. That's probably why high-toned whistles are still so widespread. They're compact and loud.

I once blow a Storm whistle in a shop, which was incredibly loud, but not as high (nor as compact) as a referee's whistle.

Luca