"Jaques d'Alltrades" wrote in message
...
The message
from "Franz Heymann" contains these
words:

They are only partially diurnal.
In Surrey, there was a group which frequently woke us at 2.00 in the
night.
I once used a large parabolic sound mirror to follow and record them.
If
anybody had been around at that time of the night, they would have had
me
certified, running.
around in the midle of the night in pyjamas with clumsy weird-looking
electronic equipment.

/snip/

I use a reflex cone loudspeaker trumpet with a microphone set where the
driver would have been. It's extremely directional and concentrates
sound marvellously. Using an amplifier you can hear clearly a whispered
conversation at a hundred yards in still air.

For equal diameters, the parabolic reflector and the exponential cone would
have just about the same directionality and sensitivity for sounds with
wavelengths small compared to the diameter of the device. Whispers and most
birdsong fulfil this criterion.

Franz