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.

--
Rusty Hinge
http://www.users.zetnet.co.uk/hi-fi/