> What arer the advantages of an array? Wouldn't the beam width be extremel=
y narrow?
Greg,
II was wondering that as well. I have experimented with a parabolic
reflector for bat detectors. It gave directionality but the gain was not
helpful as aiming the rig was difficult because the bats were too mobile to=
follow.
There was an increase in gain, but in practice the absorption of ultrasound=
in air is the dominant effect on range. The limit for bat detectors with
Lesser Horseshoe bats is about 5 to 8 metres after which an increase in
sensitivity is not much help. I once estimated that the sound pressure
levels in a bat's vocal tract is near atmospheric pressure so their calls
are near the absolute limit.
I have an Anabat detector with a mic diameter of about 2 cm which is
strongly directional at the higher bat frequencies.
The array spacing appears about the same order as the wavelength at 40KHz.=
This suggests that the centre lobe over 8 units is only a few degrees wide=
with a series of surrounding lobes.
It would be interesting to know how the increased sensitivity of the array=
responded to the thermal noise of the air. This rises with frequency and
sets a limit to useful ultrasonic mic sensitivity.
David Brinicombe
|