At 4:16 AM +0000 6/6/08, werainey wrote:
>Hi Rob,
>Thanks for suggestion, but from the recordings, bat activity at
>least seems highly sensitive
>to temperature (warmer night in a cold interval =3D> a lot more bat
>calls). This is also
>reasonable since insect availability (assessed by light traps) ramps
>up when it is warmer.
>The important issue Klas's observation raised is that low detection
>of bat acoustic activity
>at low temperatures might result in part from lowered microphone
>sensitivity and a
>resulting decrease in detection volume.
Of course, there could be disproportionate response above 20K Hz, but
I'd _guess_ that there's not enough sensitivity drop even at -20 F to
substantially affect your baseline. The air is also denser, and to my
ears, sound transmission is more efficient.
>So how much is biological response and how
>much is instrument effect?
>
>There are a couple of detailed papers from a few decades ago that
>measured attenuation
>with distance for acoustic signals over a wide frequency range at
>biologically relevant
>temperatures and humidities (though not very cold if I recall
>correctly). The results are
>surprisingly complicated in the ultrasound.
Given the attenuation with distance for these small energies, don't
most of the ultrasound sources have to be fairly close to be
recorded? How far away can you record a bat call in a very quiet
location? Rob D.
>
>Bill
> > >Hi Klas,
>> >The idea of response changes with low temperature seems entirely
>> >reasonable. Can you
>> >suggest from tests or speculate on what would happen to sensitivity
>> >and frequency
>> >response?
>>
>> Hi Bill--
>> As you are running long takes, perhaps you could extract and compare
>> some samples from bitter cold and warmer nights a few days apart with
>> the mics in the exact same spot. While you're at it, it would be
>> interesting to ponder potential air density differences! :-) Rob D.
>>
>>
>> --
>>
>
>
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