Thanks to Raimund and Dan for the graphs and illuminating discussion of
anti-aliasing and frequency response of the SD722 at high sampling rates.
> DAN DUGAN:
> A couple of months ago I recorded Mexican free-tailed bats, and they
> had a lot of chatter audible and visible at the top end of the audio
> spectrum in my 183s/MD recording.
Freetails are the most common bat in much of California (not including wet
coastal forest understory) and typical open air search phase calls are around
19-23 kHz. A few people can hear them and track them through the sky.
Their in-roost social calls extend well down into the audible. Several bat
rehabilitators in Texas closely observing small colonies of long term
captives learned the social context of particular audible calls. They started
working with a university researcher and his students are now working on
freetail 'language'.
Various sites have discussions about this and some downconverted audio clips:
http://www.utexas.edu/features/archive/2004/bats.html
http://www.batcon.org/batsmag/v18n2-2.html
Raimund has freetail in-flight samples on his site.
> DAN DUGAN:
> I examined the top ends of my three night sessions (the sunrise
> sessions had rain noise.) In one, made at the entrance (riparian
> foliage), I see a very faint series of clicks centered around 14 KHz,
> repeating two or three per second, in strings a few seconds long with
> a break and then on again, rather consistently. The hum of
> refrigeration compressors from the cafe was quite audible there, so
> it could have been something from that. Nothing in the Ben Johnson
> Trail silence (conifer forest), and nothing on the Dipsea Trail
> (riparian).
14 kHz is an unlikely frequency for a bat at Muir Woods. There are clicking
insects in that range, but, as you suggest, something like a refrigeration
non-return valve is also possible. There are two California bat species
(typically roosting in rocky areas) that echolocate largely in the audible --
down to 4 kHz for one. As far as we know, they don't occur in the Bay Area.
Bill R.
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