Randolph S. Little wrote:
> It may well be that many of the phenomena that Walter reports for his
> amphibian sounds are produced by the environment, such that both
> his microphone and his ear are receiving the product in question. A
> long shot would be that water vapor saturation in the air nearest the
> sources of the sounds is enough of a non-linearity to produce the
> reported effects. Perhaps a more plausible hypothesis is that multi-
> pathing of the signal is contributing to constructive and destructive
> interference, which would necessarily be wavelength dependent. It
> would be interesting to learn if arboreal amphibs exhibit the same
> apparent effects as terrestrial or aquatic surface amphibs.
The spring peepers that Lang mentioned also exhibit the same thing, they
call from vegetation, though usually not more than head height. Yep,
they do the same sort of thing. They also have a interesting sound when
you get many hundred of them really calling in a excited manner. Very
characteristic. Best described by saying the sound gallops.
Oh, on humidity, this occurs from saturated air (even heavy rain) down
to as dry a air as toads will call, which is pretty dry. I've had them
calling when the humidity was down very low with blowing dust. As far as
air stability and layering, this occurs from type A to type F stability,
the full range. Occurs with or without a thermocline, the mixing cells
can be a foot high or very unstable with no real top.
These sounds are not produced by the environment, that's a separate set
of sounds. You are making a fundamental error in assuming that frogs are
crystal controlled signal generators. They are not, and within a species
there is a range of variation between individuals as to frequency as
well as other parameters. The beat frequencies I'm talking about are in
the order of just a few cycles per second or less up to a few hundred or
more. They occur from the faintest competing calls to the loudest, let's
say from 120 dB or so down to the calls from a group of toads calling at
distances of several hundred yards. Too quiet to pick up with much of
anything other than a parabolic or a shotgun with a lot of gain applied.
In all cases it's normal to find beat frequencies between toad calls.
It's very obvious because their call is essentially a steady frequency
and sustained for a good fraction of a minute.
Even if two toads happen to have the exact same frequency, there is
still interference as they lack any sort of electronic syncronizer to
insure that all toads keep in phase. So are almost always out of phase
by some amount. Obviously we need to fit each of them with syncronizers.
Note I also know of some species of frogs that do sync in phase. It can
sometimes be tricky to figure if a call is one or two individuals with them.
And next time I hear a bunch of toads I'll give them a stern lecture
about keeping in sync. Or maybe I'm not hearing them at all. I spent a
lifetime as a field biologist and failed to learn how to listen.
I'm glad we cleared that up and now know that anything we hear we don't
like is a error and should not be recorded. Our equipment should
immediately be sent in for repair. The frog chorus just needs a better
chorus master to whip them into shape. And I can quit recording toads as
they always do this and we know it's a error of our ears or the recorder
and we won't get a "correct" recording. I'll have to drop some other
species as well, they never call correctly either.
Walt
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