pers
Randolph S. Little wrote:
> NatureRecordists:
>
> Hope I'm not the only one who is finding this thread to be quite producti=
ve;
> if so, please advise and I'll not pursue it further on this forum.
>
> Walter Knapp's latest reply finally sparked what I think is a highly like=
ly
> hypothesis. Whereas, with a single sound source something like multi-
> pathing can create the reported effects (e.g., ghosts on a TV image), if
> there are multiple sources at similar frequencies then the effects of
> constructive/destructive summation are inevitable (e.g., beats between
> engines on any multi-engine aircraft). No non-linearity is required. Th=
is
> is a simple consequence of convergence of multiple acoustic waves,
> and affects the instantaneous sound pressure level at the microphone
> just as at our ears. And the effect is not amplitude dependent in the
> sense that overload distortions are.
>
> This multi-source interference is not often encountered in any birds that
> I have typically recorded, but it is certainly a fact of acoustic life in
> many
> other situations such as vocalizing choruses of insects and amphibs.
>
> Mea culpa; I should have thought of this hypothesis much sooner.
I was thinking about this last night. Frog recordists are very likely to
encounter this frequently as frogs are almost always group callers. For
us it's so routine we generally don't even think about it. You really
notice it when you are sifting your recordings for clean calls for a ID
clip for the species. As I happen to be doing right now.
But bird recordists mostly record single individuals calling. This would
not occur there, at least not this particular thing. And on top of that,
not many birdcalls are long drawn out things of a single frequency where
this is easiest to hear.
It does occur in insects in my experience where you have enough of the
same species calling simultaneously.
With the frogs, particularly the toads, I'm speculating it's not just a
accidental thing, but a contrived arms race in the battle to get the
female. I've found that frogs are highly aware of their acoustic
environment. Their calling locations and how they call give every
evidence of careful choice to maximize their chances. It's probably
mostly hard wired into them rather than thinking like we might do. With
most species the optimum acoustic locations are fought over. Which is a
whole different set of calls as that's got a vocal component too.
I've spent many nights listening to and observing gray treefrogs
jockying for optimum location around the rim of a half cask water garden
they like on our front patio. Some locations along the rim the call
carries farther, and those spots are fought over all through the evening.
Walt
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