NatureRecordists:
In the following quote, I have capitalized the word ERROR because that
is the point that I wish to call into further consideration and discussion.
Walter Knapp wrote:
> There is always Occam's Razor. I say that proposing a system requiring
> two or more simultaneous ERRORS to have exactly the same result is less
> likely to be correct than that there is no ERROR. The simple explanation
> there is that it's as we hear it or record it.
>
> I'm well aware that recording equipment is not perfect. In this case,
> what I hear in the wild, what I hear in the recording are the same, and
> agree with the sonogram. If you have a method where such detailed
> perfection of multiple ERROR is proven, I might believe it. But it's
> certainly a far more complex hypothesis. Not just one ERROR, but
multiples.
>
> On top of that, most all the proposed ERRORS require that the recorder
> or some other part of the system be driven with a high signal level. But
> this records at a low signal level as well, where those ERRORS would not
> occur. And the hearing ERRORS that occur at loud calling also don't
> apply as we can hear this at low levels too.
>
> And on top of that, the "ERROR" turns on and off with the 2nd toad. The
> exact instant one toad gets it to himself, the sound is smooth and clear
> (at least for American Toads, which have a pretty musical trill). Keep
> in mind that the 2nd toad's call may be distant and weak, while the
> first toad is right at you and very strong. So these ERRORS come and go
> with the calls, does the toad throw a ERROR switch?
>
Now, whereas Occam's Razor refers to random exceptions, modulation
products are not random in the least - they are entirely predictable and
unavoidable consequences of any nonlineraity. Furthermore, they are
not necessarily amplitude dependent. We are perhaps most familiar
with modulation products which we call distortion, i.e., products of un-
intended nonlinearities, usually in amplifiers and usually exacerbated at
higher signal amplitudes than our amplifiers were designed to handle
with acceptible linearity.
Both our ears and our amplifiers are quite likely to be less linear at
both extremes of amplitude - starting to "clip" at high amplitudes and
failing to follow subtlties at low amplitudes, or starting to lose the
signal
in the noise.
When a modulation product happens to fall within our most acute range
of hearing we are more likely to detect it, especially if its frequency is
widely removed from other elements of the signal. Thus, we tend to
notice differences between two very high frequency elements more
readily perhaps than those elements themselves. (This is why even the
slightest aliasing can be noticed, but that is distortion of another sort.)
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.
Good recording,
Randy
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