Rich Peet wrote:
> --- In Dan Dugan <> wrote:
>
>>I'd be happy to take a shot at analyzing some samples of those
>
> files.
>
>>-Dan Dugan
>
>
> I am having trouble even figuring out where to begin in analyzing
> infra-sound. Such as the UFO post, thunder post, or even a simple
> bird such as my 200kb Grouse recording at
> http://home.comcast.net/~richpeet/0298.wav
>
> The spectral displays I have made with Cooledit seem to fall apart in
> determination of very low frequency.
>
> I also don't understand how waves work at low freqs. ie question 1.
>
> 1. If a wave length of 1/2 cycle audio is 2,260 feet long, can a wave
> at the frequency be detected in a short burst such as this grouse?
>
> 2. How does one determine how low this bird goes?
>
> 3. How does one determine how low my recorder goes?
>
> Rich Peet
Use as high a FFT blocksize as you can set on sonograms. It's still very
poor resolution at low frequencies. Something is there, but no good
detail. Spark XL, which I use has a max FFT size of 4096, and that can
resolve fairly well down to 100hz, with a blurry picture that get's it
down to 20hz, the lowest frequency it will display. There are other
sonogram programs that can do larger sizes, but the processing gets more
and more intensive as you go up.
A 1/2 cycle audio will take two seconds to pass a given point. For one
complete wave cycle. At this level looking at the waveform rather than
the sonogram may give a better visual image. You can see such slow
cycles pretty easy. And measuring the time from wavetop to wavetop off
that display can give you frequency.
You are equipment limited as far as determining how low the bird goes.
Or may be equipment limited. You've got mics, but anything else in the
chain is not up to what a MKH-110 can dish out. I'd be really surprised
if it's going anywere near as low as 0.1hz.
In the case of the Portadisc, HHb provided that statistic on their specs
list. Just how it was determined and what it means is a unknown. I've
seen frequencies in the waveform that looked lower, but probably exist
only as modulation of a higher frequency rather than directly recorded.
BTW, one does not hear sounds this low, but one can feel them. Not just
the ones you are aware of either. Sometimes it's just a emotional effect
that's hard to track down. Of course if it's modulating a higher
frequency sound it's much easier to hear. Such as the frequency you can
get in a car on the highway with it's windows down.
Walt
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