This recent talk of peak frequencies and spectrums, etc, has reminded me that I
didn't post a message about how I finally decided to locate all the faint
thumps in my recording.
I have discovered the maximum frequency and FFT window size settings in
Audacity's spectrogram preferences. If I set the maximum frequency to 200Hz and
the window size to 32768, then I can see them very clearly as lines running
from top to bottom on the plots. It's slow, and takes a few seconds to display
each new screen, but being able to see them clearly makes me confident I'm
finding them all. Hundreds of them :(
The only other sounds on the recording that show similar lines are wing beats
as birds flutter near the microphone. If I view just one or two minutes per
screen then I can often tell the difference by the width of the lines and by
the consistency of the colouring, but it's fairly simple to check them all. I
hold the mouse over them and press "1" to play half a second either side of the
mouse position, and label them appropriately.
Playing with these spectrogram settings has revealed detail I never realised
existed in the low frequencies, and I can scan through and spot birds calling
softly that I hadn't noticed before.
Peter Shute
> -----Original Message-----
> From:
> On Behalf Of Avocet
> Sent: Thursday, 8 November 2012 4:39 AM
> To:
> Subject: Re: [Nature Recordists] Background thumps in recording
>
>
>
> > I tried looking at the spectrogram first, but I found the low
> > frequencies a bit hard to interpret on it, unlike bird calls, which
> > make nice patterns.
>
> Peter,
>
> Spectrograms are a bit blurry at low frequencies and you
> don't get many waveforms to work on. The thumps show up
> clearly on the waveform display and you can count the waves
> to get the exact frequency.
>
> > Can you please explain "step function"? Is that like a square wave?
>
> It is a pressure wave which makes a fast high or low change
> and slowly returns to normal as with an explosion or sonic
> boom. Unlike a burst of low frequncy waves, it contains a
> wide range of frequncies. If these trigger a resonance like a
> cavity or resonant windshield, that will then form a "boom"
> sound. Similarly tapping a mic rig with a finger is a "step
> function" and will make any resonances "ring" Same way that
> bells work.
>
> > I'm going to have a play with "fixing" them, but I might
> end up just
> > leaving them in as Keith Smith has suggested. Did you apply your
> > filter to the whole recording, or just to those sections?
>
> My filter was very extreme to see what was happening. The
> main boom frequencies show up on a power spectrum and I would
> try a bass rolloff from about 200Hz. Listen out for sounds
> that you are prepared to lose along with the booms, but I'd
> leave them in at a lower level. Back to an "artistic
> judgement". Trying to filter out just the booms or editing
> them out would sound messy.
>
> Digital noise reduction is possible, but you have to define a "sample"
> of just what you want to remove. Fiddly. It's easier to
> reduce the non-bird frequencies and select the best bits and
> call it artistic judgement. :-)
>
> > Mostly, although I'll have to take your word for things like
> > "stimulated a local resonance".
>
> I should have said "ringing a bell". :-( Was there a slight
> wind blowing which could have been rocking the mic stand? I
> frequently push the idea of trying the mic rig directly on
> the ground to see what it sounds like there.
>
> David
>
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce
>
>
>
>
>
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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