> 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
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