Saturday, August 31, 2013

Using Sox spectrogram tool to analyze audio noise

While out on a walk near Lagos (Algarve, south Portugal) I noticed a loud buzzing/crackling sound. It sounded very similar to the kind of buzz/crackle you'd hear near high voltage transmission lines, but there was no power lines in sight. So I though I'd capture a few seconds of audio with a voice recorder app on my phone and look at the spectrum later to rule in/out an electrical origin: any thing electrical would have peaks at exactly 50Hz and harmonics of 50Hz.
It turns out that sox has a neat tool to create a spectrogram from any audio file:

sox recording.wav -n spectrogram -o spectrum.png

However the frequencies I was interested in were down well under 1kHz, so I first resampled at a rate twice the highest frequency of interest:

sox recording.wav -r 2k -o t.wav
sox t.wav -n spectrogram -o spectrum.png

So this is the result:

The buzz sound can be seen in the spectrograph as horizontal streaks at about 60Hz and 120Hz. Not being exactly 50Hz rules out an electrical origin. The frequency can also be seen to vary a little with time.

Here is a spectrogram of another recording I took on the way down to the beach later. There was multiple sources in this recording (audio file here) but fainter/further away.

and other (audio file here):

You can see multiple horizontal streaks  of varying frequencies.


It's clear now the noise is from an insect. What insect, I have no idea. (Update: I think it might be Cicada)

If puzzled about the origin of a strange sound, record it and create a spectrogram... it might yield clues. Anything relating to utility power will be at the AC frequency (50Hz most of the world, 60Hz US).


PJ McKenna said...

I noticed what looked like doppler effects at the beginning and end of some of the horizontal streaks so many of the emitters were moving relevant to the mic. Could we also be seeing harmonics of motors stepped down from the mains AC frequency?

Joe Desbonnet said...

I think what ever activity the critter was at needed to ramp up. Pretty sure it was some insect desperately trying to get laid.