The Spectral Analysis of Sound


Doppler shifted sound of the turbocharger on a passing truck


Most of my working life I have been involved with analyzing sound and vibration problems using very expensive, high precision microphones, signal conditioning amplifiers, and spectrum analyzers. I never got bored with it or tired of looking at those spectra in twenty nine years. There was always the prospect of seeing something in them that I had never seen before; they never let me down.

Last summer I was visiting a colleague at Michigan State University in association with my present job at Ohio State University.  During one of the presentations on environmental sound one of the spectrograms showed a beautiful s-shaped feature that immediately caught my attention.  No one seemed to know what it was.  He had the sound embedded in the Powerpoint so I asked him to play it.  It was a diesel truck driving past on the highway.  Ahh!  It was the signature of a doppler shift.  I have found out since that it is probably the doppler shifted blade passage tone from the turbocharger as the frequency of the tone is quite high.

I found a free program called Raven Lite from the Cornell University Ornithological Laboratory that allows anyone to record sounds and then display their waveform and spectrogram.  This is not on par with the analysis equipment that I used in my former life but the anticipation of what new things I might see in the next spectrogram is just as strong.

I have added a couple of new pages devoted to examples of some projects you can do with this software.  The doppler shift analysis is still in the works; it needs a diagram or two and has a lot more equations to enter before I can publish it.  I also am still looking for a signature that is as beautiful as the one I saw in Michigan but I will publish with the one I have if I don’t find one before everything else is ready.

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