Spectral Analysis with Audacity


This month’s digital recording tool excursion is into the realm of frequency spectrum analysis. As an organist who took lessons early in life on a Hammond B3, played in rock bands using a chopped M2 with percussion added, and now plays services on a model 906, I can’t avoid trying to guess drawbar settings whenever I listen to someone play a Hammond organ.

With the free digital audio tool “Audacity” (Audacity audio software site) I no longer have to guess at drawbar settings. If I can find a relatively clear section of audio in a recording that sustains a single note, Audacity’s “Plot Frequency” analysis tool can tell me exactly what I need to know.

As an example, here is a screen shot of Audacity displaying a section of Jimmy Smith’s recording of “The Cat” (Jimmy Smith, The Cat):


In this case, we have an “F” in the clear for analysis. By opening the “Plot Spectrum” window under the “Analyze” menu, we can visualize the audio frequency spectrum calculated via an FFT:


In addition to numerous options for presentation (here we use the “Log Frequency” mode), the “Frequency Analysis” window has a tool for locking onto peaks and displaying their frequency, equivalent keyboard note, and volume level. Here are the significant peaks for this analysis plot:

346 Hz (F4) = -13.4 dB
696 Hz (F5) = -18.5 dB
1046 Hz (C6) = -18.3 dB
2077 Hz (C7) = -43.4 dB

Now we turn to a reference page on the Hammond site (Introduction to Drawbars) that gives us the spectral layout of drawbars:


With this, and a reference on frequency vs. keyboard notes (Piano key frequencies), we can lay out a quick spreadsheet that gives us the equivalent drawbar frequencies for any given note:


Since each drawbar has a volume range from 0 to 8, we can use a rough estimation of the relative value for each drawbar position:

0dB 8
-3dB 7
-6dB 6
-9dB 5
-12dB 4

We can now guess that based on the spectrum of this limited section shown above, Jimmy had his drawbars set to something in the range of “87 7020 000”. So if you’re willing to spend a little time in analysis mode, Audacity provides yet another way of learning by listening to the greats.


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