# Linearizing "Plot Spectrum" Fourier Transform

Hello, I am doing research on the harmonics present in different instruments and I want a way to quantify the relative amplitude of each frequency in the different harmonics of one note and compare them on several musical instruments. I am hoping to insert each of the harmonics into a spreadsheet and create ratios for each peak in order to see how loud they are relative to the fundamental, so I am not able to use the decibel scale. I have decided to use audacity, but am having trouble figuring out how the dB scale is created for the “Plot spectrum” function under “Analyze”. Is there a constant value to which the dB are scaled, i.e. what is the ‘zero’ value? Thank you!

Unless it got changed, I think the values up the left change to keep maximum display for the loudest spike. So yes, it’s a little rough to directly compare multiple values. Further, if you’ve been working hard to minimize your studio noise and nothing seems to be happening, you should keep strict note on where the left-hand scale is. If you were successful, the values will be getting quieter and quieter, even if the purple graph itself doesn’t change much.

Someone ran into the same problem and I’m trying to think how they did it. I think they intentionally created a loud calibration tone that more or less forced the left-hand values to be stable. It has to be the loudest thing in the measurement.

Koz

Voxengo has a good free spectrogram plugin which works in Audacity***** : it’s got peak-hold …

[ ***** 32 bit version of the plugin works in Audacity, not the “VST3” version ]

NB: the ratios of the various harmonics are not necessarily constant during one note played on an instrument :
some harmonics die-off quickly, some have amplitudes which pulsate, (tremolo).

The values are normalized such that a 0 dB sine wave will measure as 0 dB.

If I recall correctly, the drawn “Spectrum graph” uses cubic interpolation to connect the data points. The raw data is available by exporting from Plot Spectrum as a text file: Plot Spectrum - Audacity Manual