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I am analyzing spectrum of recorded sound. For that purpose I am using Mathematica's built in functions Spectrogram[] and Periodogram[]. I have three questions regarding those:

  1. What is the way to set desired range on frequency axis (y-axis) in Spectrogram[]'s output, so that it "zooms in" to frequencies at specific interval? I managed to accomplish this with Show[], but it removes my x axis ticks.

  2. How to change tick positions on frequency axis (so for example that I have them every 200 Hz)?

enter image description here

  1. Regarding the Periodogram[] function, what y axis represents (what are the units)?

enter image description here

It seems to me like something in decibels, I looked in MMA's documentation, searched on internet, and I couldn't find answer. Documentation says:

plots the squared magnitude of the discrete Fourier transform (power spectrum) of list

And I understand that the periodogram tells me which frequency is most present. But what are the units?

Thank you for any helpful answers, comments or advices...

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You can zoom in; just use PlotRange ( I have combined the tick specification and zooming-in in one plot)

Spectrogram[data, 64, 2, BlackmanHarrisWindow, ImageSize -> 500,  ImageMargins -> 10,
     SampleRate -> 483, PlotRange -> All]

Mathematica graphics

ticksY = Reverse@Transpose[{30 #1, 10 #1}]& @ Range@7
ticksX = Transpose[{.25 #1, .25 #1}]& @ Range@4
Spectrogram[data, 64, 2, BlackmanHarrisWindow, ImageSize -> 500, ImageMargins -> 10,
     SampleRate -> 483, PlotRange -> {All, {10, 100}}, FrameTicks -> {ticksY, ticksX}]

Mathematica graphics

As for the Periodogram, read under Properties & Relations. The example is the following

data = Table[2 Sin[0.2 π n] + Sin[0.5 π n] + RandomReal[{-1, 1}], {n, 0, 127}];
{Periodogram[data], ListLinePlot[Take[20 Log[10, Abs[Fourier[data]]], 64]]}

Mathematica graphics

Now, there is a relationship that states $10\; log_{10}(\frac{v^{2}}{w^{2}}) = 20\; log_{10}(\frac{v}{w})$ Great; Let's put that to the test !

{Periodogram[data], ListLinePlot[Take[10 Log[10, Abs[Fourier[data]]^2], 64]]}]

Mathematica graphics

So, they are the same thing and the units are dB

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  • $\begingroup$ I will leave to you the task of reversing the ticks in the Spectrogram plot $\endgroup$ – Sektor Mar 16 '15 at 23:17
  • $\begingroup$ Thank you for your answer, it helped me a lot. Just one thing is left unsolved. The way you proposed for generating ticks works great, but for some reason they are not drawn correctly to the spectrogram. If you check on your graph from top to bottom it is 50, 60, 70 instead 70, 60, 50. Reversing the ticks list does not solve the issue. And this gives wrong values of frequencies. What I understood that happens is that frame is showing ticks relative to top, while spectrogram frequencies are drawn relative to bottom. Any ideas on how to solve this? $\endgroup$ – balboa Mar 17 '15 at 19:46
  • $\begingroup$ And now I understand your previous comment :) But it's not as easy as it seems... $\endgroup$ – balboa Mar 17 '15 at 19:47

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