A spectrogram is a visual representation of the spectrum of frequencies as they change with respect of another variable (in our case time). Matlab offers a great function to plot the frequencies against time, therefore I decided to try it on the recordings I used in the previous post. Here is the result I got

Now, some comments. This representation of the frequencies tells us that the stronger components of our signal are below 1.5 KHz (1500 Hz) which is what we expected, however it looks like there are no other significant frequencies above 7 KHz and this is weird since in our FT of the signal we found higher frequencies (although with lower magnitude). Notice how the the higher the magnitude in the FT, the longer the frequency appear in the spectrogram.

Now, by using a simple tool I generated another spectrogram which seems to contain also the higher frequencies the one in Matlab did not have.

This spectrogram is better in tune with our FT of the signal, since it shows that our signal has frequencies up to 15 KHz as the FT showed

The program I used to generate the second spectrogram is Spek, it is free, and fast, however it has very few options. Below are some useful links

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