Why I get negative values on using periodogram after Fourier analysis?

Question

sorry for bothering you with this question today. I am trying to analyze wave data that was produced using a wave tank. The period used for the waves is 1.7s, the waves encountered an obstacle at some point and then reflections are expected.

I plotted one of the sensors that are a wave gauge system. And I obtained the next plot:

The x is the time in seconds and the y the wave height in mm. After this I use the code:

ListLinePlot[Abs[Fourier[datcy]], PlotRange -> All]

And then I get the Fourier transform and I also plot just the data:

I know the plot is reflected and that is why I have a double one, but what I don't get in wolfram or maybe on signal analysis is how I get a power spectrum that is negative after this:

Periodogram[datcy[[All, 2]], Frame -> True, 

GridLinesStyle -> Directive[Red, Dashed], PlotRange -> All]

It's on DB?, if that is true then technically there are no negative values and I should interpret as a -150 DB?. I am a bit lost here. Also, have a bit of doubt about why the amplitude of the Fourier plot doest not relate to the amplitude of my wave?.

Answer 1

The periodogram plots the magnitude of the Fourier transform. The default magnitude scaling is decibels, defined in this function as $10\cdot\rm{log}_{10}|y|^2$. We can clearly see that your Fourier transform is less than 1 for a big part of the x-axis. A number that is less than 1 has a logarithm that is negative, hence the negative values.