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

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?.

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.