FFT and decibel

If I take the audio data on the iPhone (i.e. real) data, I perform the FFT, and then I take the values ​​(Re ^ 2 + Im ^ 2).

They vary from> 0 to some large numbers, so I do 10log(n)to get it in dB.

This gives me outputs that are negative (for inputs that were <1) positive.

But the examples I saw for this (and also for drawing the spectrum in Sonic Visualiser) always have a positive spectrum when measured in dB.

So what did I miss ?!

In a broader sense, since I understand that decibels are a ratio, so in this context, when you turn the FFT values ​​into dB, what are their relationships to?