Why my spectrogram have negative values?

In this figure we have a exponential sine wave and his spectrogram. My question is why the spectrogram have negative values? (from -50 to -150)

Answer 1

[Star Strider]

Assuming he spectrogram function plots the power spectral density (PSD) in decibels. The values are relative, not negative, amplitudes, so -150 dB corresponds to an amplitude of about 3.2E-8.

Answer 2

[Kaiwen Zhao]

It's Power/frequency(dB/Hz), using log.