Error with colorbar in specgram plot.

Hello everybody,
this is my first post in this list.

I'm plotting a spectrogram with

Pxx, freqs, bins, im = specgram(y, nfft=256, f_sampling=12000)

and i want to add a colorbar with


The problem is that the color scale seems to be wrong with respect to
the data in Pxx, i.e. Pxx is of the order of 1e-2 while in the colorbar
i have tick values spanning from -20 to -180. What is the problem??

Thanks in advance!