why do my spectrogram values are negative and how to read them?

I have plotted three spectrogram of audio signal "hello" in FFT and FFT written without using inbuilt fft and HDFT(hopping DFT) and values of spectrogram are in negative values. How to read them and why they are in negative values?

Answer 1

[Paul]

The power levels are in dB. Any positive number less than 1 is negative when converted to dB

pow2db(0.5)

ans = -3.0103

See pow2db.