About Mel spectrogram normalization

[Jesper-Jung]

Dear soobinseo,
Thanks for your wonderful work.
I have a one question about your mel-spectrogram normalization method, after I look carefully at your work.

From util.py,

# normalize
mel = np.clip((mel - hp.ref_db + hp.max_db) / hp.max_db, 1e-8, 1)
mag = np.clip((mag - hp.ref_db + hp.max_db) / hp.max_db, 1e-8, 1)

Could you explain about a reference dB(ref_db)? Is meaning of that related to reference value of sound pressure level?