def normalize_db_spectrogram(db_spec, high=20.0, low=-100.0):
    """
    Normalizes db spectrum with zero mean and unit variance.
    In normal cases, librosa generates db spectrum having -100dB as maximum.

    Args:
        db_spec: db spectrum
        high (float): maximum decibel value
        low (float): minimum decibel value

    Returns:
        normalized db spectrum
    """
    mid = (low + high) / 2.0
    scale = high - mid
    return (db_spec - mid) / scale
