def mel_to_hertz(mel):
    """Returns frequency from mel-frequency input.
    Parameter
    ---------
    mel : scalar or ndarray
        Mel-frequency value or ndarray in Mel
    Returns
    -------
    freq : scalar or ndarray
        Frequency value or array in Hz.
    """
    return 250.0 * 9 ** (mel / 3340.0) - 250.0
