def mel2hz(mel, formula="htk"):
    """
    Convert the mel-scale representation of a signal into Hz

    Parameters
    ----------
    mel : :py:class:`ndarray <numpy.ndarray>` of shape `(N, \*)`
        An array of mel frequencies to convert
    formula : {"htk", "slaney"}
        The Mel formula to use. "htk" uses the formula used by the Hidden
        Markov Model Toolkit, and described in O'Shaughnessy (1987). "slaney"
        uses the formula used in the MATLAB auditory toolbox (Slaney, 1998).
        Default is 'htk'

    Returns
    -------
    hz : :py:class:`ndarray <numpy.ndarray>` of shape `(N, \*)`
        The frequencies of the items in `mel`, in Hz
    """
    fstr = "formula must be either 'htk' or 'slaney' but got '{}'"
    assert formula in ["htk", "slaney"], fstr.format(formula)
    if formula == "htk":
        return 700 * (10 ** (mel / 2595) - 1)
    raise NotImplementedError("slaney")