def tonal_adjustment(tonal_audibility):
    """Adjustment :math:`K`.

    :param tonal_audibility: Tonal audibility :math:`L_{ta}`.

    See equations C.4, C.5 and C.6 in section C.2.4.
    """
    if tonal_audibility > 10.0:
        return 6.0
    elif tonal_audibility < 4.0:
        return 0.0
    else:
        return tonal_audibility - 4.0
