def phonetic_i_umlaut(sound: Vowel) -> Vowel:
        """
        >>> umlaut_a = OldNorsePhonology.phonetic_i_umlaut(a)
        >>> umlaut_a.ipar
        'ɛ'

        >>> umlaut_au = OldNorsePhonology.phonetic_i_umlaut(DIPHTHONGS_IPA_class["au"])
        >>> umlaut_au.ipar
        'ɐy'

        :param sound:
        :return:
        """
        if sound.is_equal(a):
            return ee
        elif sound.is_equal(a.lengthen()):
            return ee.lengthen()
        elif sound.is_equal(o):
            return oee
        elif sound.is_equal(o.lengthen()):
            return oee.lengthen()
        elif sound.is_equal(u):
            return y
        elif sound.is_equal(u.lengthen()):
            return y.lengthen()
        if sound.is_equal(DIPHTHONGS_IPA_class["au"]):
            return DIPHTHONGS_IPA_class["ey"]