The General Instruction of the Roman Missal (2000, #150) stipulates that a bell may be rung regularly at two places during the Mass: First, “a little before the consecration, the minister may ring a bell as a signal to the people.” Here the bell is normally rung at the time of the epiclesis in the Eucharistic Prayer. At this point, the priest joins his hands and places them over the bread and wine to be consecrated. He prays for the Holy Spirit to come down upon the gifts so that they may become the Body and Blood of our Lord. The ringing of the bell alerts the congregation to the calling down of the Holy Spirit and prepares them for the consecration that immediately follows.
Second, “according to local custom, a minister also rings the bell at the showing of both the Eucharistic Bread and the chalice.” After the priest says the words of consecration, he elevates the Sacred Host or the chalice of Precious Blood. The ringing of the bell again alerts the faithful that transubstantiation has taken place and that the Body and Blood of our Lord is truly present on the altar.
Keep in mind that the rubrics leave to the discretion of the pastor whether a bell should be used or not. Nevertheless, the use of a bell at Mass is a long standing tradition in our Church and no one should ever think that their use has been suppressed. Actually, the common practice in the basilicas of Rome is for the bell to be rung at each elevation and then in a prolonged way when the priest genuflects after the elevation of the chalice.
The tradition itself seems to arise around the thirteenth century, when the common practice became to ring a bell at the consecration. Later, records at the Cathedral of Chartres in 1399 indicated that a large bell suspended above the choir area was rung at the beginning of the Sanctus to call the people to attention for the coming consecration. Moreover, the sound of the bell was seen to give glory to God. (For this same reason, the Church bell may be rung during the Gloria sung at the Easter Vigil Mass.)
Another reason for the ringing of the bell as well as the high elevation of both the Sacred Host and chalice of Precious Blood after the consecration was to affirm that transubstantiation had occurred. In the 1100s, some theologians speculated that the transubstantiation of the bread did not occur until after the words over the chalice had also been pronounced. To counter this notion, the following practice was instituted: After the words of consecration of the bread, the priest elevated the Sacred Host so that it could be seen by all, the bell was rung, and then the priest genuflected after placing the Sacred Host back on the paten on the altar. For example, Cardinal Guido, the papal legate to Germany, promulgated this practice at Cologne in 1201, a practice which was already established in Rome (and throughout much of the Church). To highlight the solemnity of this miraculous moment, an incensation was also allowed at each elevation (as it may be today). Moreover, during the Solemn Mass or High Mass, the great bell of the Church was rung to alert the townsfolk that the consecration had occurred, at which time they would piously kneel and make an act of adoration.
In the old Tridentine Mass, the Missale Romanum, promulgated in 1570 by Pope Pius V, prescribed the ringing of a bell at both the Sanctus and the consecration. When Pope Paul VI issued the revisions to the Mass, the practice of ringing a bell was retained as an option (as stated previously).
Since we are a people of senses, sounds are important in our worship, whether in the quiet of the moment, the singing of a congregation, or the sound of the organ. The sound of bells does add to the reverence and the solemnity of the Mass. Most importantly, they highlight in a sensible way the sacred action taking place on the altar. Therefore, we have a tradition that is not only practical but also beautiful.