Commentary on Psalm 33 [Vulg. 32], vv. 2–3
Then when he [the Psalmist] says: “Give praise to the Lord [on the harp; sing to him with the psaltery, the instrument of ten strings]” (Ps 32:2), he states the manner of praise and delight [i.e., the manner in which they are to be taken]. Now, in the praise of God what is chiefly intended is that man’s affection [or feelings] should reach to God and be directed. Again, musical harmonies change man’s affective disposition. Whence seeing that a young man was driven mad by the Phrygian sound, Pythagoras changed the mode [of the music], and thus rendered most tranquil the spirit of the raging youth, as Boethius says in the preface of his work On Music. Thus in all divine worship it is contrived that certain musical harmonies are employed to lift the spirit of man to God.
Yet such harmonies have generally been used in two ways: sometimes with musical instruments and sometimes in songs. First the Psalmist shows the first use: “Give praise to the Lord on the harp…” (Ps 32:2), then the second: “Sing to him…” (Ps 32:3). For man’s affection is directed through instruments and musical harmonies in three ways: sometimes it is established in a kind of rectitude and strength of soul; at other times it is carried off to the heights [rapitur in celsitudinem]; at still other times [his affection is established] in an agreeable and pleasant condition. Concerning this matter, three types of song have been established (as the Philosopher has it in Politics VIII.7): for the first, song in the Dorian, out of the first and second tone [toni], as some have it; for the second, song in the Phrygian, which is of the third tone; for the third, song in the Hypolydian, of the fifth tone and the sixth. Others were discovered later.
This division bears on instruments, as some, such as flute and trumpet, are suited to the first [mode], others, such as the organ, to the second, and others still to the third, for example the psaltery and harp: “Bring hither the . . . pleasant psaltery with the harp” (Ps 80:3). Since at this point in the Psalm the Psalmist intends to lead us to exultation, he mentions only the psaltery and harp. Yet as “all these things happened to them in figure” (1 Cor 10:11), these instruments are used not only for the aforesaid purpose, but also figuratively. The harp has a deep sound, and signifies praise which rises from the deepest places, that is, from the earth, while the psaltery [or ten-stringed lyre] has a higher sound, and signifies praise concerning heavenly goods. He adds “the instrument of ten strings” because through these are signified the ten commandments of the Decalogue, in which the totality of spiritual doctrine consists.
Then when he says “Sing [to him a new song, sing well unto him with a loud noise]” (Ps 32:3), he treats of the song of a human voice. Yet, speaking to the biblical text [secundum litteram], there are two types of singing: by simple song [i.e., a cappella] and with instrumental accompaniment. He refers to the first when he says “new song”; to the second, “with a loud noise.” Understood according to the spiritual sense, man ought to exult over two things: the good things of grace already received and the good things of glory awaited. By the first good things we are made new: “Be renewed in spirit of your mind” (Eph 4:23); “As Christ is risen from the dead by the glory of the Father, so we also may walk in newness of life” (Rom 6:4). Thus, he sings a new song who exults in God’s making us new by grace: “The saints sang a new song” (Rev 14:3).
While he “sings well with a loud noise” who sings of the good things of glory, and the song that man conceives in his heart he expresses in words—or in jubilation or jubilant melody [jubilo], according to Jerome. Such a jubilant melody is an ineffable gladness that words cannot express, but by the voice is given to be understood an immense breadth of joys. Now what cannot be expressed are the good things of glory: “No eye has seen, nor ear heard, [nor the heart of man conceived, what God has prepared for those who love him]” (1 Cor 2:9). And therefore the Psalmist says “sing well unto him” with jubilation because they cannot be expressed with [ordinary] song.
But you may object that in the Old Testament there were musical instruments as well as songs with words. Why then does the Church relinquish the instruments, though she takes up the songs? Two reasons exist on the mystical side: first, those instruments were figurative; the second reason is that God should be praised with mind and voice, not with instruments. Another reason is had in the words of the Philosopher, who says it is against wisdom that men be instructed in lyric poetry and musical technique, because these [have a tendency to] preoccupy the soul in its activity. But music ought to be simple, that it may draw us away from bodily concerns, yielding us up to praises of divinity.
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Jubilus [rendered above as “jubilation or jubilant melody”] is the name given in Latin antiquity to a joyful work song without text. First applied to melismatic Christian chant by Amalarius of Metz (9th century), it has customarily been defined narrowly by modern scholars as the long melisma on the final syllable of the refrain ‘alleluia’ in the alleluia chant. These melismas are often much freer in melody than the rest of the chant and have their own internal forms based on various patterns of repetition. (definition of jubilus courtesy of http://arts.jrank.org/)