Sing unto him a new song. As the Psalmist afterwards treats of the mighty works of God, and particularly concerning the preservation of the Church, it is not wonderful that he exhorts the righteous to sing a new, that is, a rare and choice song. The more closely and diligently that believers consider the works of God, the more will they exert themselves in his praises. It is no common song, therefore, which he exhorts them to sing, but a song corresponding to the magnificence of the subject. This is also the meaning of the second clause, in which he urges them to sing loudly. In this sense, I understand the Hebrew word hytyv, heytib, although others refer it rather to the proper setting of the notes.

Sing unto him a new song - A song specially composed for this occasion; expressive of the special feelings suggested by this occasion, or appropriate to this new manifestation of the divine goodness and mercy. Such occasions, exhibiting some new phase of the divine goodness, demanded new language appropriate to them. So now, new hymns of praise, and new tunes in music, are demanded to meet the ever-varying manifestations of the mercy of God; and as the church is extended in the world, its modes of praise must be adapted to the new state of things which will arise. Nothing could be more absurd than to attempt to restrict the church in its praises to the exact words which were used in the time of David, or to the music which was employed then. Compare the notes at Revelation 5:9. The expression "new song" occurs several times in the Psalm, showing that new hymns of praise were composed as adapted to some new manifestation of the goodness of God: Psalm 40:3; Psalm 96:1; Psalm 98:1; Psalm 144:9; Psalm 149:1. Compare also Isaiah 42:10.
Play skillfully with a loud noise - literally, "Do well to play;" or, "do well in playing." That is, do the work well, or with all the skill of music. The word rendered "loud noise," means properly "a shout of joy" or "rejoicing:" Job 8:21; 1-Samuel 4:5. It is especially applied to the sound or clangor of trumpets: Leviticus 25:9; Leviticus 23:24; Numbers 29:1. There is rather the idea of "rejoicing" than of "noise" in the word. The meaning is that the music should be such as would be expressive of the highest joy.

Sing unto him a new song - Do not wear out the old forms: fresh mercies call for new songs of praise and gratitude.
Play skilfully with a loud noise - Let sense and sound accompany each other; let the style of the music be suited to the words. This skill is possessed by few singers. They can make a loud noise, but they cannot adapt sound to sense.

Sing unto him a new song,.... One newly composed on account of recent mercies received; and as the mercies of God are new every morning, there ought to be a daily song of praise to him; and so a new song is a continual song, as Christ is called the "new and living way", Hebrews 10:20; because he is the everliving way; or the constant and only one, which always was, is, and will be. Or it may denote some famous and excellent song, as a new name is an excellent name, an unknown and unspeakable one; see Revelation 2:17; compared with Revelation 14:2; or respect may be had to the New Testament dispensation, in which old things are passed away, and all things become new; a new covenant is exhibited, a new and living way opened, and new ordinances instituted, and at the end of it there will be new heavens and a new earth; and so here is a new song made mention of, as suited to it;
play skilfully with a loud voice: either with the quill upon the harp, and the instrument of ten strings; or with the fingers upon the psaltery, at the same time, vocally, and aloud, expressing the new song.

a new song--fresh, adapted to the occasion (Psalm 40:3; Psalm 96:1).
play skilfully--(Compare 1-Samuel 16:17).

A new song - Renewed or continued from day to day.