Moreover four thousand were porters; and four thousand praised the Lord with the instruments which I made, said David, to praise therewith. (1 Chronicles 23:5)
Not only did David sing and play upon musical instruments, but he seems also to have been an inventor of such instruments. Later in the history of Israel mention is made of those who invented “to themselves instruments of musick, like David” (Amos 6:5). After the Exile such instruments invented by David were apparently still in use (Neh. 12:36).
Porters.
That is, doorkeepers. They did not all serve at one time, but took their turns.