Jamieson, Fausset and Brown on Nehemiah 11:22

22. the sons of Asaph, the singers were over the business of the house of God--They were selected to take charge of providing those things which were required for the interior of the temple and its service, while to others was committed the care of the "outward business of the house of God" (Ne 11:16). This duty was very properly assigned to the sons of Asaph; for, though they were Levites, they did not repair in rotation to Jerusalem, as the other ministers of religion. Being permanent residents, and employed in duties which were comparatively light and easy, they were very competent to undertake this charge.