THE ACHIEVEMENT OF EXTREMELY LOW AIR-CONDITIONING NOISE LEVELS REQUIRED FOR MODERN AUDITORIUMS ARE THE RESULT OF CAREFUL PLANNING AND THOROUGH DETAILING. PROBLEMS FACED AND TECHNIQUES USED IN ARRIVING AT LEVELS AS LOW AS NC-15 FOR A SINGLE SYSTEM SERVING A HALL ARE DESCRIBED. SIX CASE HISTORIES ARE EXAMINED AND THE FOLLOWING OBSERVATIONS ARE MADE--(1) LEVELS BELOW AN NC-15 CAN BE ACHIEVED WHEN DIFFUSERS ARE OMITTED, WHEN VELOCITIES OF ABOUT 400 FT/MIN ARE USED IN DISTRIBUTION DUCTS, (2) THE NC-15-20 CAN BE ACHIEVED BY USING DIFFUSERS, ESPECIALLY IF NECK GEOMETRY IS GOOD AND IF NECK VELOCITIES DO NOT EXCEED 450 FT/MIN, SUGGESTING THAT NECK VELOCITIES OF 500 FT/MIN ARE MARGINAL, AND (3) IF NECK VELOCITIES ARE KEPT AT 300 TO 350 FT/MIN, IF 2 INCH LINING IS USED IN THE NECKS, AND IF MAIN BRANCH DUCT VELOCITIES NEAR SPLITTER DAMPERS DO NOT EXCEED THE RANGE OF 700 TO 800 FT/MIN, IT IS EXPECTED THAT LEVELS BELOW AN NC-15 CAN BE ACHIEVED EVEN WITH DIFFUSERS. THIS ARTICLE APPEARED IN THE MAY 1968 ISSUE OF SOUND AND VIBRATION. COPIES MAY BE OBTAINED BY WRITING ACOUSTICAL PUBLICATIONS, INC., P.O. BOX 9665, CLEVELAND, OHIO 44140. (RH)