Certification testing of air handling unit sound power (PWL) indicates that individual unit PWL can vary significantly from manufacturer published data. Published data are typically based on a limited number of actual tests with results extrapolated for other fan sizes and operating conditions. Although published data are normally acceptable for routine applications, indiscriminate use for sensitive designs (studios, theaters, conference centers, etc.) can result in excessive finished space noise levels. The fact that the design goals have been exceeded can be accurately documented. The exact reason for the exceedance, however, cannot be as firmly established in a complex system. Certification PWL testing of air handling units prior to installation can detect PWL variations from the published data. Individual unit PWL certification was often not cost effective prior to the developement of sound intensity and the establishment and use of recognized standards for in situ PWL testing (e.g., ASA 104-1992, ANSI S.12.12.1992). Case histories to be presented document fan PWL variations from published data of up to 10 dB and higher in certain octave bands.