Sound‐power ratings of machinery are becoming more important for a number of reasons. Industrial installations must comply with recently enacted state and local noise control codes, and sound power ratings are needed to predict compliance. In addition to this, The Federal Noise Control Act of 1972 requires labeling of machinery to show maximum noise emission, and this, logically, may be in terms of sound power. There are several ways to determine the sound power of machinery under laboratory conditions, but these procedures are usually not applicable in industrial environments. The two‐surface method offers the best practical approach to the problem of calculating sound power ratings of large machinery installed indoors, under actual operating conditions.