Sound Testing Standards
NEMA ST 1-4 (ANSI-C89.1) section 2.7 covers “Audible Sound Level Test.” For a thorough understanding of these tests it should be read in its entirety.
Briefly, the transformer is tested at its rated frequency and voltage under no-load conditions in a room which is 10 feet larger on all sides than the transformer. The ambient sound level of the room must be at least 5 db, and preferably 10 db, below the ambient level plus the transformer level. Five sound readings are taken with an approved sound meter one foot from each side of the transformer enclosure and one foot above the enclosure. The sound rating is the average of these five readings.
For three-phase transformers, the NEMA maximum allowable averages of the readings in decibels are shown in the chart to the left.