Options for Converting Sound Power Levels to Sound Pressure Levels at 3 Feet

Active motor sound level limit specifications NEMA MG1-2016 & IEC60034-9 require that motor manufacturers meet noise levels in terms of sound power levels in decibels. Prior to the last couple of decades, the maximum permissible sound levels were determined based on sound pressure levels typically at three feet or one meter from the motor’s suffices per outdated sound specification IEEE 85-1973. Subsequently, and contrary to what they had done for decades, motor manufacturers may no longer record sound pressure levels, or they may record them at a distance other than 3 feet, of with a measurement surface other than a rectangular box. To accommodate customers that still request noise tests in terms of sound pressure at 3 feet / 1 meter, and also to compare with decades of legacy motor noise test data, two traditional methods and two alternate methods for converting sound power to sound pressure at three feet have been compared to the measured noise test results of ten medium AC motors. One traditional conversion method and one of the two alternate methods yield the closest sound pressure levels compared to the measured values at 3 feet. The half-cylinder conformal surface method and the average distance to each rectilinear surface method underestimate the sound pressure at 3 feet by less than 1 dB on average.