Extensions of earlier studies [J. Acoust. Soc. Am.26, No. 2(1953)] on ventilation system noise are reported. It is shown here that in properly engineered ventilation systems the over‐all sound power level in decibelsre1013a watt is related to the horsepower rating of the fan motor by the formula:PWL = 100+10 1og10hp. The octave frequency band spectrum slopes off with increasing frequency at the rate of 5 decibels per octave. The level in the first band is one decibel below the over‐all level. Measured data on fourteen fans in actual installations yield a spread of about ±4 db around these relations.