Octave Frequency Bands Research Articles

Measurements of Noise from Cooling Towers

During the past few years, noise measurements have been made on a number of water-spray cooling towers used with air conditioning systems or in conjunction with industrial processes. For these particular cooling towers, the water flow rate has been in the range of 300 to 1500 gallons per minute and the power rating of the air circulating fan in the range of approximately 712 to 40 hp. The over-all acoustic power level of these cooling towers has been computed from the measurements and found to lie in the range of approximately 115 to 120 db re 10−13 watt. The peak band power level falls in either the first (20–75 cps) or second (75–150 cps) octave frequency band, and the band power level appears to decrease at a rate of approximately 4 db per octave for the higher frequency bands. Data are presented to indicate the approximate proportion of noise contributed by the various noise sources in a typical cooling tower.

Noise of Centrifugal Fans

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 decibels re 1013a 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.