THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

14.4 Fan or Blower Noise 365Table 14.3. Calculation Results for Sound Power Level.Octave-Band Center Frequencies, HzProcedural Step 63 125 250 500 1000 2000 4000 80001 98 98 88 81 81 76 71 662 16 16 16 16 16 16 16 163 0 0 2 0 0 0 0 04 −3 −3 −3 −3 −3 −3 −3 −3Radiation form inlet 111 111 103 94 94 89 84 79The blade frequency is from Equation (14.2)f B = nN60=750 × 2460= 300 Hzwhich lies in the octave band with the center frequency of 250 Hz. The BFI is 2 dB.Step 3 is now completed. Step 4 consists of subtracting 3 dB from each octaveband to obtain the noise from the inlet alone.Fan LawsFan laws can predict fan performance quite well over a wide range of size andspeed. These laws are as follows:( ) 3 ( )da naQ a = Q b (14.4)d b n bp ta = p tb(dad b) 2 (nan b) 2(14.5)where( ) 5 ( ) 3 da naP a = P b (14.6)d b n b( ) ( )danaL wa = L Wb + 70 log + 50 log(14.7)d b n bQ = volume flow rate, m 3 /sp t = total pressure, kPaP = fanpower,kWL W = sound power level, dB re 1 pWd = rotor diameter, mn = rotor speed, rpmSubscript a denotes the parameters for the base curve performance conditions, andsubscript b denotes the parameters for the desired performance conditions.