Document RjZG3dB061pDQNyJY786GmbyE

i: I !! f i' * i i!:! n I! i Hit !iJ{ ! I Ft ;!| i-. 1018 CHAPTER 40 1957 Guide power level to yield the sound pressure level at the distance r and the angle 0 in the room (See Equation 24). It is seen from Fig. 18 that the noise from a ventilating duct is less at distances far from it than nearby. Moreover, at large distances, the levels produced in the room depend only on the power level of the source and the room constant and not on the directivity factor arid the distance r. Example S: To illustrate the application of the noise criteria and control procedures outlined in this chapter, calculation will be made for the required treatment for the ventilation system supplying the library reading room of Fig. 19. This room has a total volume of 80,000 cu ft and may be considered as an average room for acoustic properties (see Fig. 17). The total air supply to the reading room is 6000 cfm which is equally divided between four 12 X 18 in. supply grilles located along one wall. The supply of outdoor air is from a central fan system which handles a total of 25,000 cfm at 2 in. water static pressure and requires a 15 hp motor. Solution: It is necessary to determine the noise level in the reading room as a result of the ventilating system, both for the position of the nearest listener to a grille, and for a position remote from the supply grilles since it is impossible to tell beforehand which will be greater. The required attenuation for acceptable octave band noise levels will then be determined. Analysis of Fan and Duct System The acoustic power level of the fan should be computed from Equation 18 when the specific power level ie available. In this example it is assumed that the specifio sound power level is not known. Therefore, employ the approximate Equations 19 or 20. From Equation 19: PWL,dact AB) = 100 + 10 log 15 + 10 log 2 = 114.7 dbe Sound Control 1019 Table 7. Summary of Calculations for Example S on Ventilating Noise Levels in Library Reading Room Item Octave Frequency Bands 20-75 cps 75-150 cps 150-300 300-600 600-1200 cps cps cps 12002400 cps 24004800. cps 480010,000 cps 1. PWLfduct DH).................. 101 96 91 86 81 76 71 66 3. Att n^bend B)................. 5. Att'nfond losa).................... 5 -- *--13 5 -- -- 7 555555 2 7 7 6 11 12 1 4 7 7 8 10 41 ---- 6. Total duct att'n........... 18 12 12 17 19 18 24 27 83 84 79 69 62 58 47 39 8. fL/1000 cps-in............... 0.5 1.3 2.5 5.1 10.2 20.4 40.8 81.6 9. Q............ ........................ 2 2 2.2 2.7 3.2 3.6 3.9 4.1 10. Rel.Lp at 7 ft................ -22 -22 -22 -21 -20 -20 -20 -20 11. h, at 7 ft..................... 61 62 57 48 42 38 27 19 12. PWL(*11 grille*)................ 89 90 85 75 71 67 56 48 13. Rel.Lp far field............ -26 -26 -26 -26 -26 -26 -26 -26 14. Reverberant L,,............ 63 64 59 49 45 41 30 22 15. Criterion Lp.................. 60 51 .43 37 32 30 28 27 16. Required Att'n............. 3 13 16 12 13 11 2-- 17. Att'n 4 ft pkg. unit___ 7 11 14 30 40 40 38 33 or from Equation 20: PWLfdoct AB) = 65+10 log 25,000 + 20 log 2 = 115 dbe which agreement is better than the accuracy expected from these equations. Of this total acoustic power generated by the fan the main interest is in the part that reaches the reading room. At each duct branch the power will divide approxi mately as the ratio of the branch duct area to the total duct area after the branch. Thus at station B (Fig. 19) the acoustic power delivered to the 18 X 48-in. duct, neg lecting any attenuation in the system, will be PWL(duct BC) = PWLfduct AB) + 10 log (Abranch/-^total) r 18 X48 "I ,, = n11o5 --+L- i1nU lloogg |^--(24--x---4-^-----+----(;-1-8---x---4--8-) J = 111 dbe At station C the power division gives [ J12 x 48 "1 (18X24) + (12 X 4i) = 108 dbe Thus neglecting, at first, the attenuation in the connecting duct system, the total acoustic power delivered to the room by the fan will be 108 dbe. Assuming that the branch fittings at D, E, and F are similar to that at C in Fig. 19, the unattenuated fan acoustic power delivered to each of the four 12 X 18-in. grilles will be approxi mately PWLfduct dh) = 108 - 10 log 4 = 102 dbe The octave band spectrum of the centrifugal fan noise is given by Fig. 6. This chart indicates that the power level in the first band (20-75 cps) will be about 102-1 r 101 dbe, and the power level in each succeedingly higher octave band will be 5 dbe less. These levels are entered in line 1 of the calculation Table 7. The next step is to determine the natural duct attenuation between the fan dis charge and the room supply grilles. From Table 3 the attenuation for straight sheet