Document 99KKk15Yv07rO6K3vZBX39MZ7

342 CHAPTER 25 1959 Guide Hmm data am for a particular fan. 77m qwenfitiei i>,, and PWL* are diteutrad in fiw text. Sudi amt apply to eO font in a deodar taria*. Fig. 5 .... Specific Fan Characteristic Curves, Including Specific Sound Power Level P(g)(D)(rpm)~l I static pressure /wheel peripheral\ yveloeity pressure ) (cfm)/(Outlet Area) outlet-velocity ()/(P)(rpm) wheel peripheral velocity (16) D = wheel diameter, feet. p, = 'static pressure. p, = total pressure. Such curves apply to all fans in a similar series, Le., a series of fans differing in size only while preserving complete' simi larity in shape by having all linpax dimensions rhangpd in the same proportions. An overall specific sound power level PWL, in dbe as shown in Fig. 5 can also be defined that will be applicable to all fans in a family.7 It is obtained by means of Equation 18 which follows: PWL. - PWL - 10 logiefe-p1) dbe (18) Although few data are available to confirm the conclusion, it is indicated that such a plot also holds throughout a series of fans for all eight of the frequency bands provided rotor or blade passage frequencies are not predominant. Approximate Overall Sound Power Level In practice, data of the type shown in Fig. 5 are usually not available. Hence, in design an estimate has to be made of the overall sound power level and of the shape of the noise spec trum. Tests on centrifugal fans ranging in size from 0.01 hp to 100 hp show that the sound power level in a duct either upstream or downstream from the fan can be found within 4 db from the simple relation:*-,# where PWL -100+10 logtWs + 10 log!p dbe (19) Wg -- nameplate electrical horsepower of the motor driving - fan (assuming that the motor is operating near rated bp). p = pressure head, inches of water. dbe -- number of decibels re 10~u watt. (See discussion following Equation 18.) The assumption is made that the fan is being operated near the knee on its operating characteristics, i-e., near maximum efficiency or near the minimum in the curve of specific sound power (upper curve on Fig. 5). By assuming a fan efficiency of. 50 percent, Equation 19 can be put in the form of Equation 18 with PWL. - 65 dbe, i.e., PWL = 65 + 10 logig + 20 logip dbe (20)] q -- air quantity, cfm. p -- pressure head, inches of water. (See discussion after Equation 18.) It should be emphasized that Equations 19 and 20 are in tended to be used only when actual noise data are not availa- q -- quantity of air discharged, cfm. p = pressure, inches of water. PWL -- sound power level actually measured. Since static presure p, is more frequently measured for fans than total pressure pt, p> may be used under most normal operating conditions (when the difference between p, and p, is less than 20 percent) and the error will then be less than 1 db. Near free discharge, p becomes zero and p, must be used. In other words, PWL. is the sound power level that a similar (although hypothetical) fan would produce when operating at 1 cfm and a total pressure of 1 in. of water. As an example, the specific power level for one fan tested is shown on Fig. 5. TS 194 SOC MO IHO *0O MO tftOOO mcaueNCr bu m cvclcs k* ttcCMi 77m overall power level it firs* determined from Equation 19 or 20 and h added algebroicaUy to (he ordinate in fig. 6. Acoustics by 1. L Bomnofc (McGrow-Hid, 1954 Second Printing), (feed by pereutrioA. Fig. 6 .... Chart for Determining the Sound Power Band Levels for Ventilating Fans of Two Types : w* i" Sound Control 343 ble. All fans of the same power input are not equally noisy. The example illustrated in Fig. 5 demonstrates this fact since the minimum value of PWL. for this particular fan is 69 db, 4 db higher than the value used in Equation 20. It is on the edge of the stated limit of approximation 4 db. Approximate Octave Band Spectrum Within about 4 db approximation, and assuming no tur bulence in the duct or the coupling between it and the fan, the octave band spectrum for a centrifugal fan, relative to the overall sound power level, is given in Fig; 6. limited data on vane-axial fans,11 where sound pressure level measurements were made in the discharge duct, indicate that the overall sound power level is about the same as that for centrifugal fans. However, the spectrum shape of the vaneaxial fan noise has a shape something like that shown in Fig. 6. More recent free field measurements on axial fans indicate that perhaps the drop in level at the high and low ends of the spectrum are more pronounced.* The solid line in Fig. 7 is typical of the sound power spectrum for the axial fans re ported, as taken directly from the free field measurements. Application of a correction for the end reflection losses from tiie discharge opening (see Fig. 11) gives the dashed curve in Fig. 7, which corresponds to the relative sound power level in tiie duct. This spectrum shape for the avia! fan appears to be based on more extensive data than those of Fig. 6. NOISE GENERATED BY GRILLES AND DIFFUSERS Grille Noise Hie subject of grille noise has not been covered thoroughly in the literature. The information given in thiw chapter hn!a been taken from the advertising literature of several grille manufacturers and from recent experiments at the Massachu setts Institute of Technology. In general, grille noise at tiie lower frequencies is dependent primarily on tiie size and shape of the grille opening, on tiie mass flow per minute, and on the temperature of the air. At the higher frequencies, the noise is determined primarily by the area of the grille, the velocity of air through the free (open) areas (not the core or nominal area), and the pressure drop across the grille. - For two simple types of grille, stamped and deflector vane, power levels are given in Fig. 3;'These power levels are given 77m nofee fe expressed at (he sound power (ere/ ea dbe in eoeb of (be three speech interference bands (PWlj/t). Rg. 8 . -- Grille Noise for a Grille Area of One Square Foot in terms of the speech interference level, PWLsil . Conver sion to the speech interference level in db in the room at a listener's position is described in the section on Determina tion of Room Levels. In the absence of more complete infor mation, it may be assumed that the power levels of the noise produced by the grilles themselves at low frequencies are in consequential. Fig. 8 is for a grille core area of 1 sq ft. Correction to other areas of grille is made by the relation: where Decibel addition -- 10 logj*A db A -- core area, square feet. Values of the decibel addition for various values of A are given in Table 3. Table 3 .... Decibel Correction for Area A *9 ft Deo'be/ Addition A cq ft Dedbe/ Addition A Cq fi Oedfee/ Addition 0.5 . -3 10 23 46 8 16 20 40 9 70 18 12 100 20 13 200 23 16 400 26 .Fig. 7.... Sound Power Spectrum for Single-stage Axial Fan* Example S: Assume a deflector vane type of grille with a core area of 2 sq ft. Determine the sound power level in the speech interference bands for an air velocity of 2000 fpm. Solution: From Fig. 8. the PWLqu. for a 1 sq ft deflection grille is 59 dbe. From Table 2, an addition of 3 more dbe is re quired, so that PWLl -- 62 dbe. Diffuser Noise Aside from the information furnished by diffuser manu facturers, to their users, on sound levels generated by air dif fusers, few other data are available. Recently, however, some test results on several types of ceiling air diffusers have yielded a satisfactory correlation of the sound generating characteris tics of these devices." The data, covering ceiling diffusers ranging in size from 4- to 18-in. neck diameter, show that the X