Document 3QDxZdq9ppqLnkqvm0yLxpK6

340 CHAPTER 25 1959 Guide Eads NC curve ho* o !oodmss level in pbonj that h 32 emit* greater Aon (he Spevdi Interference Level in dedbel* [expressed by the NC number of (be curve). These curve* should be mod in design or (pecrficofioo* wherever a favorable relation between (be low frequency and the high frequency portion of (be tpecfruw is desred {tee text). Rg. 3 .... Criterion Curves for Use with Table 1 in Determining"the Permissible (or Oesirable) L,, Values in 8 Octave Frequency Bands blades. It is random in character and has a continuous spec* truxn over a wide range of frequencies determined by the fan geometry and operation. It may also have many nonh&rmonic single-frequency components determined by blade geometry nnH local air velocity. Since the laws of generation of these two types of noise are different, they will vary in importance for different types of fans and operating conditions.*'7 In addition to aerodynamic noise, there are usually several non-aerodynamic sources of noise in equipment involving fans. Such sources include noise resulting from unbalance, bearing noise, brush noise, magnetic noise, and belt noise. The number of blades of a centrifugal fan generally is governed by optimum air-flow design. The noise generation decreases but slightly for more than the optimum number of blades. Powerful tones of definite pitch may be generated in fans when the wakes from the impeller blades impinge on an ob struction such as a badly located or shaped cut-off in a centrif ugal fan, or struts or motor supports which are badly shaped and too close to the impeller of an axial flow or propeller fan. This interference noise has the same frequency characteristics as rotation noise.* A shroud around a propeller fan may serve to reduce noise considerably if it is working properly. Such reduction is gen erally most effective at the higher harmonics. However, if the flow breaks down over part of the shroud, the noise may be come considerably worse than for an unshrouded case. As the operating pressure across axial fans is increased the maximum sound intensity is shifted from the fundamental Same at fig. 3 excepI (hot each NCA curve has a loudness lave! to pfioft* (hoi it 30 umlf graoiar Ifton (ha Speech Interference Laval m decibel* (expressed by the NCA number of (he curve). Theta curve* noy be used In design or specifications its place of those of fig. 3 wherever economy' diefafe* a soaxianrm compromise and whore, in addition, Bit fan and (urbulence noise is steady end free of lew frequency pure lone component* (tee texf). Rg. A .... Criterion Curves for Use with Table 1 in Determining the Permissible (or Desirable) Lp Vatues in 8 Octave Frequency Bands to higher harmonics. This effect is not observed for centrifugal fans. Effect of Fan Parameters on Noise vs. Performance Fan Laws. The principal laws governing the-mechanical performance of a fan, whether centrifugal or axial, are fairly well established and are given in Chapter 22. Fixed Point of Rating. A fixed point of rating of a given size fan at a given speed is one for which the capacity (volume flow) is chosen as a given fraction of the free delivery capacity. Having chosen this capacity ratio, the ratio of pressure to the static no-delivery pressure is determined. This fixed point of rating will have a fixed efficiency within the limitation of cer tain manufacturing details. The fan laws serve to relate the performance of two members of a symmetrical series of fans operating at the same fixed point of rating whether at the same or different speeds. Correlation of Fan Noise with Size, Static Pressure, Speed, and Capacity. The empirical relations between the noise gener ated by a fan and its size, static pressure, speed, and capacity are as follows: Change in overall sound power level varies as: 70 logi(sise*/sisei) + 50 logn(speedi/speedi) (12) 20 logi(6ue*/sizei) + 25 logii(pressuret/pressurei) (13) Sound Control 341 10 logn(capacityi/c&p&eityi) + 20 logit(pressuret/pressurei) (14) These relations also apply only for a fixed point of rating. A double width fan is essentially two fans of the same size, speed, and sound pressure level and therefore its soundpower level will be 10 logit 2 or 3 db greater than for the single one. Example 1: Consider s 36>-in. diameter fan operating at 700 rpm. If it produces a noise level of 70 dbc what will be the level if a 49-in. diameter fan operating at 1000 rpm is substi tuted for it? Solution: From Equation 12, 49 1000 Change in level -- 70 Iog --36---5 + 50 login 7--0--0 * 168 db Noise level of second fan = 70 -f 188 -- 868 dbc Fan noise does not follow equations exactly. The radiated sound power of a centrifugal fan, for example, is not all con centrated on the blade frequency. While the overall sound power level seems to follow these equations fairly closely, the various components in the different frequency- bands may show greater deviation. In the case of axial flow fans there is a wider range in characteristics of the vortex noise, so it would be expected that a slightly greater deviation from these equa tions exists than for centrifugal fans. This is particularly true as speed is increased. The fundamental or blade frequency exists over a wider range of pressure in the case of the cen trifugal fan. In the axial fan, the harmonics are more easily excited and often the second and third harmonics.exceed the level of the fundamental.* Specific Plots Curves which describe the characteristics of an entire series of similar fans can be plotted by using dimensionless,coor dinates.7 To describe the pressure characteristics the pressure coefficient <p can be plotted against the flow coefficient <j> as shown in fig. 5, where p, r(*)(I))(rpm)~| 4005 J total pressure J/wheel peripheral\ ^velocity pressure (15) Table 1 ... .Recommended Noise Criteria for Rooms* famft te be Afeswrerf fi Lfesscaparf Xec s Type of Space fiscommended Note Criterion Curve Computed Fquivofen! Sound level Meter ' Keodmgs* Weighting Scale-A dba Broadcast studios................................ Concert balls........................................ Legitimate theaters (500 seats, no amplification)................................... Music rooms.......................................... Schoolrooms (no amplification)___ Conference rooms for 50................... Apartments and hotels...................... Assembly halls (amplification).... Homes (sleeping areas)................... Conference rooms for 20................... Motion picture theaters.................... Hospitals................................................ Churches................................................ Courtrooms.................................. Libraries................................................ Small private offices.......................... Restaurants........ .............. ............... Coliseums for sports only (ampli- fication).............................................. Slenoeranhic offices (tvoine and business machines)......................... Factories................................................ NC 15-20 NC 20 . NC 20-25 NC 25 NC 25 NC 25 NC 25-30 NC 25-30 NC 25-35 NC 30 NC 30 NC 30 ` NC 30 NC 30 NC 30 NC 30-35 NCA 45 NCA 50 NC 50 NCA 40-65 25-30 30 30-35 35 35 35 35-40 35-40 35-45* 40 40 40 40 40 40 40-45 55 60 60 50-75 Nets: Etch bomb criteria curve is s eede (or *pecitying permissible sound pres sure levels in eight octave bind*. It U intended that in no one frequency band should the specified level be exceeded. The computed equivalent dbe number* in the right-band column are presented for information only and are no longer recommended for use in specifications. Ventilating ayetems ehould be operating endoutside ooiaesources, traffic condition*, etc., should be normal when measure ments are * If the noise is predominantly in the speeeh bands so that the spectrum does not have the shape of the recommended noise criterion curve, the recommended dbe numbers should be lower by S dbe. b Room sir conditioners made prior to 1SS7 commonly produce levels uf SO to 0 dbe in sleeping areas. Table 2 .... Recommended Noise Criteria for Offices* NC Curve of fig. 3 Communication Environment Typical Applications NC20 to NC30 NC30 to NC35 NC35 to NC40 NC40 to NC50 NC50 to NC55 Above NC55 Very quiet office--telephone use satisfactory--suitable for large conferences. Quiet office; satisfactory for conference at a 15-ft table; nor mal voice 10 to 30 ft; telephone use satisfactory. Satisfactory for conferences at a 5-8 ft table;'telephone use satisfactory; normal voice 6 to 12 ft. Satisfactory for conferences at a 4-5 ft table; telephone use occasionally slightly difficult; normal voice 3 to 6 ft; raised voice 6 to 12 ft. Unsatisfactory for conferences of more than two or three people; telephone use slightly difficult; normal voice 1 to 2 ft; raised-voice 3 to 6 ft. Very noisy; office environment unsatisfactory; telephone use difficult. Executive offices and conference rooms for 50 people. Private or semiprivate offices, reception rooms, and small conference rooms for 20 people. Medium-sised offices' and industrial business offices. Large engineering and drafting rooms, etc. Secretarial areas (typing), accounting areas (business machines), blueprint rooms, etc. Not recommended for any type of office. * Noise measurements made for the purpose of rating the noise level in an office by oomperison with these criteria should be performed with the office in normal operation, but with no one talking at the particular desk or conference table where speech communication is desired (he., where the measurement is being made). Background noise with the office unoccupied should be lower by approximately S to 10 unite. J