Document pYJrZMM92Mx37n468LgG6GaX

350 CHAPTER 25 1960 Guide Eodt NC curve has o leodnta Iml in phons Did is 22 unif* greater Aon At SpttcJi iuimrfeience Uwi in dtabtb leipnatd by the NC wnbtr of (he curve). Tbe*e curvet theM be ased m desgn or specsfkatians wherever a favorable relation between (he low frequency and the high frequency portion of (he spectrum it dedred (mo fexft. fig. 3 .... Criterion Curves for Use with Table 1 in Determining the Permissible (or Desirable) Lp Values in 8 Octave Frequency Bonds such cases it is advisable to make an octave band analysis of the noise, if possible. KINDS OF NOISE Ute noise introduced into a room or building by ventilating or air-conditioning equipment may be divided into two gen eral Irinrta^ depending on how it reaches the room: 1. Noue transmitted through the duett. . From equipment such as fans, motors, sprays, etc. . From outside, and transmitted through duct walls into air stream. e. From duct wall vibrations, transmitted into air stream. d. From air currents, including eddying noises. e. Cross talk and cross noises between rooms connected by the same duct system. /. From dampers and air valves located near branch duct take-offs and outlets. g. Noise produced by grilles or diffusers. 2. Noise transmitted through the building construction. . From machine mountings as vibration. . From equipment through room w&U surfaces. The following text will present data and discussion of meth ods whereby solutions of the noise problem can be obtained when the allowable room noise level, and the path through winch the noise reaches the room, are known. NOISE GENERATED BY FANS Aerodynamic noise from all types of fans may be broadly divided into a rotational component and a vortex component. Sea* as Fig. 3 except that eoch NCA curva ho* a loudness love/ in pfoas that is 30 units greater than the Speech Interference Level m dwcAets (ex pressed by the NCA number of the curve). These curves may be used in design or speaftmti'ons m place of these of Fig. 3 wherever economy drrtates o maximum compromise end where, in addition, the fen and turbulence noise is steady and free of low frequency pure tone components (see text). fig. 4 .... Criterion Curves for Use with Table 1 in Determining the Permissible (or Desirable) L,, Values in 8 Octave Frequency Bands The rotational component is associated with the impulse given to the air each time a blade passes a given point and is honnn a series of discrete tones at the fundamental blade pass ing frequency and harmonics thereof. The vortex component of noise is largely due to the shedding of vortexes from the fan blades. It is random in character and has a continuous spec trum over a wide range of frequencies determined by the fan geometry and operation. It may also have many nonharmonic single-frequency components determined by blade geometry and 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.7'8 In addition to aerodynamic noise, there are usually several non-aerodynamic sources of noise in equipment involving fens 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 tile 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. Sound Control 351 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 (he fundamental 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 rise 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. Tins 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 sue, static pressure, speed, and capacity are as follows: Change in overall sound power level varies as: 70 logi(sizet/sizei) + 50 logi*(speedj/speedi) (12) 20 logi(size*/sizet) + 25 logto(pressure*/pressurei) (13) 10 ]ogt(capacity</capacityi) + 20 logi(pressure/pre8surei) (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. Its sound power level will be 10 logio 2, or 3 db greater than for the single one. Table 1 ....' Recommended Noise Criteria for Rooms* Noise Levels to be Measured m Unoccupied Rooms Typo of Space Recommended Note Criterion Computed Equivalent Sound level Meter Readme!* Weighting Scale-A db Legitimate theaters (500 seats, no 30 30-35 35 Assembly halls (amplification).... NC 25-30 35-40 Schoolrooms (no amplification)___ NC 25-35 35-45 Coliseums for sports only (ampli- Stenographic offices (typing and NCA 40-65 60 50-75 Nets: Each noise eriteris curve b s code for specifying permissible sound pres sure levels in eight octave bends. It is intended tbst ia no one frequency bond should the specified level be excoeded. The computed equivalent db* aumbers in the right-hand u prwr ntad for tnlamatton only and are no longer recommended fee use in speeificuions. Ventilating systems should be operating snd outside noise sources, traffic conditions,etc., should be normal when measure- meats are * If the noise b predominantly in the speech bands so that the spectrum does not have the shspe of tbs recommended noise criterion curve, the recommended values should be tower by 5 db. b Room air conditioners made prior to 1957 commonly produoe levels of SO to 60 db in sleeping arcus. Table 2 .... Recommended Noise Criteria for Offices* NC Curve of Fig. 3 Communication Environment Typical Application! 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 6-8 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-sized 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 messuremeats mode (or the purpose of rating the noise level is ss office by comperinon with these criteria eboald be performed with the office is oormaJ operation, hot with no one tsllring st the particular df*k or conference table where rpeoch communication b desired (Le^ where the manurerue ot is being made). Background noise with the office unoccupied should be lower by sppnsimstely 5 to 10 units.