1、NEMA Standards Publication National Electrical Manufacturers Association NEMA MG 3-1974 (R2014) Sound Level Prediction for Installed Rotating Electrical Machines 1974 National Electrical Manufacturers Association. All rights, including translation into other languages, reserved under the Universal C
2、opyright Convention, the Berne Convention for the Protection of Literary and Artistic Works, and the International and Pan American copyright conventions. NEMA MG 3-1974 (R1995, R2000, R2012, R2014) Sound Level Prediction for Installed Rotating Electrical Machines Published by National Electrical Ma
3、nufacturers Association 1300 17 thStreet N, Suite 900 Rosslyn, VA 22209 www.nema.org NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document at the time it was developed. Consensus
4、does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. NEMA standards and guideline publications, of which the document contained herein is one, are developed through a voluntary consensus standards development process. This
5、process brings together volunteers and/or seeks out the views of persons who have an interest in the topic covered by this publication. While NEMA administers the process and establishes rules to promote fairness in the development of consensus, it does not write the document and it does not indepen
6、dently test, evaluate, or verify the accuracy or completeness of any information or the soundness of any judgments contained in its standards and guideline publications. NEMA disclaims liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect,
7、consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on this document. NEMA disclaims and makes no guaranty or warranty, express or implied, as to the accuracy or completeness of any information published herein, and disclaims and ma
8、kes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or sellers products or services by virtue of this standard or guide. In publishing and making this document
9、 available, NEMA is not undertaking to render professional or other services for or on behalf of any person or entity, nor is NEMA undertaking to perform any duty owed by any person or entity to someone else. Anyone using this document should rely on his or her own independent judgment or, as approp
10、riate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and other standards on the topic covered by this publication may be available from other sources, which the user may wish to consult for additional views or infor
11、mation not covered by this publication. NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this document. NEMA does not certify, test, or inspect products, designs, or installations for safety or health purposes. Any certification or other statement of comp
12、liance with any health- or safety-related information in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. NEMA MG 3-1974 (R1995, R2000, R2012, R2014) 1 1974 National Electrical Manufacturers Association FOREWARD In recent ye
13、ars noise has had an increasing influence on users of machines. Exposure to excessive noise levels may cause permanent hearing damage and, also, affect peoples efficiency, moods, temperament and comfort. Local, State and Federal Governments have enacted legislation to control noise level exposure. D
14、ue to the recent emphasis on health and safety considerations, a large number of people with no special training in sound have had noise control problems thrust upon them. This booklet provides a method of predicting approximate sound pressure levels in industrial and commercial areas. The method is
15、 intended for estimating sound pressure levels and should not be construed as a guarantee of results. It requires a knowledge of the sound levels, location of all sound sources, and room characteristics. Throughout this booklet, for simplicity, emphasis is placed on the use of overall dBA. A more de
16、tailed analysis can be obtained by using the same method for each octave band sound level as shown in the Appendix. Machine sound level data are normally expressed as single dBA levels or in full octave band dB levels, unweighted, in terms of sound pressure or sound power.* An example is included to
17、 assist the reader in understanding the method described in the publication. Instructions for calculation are followed immediately by corresponding sample calculations for convenient reference. Comments will be welcomed. They should be sent to: Senior Technical Director, Operations 1300 17 thStreet
18、N, Suite 900 Rosslyn, VA 22209 _ * The NEMA “Standards Publication for Motors and Generators, “ MG 1, has for a number of years included sound levels for Design A, B and C alternating-current squirrel-cage induction motors, q1 through 250 horsepower, in terms of sound power levels as measured in acc
19、ordance with the IEEE Test Procedure for Airborne Sound Measurements on Rotating Electric Machinery, Publication No. 85NEMA MG 3-1974 (R1995, R2000, R2012, R2014) 2 1974 National Electrical Manufacturers Association CONTENTS FOREWARD. 1 Section I ABBREVIATIONS 3 Section 2 DEFINITIONS OF TERMS. 4 Sec
20、tion 3 STEPS REQUIRED TO DETERMINE SOUND LEVELS. 5 3.1 Outline.5 3.2 Defining the Example Used in the Text. 5 3.3 Room Constant. 6 3.4 Sound Pressure Level at a Desired Point Due to Each Sound Source. 9 3.5 Combining Sound Pressure Levels at a Point Due to Several Sources.11 3.6 The Effect of Ambien
21、t Sound Pressure Level.12 3.7 Concluding Comments. 13 Table 1 Sound Absorption Coefficients of General Architectural Materials.14 Figure 1 Attenuation of Sound Level (L) as a Function of Distance and room Constant Over a Reflection Floor.14 Figure 2 Estimation of Room Constant from Sound Pressure Le
22、vel Measurements at Two Points 15 Figure 3 dB Adjustments for Ambient Correction.15 Figure 4 dB Adjustments for Combining Sound Sources.15 Appendix Additional Data Required When Using Octave Band Center Frequencies to Determine Sound Pressure Levels.i Referencesiv NEMA MG 3-1974 (R1995, R2000, R2012
23、, R2014) 3 1974 National Electrical Manufacturers Association SOUND LEVEL PREDICTION FOR INSTALLED ROTATING ELECTRICAL MACHINES (Approved as Authorized Engineering Information 7-17-1974.) SECTION 1 ABBREVIATIONS A Total area of room surfaces (sq. ft.) (A 1+A 2+A 3 A n=A) d Distance from the sound so
24、urce center to the point of interest (ft.). dB Decibels, unweighted scale. dBA Decibels, A-weighted scale. D (eff) Effective distance from the sound source to the point of interest, allowing for reflective surfaces Lp Sound pressure level (dB) (reference pressure is 0.0002 microbars or 0.0002 dynes/
25、cm 2 ) Lw Sound power level (dB) (reference power is 10 -12watts). R Room constant (sq. ft.) X Point of interest for sound level determination. Absorption coefficient of material of interest. Effective sound absorption coefficient of the room being considered. L Sound level attenuation (dB). NEMA MG
26、 3-1974 (R1995, R2000, R2012, R2014) 4 1974 National Electrical Manufacturers Association SECTION 2 DEFINITIONS OF TERMS Absorption Coefficient () The ratio of energy absorbed by a surface to the energy incident upon that surface. Ambient Sound Pressure Level The sound level existing in a room with
27、all identified sources removed. Anechoic (Free) Field A room whose boundaries absorb effectively all sound incident thereon. Attenuation (L) A reduction in sound pressure level in dB. It is a function of distance, room constant and location of the source with respect to reflecting and absorbing surf
28、aces. Machine In this publication, a rotating electrical device. (In general, it is any sound source.) Noise Undesired sound. Reflecting Surfaces In this publication, all large surfaces of the room, i.e., walls, floor and ceiling. The indicated corrections to the effective distance for the nearness
29、of major reflecting surfaces do not apply to the floor, unless the motor is at least 5 ft. above the floor; such corrections have been incorporated in the curves. Room constant (R) A number rating a room according to its size and the sound absorption properties of the enclosing surfaces. Sound Power
30、 Level ( ) A measure of sound power (w) radiated from a source ratioed with a reference power of 10 -12watts. Lw = 10Log 10 10 12 Sound Pressure Level ( ) A measure of the pressure (p) at a distance from the source ratio with a reference pressure of 0.0002 microbars. Lp = 20Log 10 0.0002 NEMA MG 3-1
31、974 (R1995, R2000, R2012, R2014) 5 1974 National Electrical Manufacturers Association SECTION 3 STEPS REQUIRED TO DETERMINE SOUND LEVELS* 3.1 OUTLINE A. Define the problem (example included in text). B. Measure or estimate the “room effect” in terms of the room constant determined by one of the foll
32、owing methods: 1. Measured with a sound source of unspecified sound power level. 2. Computed from architectural materials. 3. Measured with a specified white sound source. C. Determine the sound pressure level at a desired point due to each noise source. Using Figure 1, locate the room constant curv
33、e for the room constant being considered. 1. If the machine is rated in sound power level, locate the site distance intercept with the R curve selected and read L; then Lp (R) = Lw L. 2. If the machine is rated in sound pressure level (Lp) at a specified distance in an anechoic or semi anechoic cham
34、ber: a. Locate the specified distance intercept with curve R = , and read L (noted as = L ().) b. Locate the site distance intercept with the R curve selected and read L (noted as L (R). c. Sound pressure level due to this source is given by: ( ) = + () = ( )3. If the machine is rated in sound press
35、ure level (Lp) at a specified distance in a room having a room constant of 1 : a. Locate the specified distance intercept with curve 1and read L (noted as 1). b. Locate the site distance intercept with the R curve selected and read L (noted as L (R). c. Sound pressure level due to this source is giv
36、en by: Lp (R) = Lp + =L (R 1) - =L (R). D. Combine the sound pressure levels due to identified sources at the point of interest. E. Consider the effect of the ambient sound pressure levels. F. Concluding comments. 3.2 DEFINING THE EXAMPLE USED IN THE TEXT Predict the sound pressure level at point X,
37、 6 ft. above the floor, due to three machines placed in a room whose dimensions and material are given. Floor is of asphalt tile. Ceiling is of acoustical tile, 12 feet above the floor. Machines A and B are mounted on the floor; Machine C is mounted 5 ft. above the floor. The ambient sound level of
38、the room = 70 dBA. NEMA MG 3-1974 (R1995, R2000, R2012, R2014) 6 1974 National Electrical Manufacturers Association The dimensions of the machines (supplied by the manufacturer) are: Machine Length Width Height A 3 2 2 B 3 2.5 2 C 2.5 1.5 2 Dimensions, feetThe machine sound levels (supplied by the m
39、anufacturer) are: Machine A, sound power level . . . . . . . . . . . . . . . 90dBA Machine B, sound pressure level at 3 ft. (from a major machine surface * ) R = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85dBA Machine C, sound pressure level at 5 ft. (from a major machine surfa
40、ce ) R = 2000 sq. ft. . . . . . . . . . . . . . . . . . . . . . . . . 82dBA 3.3 ROOM CONSTANT The use of a quantity known as “room constant” permits simple and realistic corrections for deviations from ideally anechoic conditions. Three alternative methods for determining the room constant are descr
41、ibed, but only one need be used. Strictly speaking, a room constant value should be obtained for each octave band center frequency. Use of the octave band method is recommended primarily if sound spectra of widely differing character are to be combined. The example, therefore, will be worked through
42、 first with overall sound levels as a first choice, and then with the octave band sound levels given in the Appendix. 3.3.1 Measured with a Sound Source of Unspecified Sound Power Level The sound source is placed as near the center of the room as possible but not less than wavelength distance from a
43、 major reflecting surface for the lowest frequency considered. (Normally 125 Hz is the lowest center frequency to be considered; it corresponds to a minimum of approximately 3 ft.) The wavelength rule applies to all microphone positions; moreover, no microphone shall be closer to the sound source th
44、an a distance equal to the source length. EXAMPLE: Sound source is centrally located in the room. Sound pressure measured at 5 ft. from the center of the source is 80.8 dBA. Therefore, L = 80.8 78.2 = 2.6 dBA. Interpolating from Figure 2, room constant R = 940 sq. ft. Two sound pressure level readin
45、gs are taken at different distances from the sound source on an approximately straight line passing through the center of the sound source. See par. 3.4.1 Sound Power Levels. Distances are from the center of the source and not its surface. Preferably, the distance between the two microphone position
46、s should be *Exclusive of terminal housing. Exclusive of terminal housing. NEMA MG 3-1974 (R1995, R2000, R2012, R2014) 7 1974 National Electrical Manufacturers Association equal to the distance from the sound source center to the closer microphone. The difference in sound levels, the two distances i
47、nvolved, and the use of figure 2 permit the determination of the room constant. 3.3.2 Computed from Architectural Materials This is done in four steps: A. Calculate the surface are (sq. ft.) of each of the enclosing surfaces in the room, i.e., walls, floor, ceiling, windows, etc. B. Find the effective sound absorption coefficient for each of the major surfaces in Table 1. In the event that the material is not covered in the table but that the absorption coefficient can be found elsewhere, it may be expressed as a function of frequency; i.e., center frequency octave bands. In this event, take
