Acoustics-Determination of sound power levels of noise sources using sound pressure-Survey method using an enveloping measurement surface over a reflecting plane
Some standard content:
GB/T3768-1996
This standard is a revision of GB3768-83 based on the international standard ISO3746:1995 "Acoustics - Determination of the sound power level of noise sources by the sound pressure method - Simple method using an envelope measuring surface above a reflecting surface". This standard is equivalent to the international standard in terms of technical content. This makes the measurement results of the sound power level of noise sources comparable internationally, which is conducive to the trade and technical exchanges of my country's import and export of mechanical products.
When revising GB3768-83 based on international standards, this standard adds some new content in terms of technical content, which is more specific than the original standard requirements, and is conducive to the understanding and implementation of the standard. The "Sound Pressure Level Measurement" has detailed provisions on the selection of the measurement surface, the location of the basic microphone and the additional microphone, the influence of environmental conditions, the requirements of the measuring instrument, and the measurement method. The "Calculation of A-weighted Surface Sound Pressure Level and A-weighted Sound Power Level" adds background noise correction, test environment correction, calculation of A-weighted surface sound pressure level, and determination of optional quantities, thereby further improving the calculation accuracy of the standard. The measurement environment is changed from allowing only one reflective surface to the sound source close to two reflective surfaces, which is more suitable for the complexity and diversity of the actual workplace. The corresponding measurement point layout principles and schematic diagrams are given for each situation to facilitate the understanding and use of the standard. Four appendices have been added to provide favorable conditions for the implementation of the standard. The measurement uncertainty is expressed as the reproducibility standard deviation; the accuracy of the sound calibrator used for the calibration of the measuring instrument is increased to ±0.3dB, thereby improving the accuracy of the measurement system.
This standard replaces GB3768-83 from the date of entry into force. Appendix A, Appendix B, and Appendix C of this standard are all standard appendices. Appendix D and Appendix E of this standard are all suggestive appendices. This standard is proposed and coordinated by the National Technical Committee on Acoustics Standardization. The drafting unit of this standard is China National Institute of Metrology. The main drafters of this standard are Shen Yang, Chen Jianlin and Zhang Mei'e. This standard was first issued on June 23, 1983 and revised on May 27, 1996. This standard is entrusted to the National Technical Committee on Acoustics Standardization for interpretation. 41
GB/T3768-1996
ISOForeword
ISO (International Organization for Standardization) is a world alliance of national standards organizations (ISO member countries). The formulation of international standards is carried out through ISO technical committees. Each member has the right to send representatives to participate in the committee when it is interested in the topic established by the technical committee. International organizations associated with ISO, both official and unofficial, can participate in this work. The draft international standard adopted by the technical committee shall be circulated to the member countries for voting. As an international standard publication, at least 75% of the member countries must vote in favor.
International Standard ISO.3746 was approved by ISO/TC43 Acoustics Technical Committee SC1 Noise Technical Committee This second edition replaces the first edition (ISO3746:1979) on the basis of technical revision of the first edition. Appendices A, B and C constitute an integral part of this international standard. Appendices D and E are for reference only.
GB/T3768--1996
This standard is one of a series of standards for determining the sound power level of noise sources. This series of standards specifies different methods for determining the sound power level of machinery and equipment or their combinations. When selecting these standards, the best choice should be made based on the purpose and conditions of the noise measurement and the general guidelines given in GB/T14367. The above series of standards only give general principles for the installation and working conditions of machinery and equipment. For a specific type of machinery and equipment, the technical requirements for its installation and working conditions are as follows. It is necessary to refer to the corresponding noise test specifications. This standard specifies a method for measuring the sound pressure level on the measurement surface of an envelope sound source to calculate the sound power level. The envelope surface method is applicable to all three levels of accuracy (see Table 0.1), and the accuracy level of this standard is Level 3. When using this standard, it is required to meet some of the identification standards listed in Table 0.1. If the corresponding standards cannot be met, the methods of ISO3747 or ISO9614 can be tried.
Specific to a certain type of machine and equipment, its noise test specifications should be based on the series of standards for the determination of noise source sound power levels or ISO9614, and there should be no contradictions.
When measuring between typical machines where the sound source is placed, corrections need to be made for background noise or unwanted sound reflections. The method specified in this standard allows The A-weighted sound power level may be determined directly from the measured A-weighted sound pressure level. This standard calculates the sound power level from the measured sound pressure level based on the premise that the sound power output of the sound source is proportional to the mean square sound pressure averaged over time and space.
Table 0.1 List of national standards with different accuracy levels for the determination of the sound power level of noise sources on reflecting surfaces using the envelope surface method GB6882
Test environment
Test environment suitability assessment criteria 1)
Sound source volume
Noise characteristics
Limitation on background noise 1)
Number of measuring points
Instrument:
Sound level meter at least meets
Precision method
Semi-anechoic chamber
Kz≤0.5 dB
It is best to be less than 0.5% of the test
room volume
GB3767
Engineering method
Outdoor or indoor
K2≤2dB
Unlimited
Limited only by the effective test environment
All types of noise (broadband, narrowband, discrete frequency, steady state, non-steady state, pulse) AL≥10 dB
(If possible, greater than 15dB)
Ki≤0.4dB
a) Type 1 specified in GB3785
Integrating sound level meter at least meets the requirementsb) Type 1 specified in IEC804c) GB3241 regulations
Bandpass filter at least meets the requirements
AL≥6dB
(If possible, greater than 15dB)
Ki≤1.3 dB
a) Type 1 specified in GB3785
b) Type 1 specified in IEC804
c) Regulations of GB3241
GB3768
Simplified method
Outdoor or indoor
K2≤7dB
Unrestricted
Limited only by effective test environment
AL≥3dB
Ki≤3dB
a) Type 2 specified in GB3785
b) Type 2 specified in IEC804
GB/T 3768-1996
The accuracy of the LWA determination method is expressed by
the standard deviation of the reproducibility
GB6882
Precision method
Table 0.1 (end)
GB3767
Engineering method
Or≤1.5dB
GB3768
Simplified method
When K2<5 dB,
When 5dB≤K≤7 dB,
Or≤4dB,
When discrete pure tones account for the main components
R increases by 1dB
1) When measuring the sound power spectrum, K, and K2 should meet the requirements in each frequency band within the test frequency range. When measuring the A-weighted sound power level, the above values of K1A and KzA should also be used.
2) Under given conditions, the number of measuring points may be reduced44
1 Scope
1.1 General
National Standard of the People's Republic of China
Acoustics-Determination of sound powerlevels of noise sources using soundpressure-Survey method using anenveloping measurement surface over areflecting plane
GB/T 3768-1996
eqvIso3746:1995
Replaces GB3768-1983
This standard specifies the method for measuring the sound pressure level on the measurement surface of the envelope sound source to calculate the sound power level of the noise source. At the same time, it gives the requirements for the test environment, measuring instruments and the calculation method of the surface sound pressure level and sound power level. The accuracy level of the sound power level measurement result is Class 3.
For various types of equipment, it is very important to formulate and use their special noise test specifications according to this standard. The noise test specifications should give detailed instructions on the installation, load, working conditions, measurement surface and microphone array selection of the measured sound source. Note 1: For a specific type of equipment, its noise test specification should give detailed information on the special measurement surface selected, because the use of different shapes of measurement surfaces will result in different evaluations of the sound power level of the sound source. 1.2 Types of noise and noise sources
The method specified in this standard is applicable to the measurement of various types of noise. Note 2: For noise classification (steady state, non-steady state, quasi-steady state, impulse, etc.), see GB/T14259. This standard applies to sound sources of various types and sizes (equipment, machines, components, etc.). This standard does not apply to very high or very long sound sources (smoke stacks, pipes, conveying machinery, multi-source industrial equipment, etc.). 1.3 Test environment
This standard applies to indoor or outdoor test environments that meet the requirements and have one or more reflecting surfaces. 1.4 Measurement uncertainty
For sound sources that radiate steady-state narrowband noise, the results measured in accordance with this standard, except for individual cases, the reproducibility standard deviation of the A-weighted sound power level is equal to or less than 3dB (K2A<5dB) or 4dB (5dB≤K2A≤7dB). For sound sources that radiate discrete pure tones, the reproducibility standard deviation is generally increased by 1dB (see Table 1). There is a high probability that there is a difference between the single value of the sound power of a noise source measured in accordance with this standard and its true value that is within the uncertainty range. The uncertainty of the sound power level measurement comes from the combined influence of the environmental conditions and experimental techniques of the measurement laboratory. If a specific noise source is measured at different test sites according to this standard, the measurement results will show discreteness. The calculation of the standard deviation of the measurement results is shown in GB/T14573.4. Except for individual cases, the above standard deviation does not exceed the value in Table 1. The reproducibility standard deviation 6R given in Table 1 takes into account the cumulative effect of uncertainty in the measurement process, but does not include changes in sound power output caused by changes in working conditions (speed, power supply, voltage) or installation conditions. Approved by the State Administration of Technical Supervision on May 27, 1996, and implemented on December 1, 1996
GB/T3768-1996
Measurement uncertainty is not only related to the reproducibility standard deviation, but also to the required confidence level. For example, for a normally distributed sound power level, when the confidence level is 90%, the true value of the sound power level of the sound source is within the range of ±1.645gk of the measured value. When the confidence level is 95%, the true value is within the range of ±1.960gk of the measured value. See GB/T14573 series and ISO.9296 for details. Table 1A The maximum value of the reproducibility standard deviation of the weighted sound power level applies to
Relatively "flat" noise spectrum in the test frequency range. The sound source with discrete pure tones as the main component
3 If K2A is greater than or equal to 5dB, oR may be 1dB larger than the value in Table 1. The maximum value of the reproducibility standard deviation, r
4 The noise test specification for a specific type of noise source may give a lower reproducibility standard deviation. 5 The standard deviations listed in Table 1 are the combined effects of the measurement conditions and methods defined in this standard, and do not include the influence of the sound source itself. They are caused by the following aspects: changes between measurement sites, including outdoor environment and climate conditions; indoor test room geometry, absorption of test room boundaries, acoustic properties of reflecting surfaces; background noise; instrument calibration form; and changes in experimental techniques, including the shape and size of the measurement surface, the number of measurement points and microphone positioning, sound source location, integration time, and determination of environmental corrections (if any). The standard deviation is also affected by the error introduced by the near-field measurement, which is related to the characteristics of the sound source and generally increases at smaller measurement distances and lower frequencies (below 250 Hz). 6 If a given sound source is measured at all test sites using similar instrumentation, the consistency of the measured sound power level results may be better than the consistency reflected by the standard deviation in Table 1.
7 For a specific type of sound source with similar size, similar sound power spectrum and similar operating conditions, the reproducibility standard deviation may be less than the value in Table 1. When developing noise test specifications based on this standard, smaller standard deviations than the values in Table 1 may be indicated in the noise test specifications if it is proven feasible through appropriate laboratory verification.
8 The reproducibility standard deviation in Table 1 includes the uncertainty of repeated measurements of the same noise under the same conditions (the standard deviation of repeatability), which is generally much smaller than the uncertainty caused by exchanging laboratories. For special sound sources, if it is difficult to maintain stable working conditions and installation conditions, the standard deviation of repeatability may not be less than the value given in Table 1. In this case, it is difficult to obtain a reproducible sound power level, which should be recorded and explained in the test report.
9 The methods of this standard and the standard deviations given in Table 1 are applicable to the measurement of a single machine. The characterization of the sound power level of a batch of machines of the same type involves random sampling techniques with specified confidence intervals, and the results are expressed as statistical upper limits. When applying these techniques, the total uncertainty must be known or estimated, including the product standard deviation defined in GB/T14573.2, which is the deviation of the sound power output between individual machines in a batch of machines. The statistical method for machine batch characterization is shown in GB/T14573.4. 2 Referenced standards
The provisions contained in the following standards constitute the provisions of this standard through reference in this standard. At the time of publication of this standard, the versions shown are valid. All standards are subject to revision, and parties using this standard should explore the possibility of using the latest versions of the following standards. GB3241-82 1/1 and 1/3 octave filters for sound and vibration analysis (neqIEC225-1966) GB3785-83 Electrical and acoustic properties and test methods of sound level meters (neqIEC651-1979) GB3947-83 Acoustics Terminology
GB6882-86 Determination of sound power levels of acoustic noise sources - Anechoic chamber and semi-anechoic chamber precision method (neqISO3745-1977) GB3102.7-93 Acoustic quantities and units (neqISO31-1:1992) GB/T14259-93 Acoustics - Guide to calibration for the measurement of airborne noise and the assessment of its impact on humans (neqISO2204:1979)
GB/T14573.1-93 Acoustics - Statistical methods for determining and verifying the specified noise radiation values of machinery and equipment - Part 1: Overview and definitions (neq ISO7574.1:1985)
GB/T14573.4-93 Acoustics Statistical method for determination and verification of specified noise radiation values of machinery and equipment Part 4: Batch 46
GB/T 3768-1996
Method for determination and verification of machine plate values (neqISO7574.4:1985) GB/T14574-93 Acoustics. Noise plate for machinery and equipment (neqISO4871:1984) GB/T15173-94 Sound calibrator (eqvIEC 942:1988) GBJ47-83 Reverberation chamber method Sound absorption coefficient measurement specification IEC804-85 Integrating average sound level meter
GB/T 16538--1996
Sound pressure method Determination of sound power level of noise source Simple method using standard sound source (neq ISO3747—87)
GB/T 4129—1995
Standard for determination of sound power level of noise source Performance requirements and calibration of sound source. (egv ISO6929—1990)
GB/T 3767--1996
3 Definitions
Determination of sound power level of noise source by sound pressure method Engineering method of approximate free field above reflecting surface. (eqv ISO 3744—1994)
This standard adopts the following definitions. Other acoustic terms, quantities and units shall comply with the provisions of GB3947 and GB3102.7. 3.1 Time-averaged sound pressure level Lpea,r time-averaged sound pressure level Lpe The sound pressure level of a continuous steady-state sound, which has the same mean square sound pressure as the measured sound that varies with time during the measurement time interval T. It is also called equivalent continuous sound pressure level.
Lpeg,7(dB) =10lg
[100.1podt
Note 10: Time average sound pressure level - generally A-weighted, using LAa,r represents. 3.2 Single-event sound pressure level Lrlssingle-event sound pressure level Lp,1sThe time-integrated sound pressure level of an independent single event at a specified time interval T (or a specified measurement time T), T. Normalized to 1s, the formula is as follows:
pz(t)d
L(dB) =10lg[]
= Lpea.r + 10lg T.
3.3 Surface sound pressure level L, surface sound pressure level L, dB.
·（2）
The energy average of the time-averaged sound pressure levels at all microphone positions on the measurement surface plus the background noise correction K, and the environmental correction K2, 3.4 Measurement surface
measurement surface
An imaginary geometric surface that envelops the sound source and has an area S, on which the measurement points are located. The measurement surface terminates on one or more reflecting surfaces. 3.5 Frequency range of interest Generally speaking, the frequency range of interest includes the octave band with a center frequency of 125 to 8000 Hz. 3.6 Reference box
The smallest rectangular parallelepiped imaginary surface that just envelops the sound source and ends on one or more reflecting surfaces. 3.7 Characteristic source dimension d. Characteristic source dimension do Half the length of the diagonal of the box formed by the reference box and its virtual image in the adjacent reflecting surface. 3.8 Measurement distance d The vertical distance between the reference box and the box-shaped measurement surface. 3.9 Measurement radius r Measurement radius r47
The radius of the hemispherical measurement surface.
3.10 Background noise GB/T3768-1996
Noise from all other sound sources other than the sound source being measured. Note 11: Background noise includes airborne sound, structure-borne vibration, instrument electrical noise, etc. 3.11 Background noise correction K, background noise correction K, a correction term introduced by the effect of background noise on the surface sound pressure level, dB. K; related to frequency, in the case of A-weighting, it is represented by KiA.
3.12 Environmental correction K, environmental correction K2 A correction term introduced by the effect of sound reflection or sound absorption on the surface sound pressure level, dB. K; related to frequency, in the case of A-weighting, it is represented by K2A.
3.13 Impulsive noise index (impuisiveness) Impulsive noise index (impuisiveness) A quantity used to characterize the "impulsiveness" of the noise radiated by the sound source, dB. 4 Acoustic environment
4.1 General
The test environment applicable to this standard includes a room or outdoor flat open space that meets the requirements of 4.2, and the background noise should meet the requirements of 4.3. 4.2 Criteria for judging the suitability of the test environment
Appendix A (Appendix to the standard) gives the determination method of environmental correction K2A. This standard requires that the environmental correction K2A is less than or equal to 7dB. Note 12: If the environmental correction K2A is greater than 7dB, it is recommended to use the method of GB/T16538 or ISO9614. 4.3 Background noise standard
The background noise A-weighted sound pressure level averaged at the microphone position should be at least 3dB lower than the measured sound pressure level. 5 Measuring instruments
5.1 General
The instrument system including microphones and cables shall meet the requirements of Type 2 in GB3785. When using an integrating sound level meter, it shall meet the requirements of Type 2 in IEC804. The filters used shall meet the requirements of GB3241. 5.2 Calibration
Before and after each measurement, the entire measurement system shall be calibrated at one or more frequency points within the test frequency range using a sound calibrator with an accuracy better than ±0.3dB. The sound calibrator and measurement system shall be qualified by metrological verification every year. 5.3 Microphone windshield
When measuring outdoors, it is recommended to use a windshield to ensure that the measurement accuracy of the instrument is not affected by wind. 6 Installation and operating conditions of the sound source under test
6.1 General
The installation and operating conditions of the sound source under test have a great influence on the sound power radiation of the sound source. This chapter specifies the conditions to minimize the changes in the sound power level caused by installation and operation. If the sound source under test has a noise test specification, the relevant regulations should be followed. Especially for large sound sources, the noise test specification is more important. It lists in detail the components, assemblies, auxiliary equipment, power sources, etc. included in the reference body.
6.2 Location of the sound source Www.bzxZ.net
The sound source under test should be installed at one or more locations on the reflecting surface according to normal use conditions. If there are several possibilities or the typical installation conditions are unknown, special treatment should be made and stated in the test report. When positioning the sound source in the test environment, sufficient space needs to be left so that the measurement surface can meet the requirements of 7.1. 48
6.3 Installation of sound source
GB/T3768-1996
In many cases, the radiation of sound power is related to the support and installation conditions of the sound source under test. When the equipment under test has typical installation conditions, this condition should be used or simulated if feasible. If typical installation conditions are not available or cannot be used for testing, care should be taken to avoid changes in the sound power output of the sound source due to the installation system used for testing. And take measures to try to reduce the sound radiation of the equipment installation structure. Note
13 Many small sound sources, although they radiate little low-frequency sound themselves, after installation, their vibration energy is transmitted to a large enough surface, and then more low-frequency sound may be radiated from the surface. If possible, elastic support should be added between the equipment under test and its base surface to minimize the transmission of vibration to the base and the reaction force of the sound source. In this case, the mounting base should have a sufficiently high mechanical impedance to prevent additional sound radiation due to vibration. When the typical installation conditions of the sound source under test are not elastic, the above method should not be used. 14 Coupling conditions (e.g. between the prime mover and the driven machine) may also have a significant effect on the sound radiation of the measured sound source. 6.3.1 Handheld mechanical equipment
Such mechanical equipment shall be suspended or handheld. So that the structure does not transmit through any accessories that are not part of the measured sound source. If the measured sound source requires a support when working, the structure of this support shall be small and can be regarded as part of the sound source and described in the noise test specification. 6.3.2 Floor-mounted and wall-mounted mechanical equipment Such mechanical equipment shall be placed on a reflecting surface (floor, wall). When referring to a machine installed on the ground in front of a wall, the mechanical equipment shall be placed on the floor in front of the wall. Table equipment shall be placed on the floor, at least 1.5 meters away from any wall in the room, unless the test specification of the measured equipment stipulates that a workbench is required, in which case the equipment shall be placed in the center of the test table. 6.4 Auxiliary equipment
Any cable ducts, air pipe joints, etc. connected to the measured sound source shall not radiate significant sound to the test environment. All auxiliary equipment required for the operation of the sound source under test but not part of the sound source should be located outside the test environment. Otherwise, the auxiliary equipment should be included in the reference body and its operating conditions should be expressed in the test report. 6.5 Working conditions of the sound source during measurement
During measurement, if the mechanical equipment has noise test specifications, the working conditions specified in the corresponding test specifications shall be followed. Otherwise, the sound source works in a typical way in normal use, and the following working conditions should be selected: under the specified load and working conditions;
--full load,
...no load (no load)
--under working conditions corresponding to the maximum noise output in normal use; working with simulated load under set conditions,--under conditions with characteristic working cycle. The sound power level of the sound source can be measured under any desired working conditions (i.e. load, speed, temperature, etc.). The test conditions should be selected in advance and kept constant during the test. Before the measurement begins, the sound source should be under the required working conditions. If the noise radiation is related to secondary working parameters, such as the type of material being processed or the model of the tool used, appropriate parameters should be selected as much as possible to minimize the change in sound radiation and be in a typical working state. For a specific mechanical equipment, the noise test specifications should provide detailed descriptions of the tools and materials used during the test. For special purpose tests, one or more operating conditions may be appropriately set. It may allow for a high reproducibility of the noise radiation of machines of the same type and it may be the most common and typical for the type of machinery involved. Such operating conditions shall be set in the specific noise test specification.
If simulated operating conditions are used, they shall be chosen so that the source under test is in normal use and the radiated sound power is maximum. For cases where the operating conditions are divided into several, each condition lasts for a defined time interval, if appropriate, and the results of the entire operating procedure are combined using an energy-homogeneous method.
The operating conditions of the source during the acoustic measurement shall be described in detail in the test report. 49
7 Measurement of sound pressure levels
7.1 Selection of the measuring surface
GB/T3768--1996
In order to facilitate the positioning of the microphone on the measuring surface, a base body shall be set. When setting the base body, units protruding from the source but not radiating significant sound energy may be disregarded. For different types of equipment, the protruding units shall be noted in the specific noise test specification. The position of the sound source under test, the measurement surface and the microphone position are all set by a coordinate system. The X and Y axes of the coordinate system are located on the ground and are parallel to the length and width of the reference body, with a specific sound source dimension d. As shown in Figure 1. The measurement surface can be of the following two shapes: a) a hemispherical or partially hemispherical surface with a radius of r; b) a rectangular parallelepiped surface with each side parallel to the corresponding reference body. For sound sources located in rooms or spaces with unfavorable acoustic conditions (e.g. many reflectors, high background noise), a smaller measurement distance can be selected. A parallelepiped measurement surface is generally specified. Sound sources are often installed or tested in large outdoor spaces with satisfactory acoustic conditions, and a larger measurement distance is generally selected. A hemispherical measurement surface is preferred. Directivity measurements require a hemispherical or partially hemispherical measurement surface. Measurements of a series of similar sound sources (e.g. machines of the same model or the same type) require the use of a measurement surface of the same shape. Note 15: For detailed information, refer to the special noise test specification for the sound source under study. The composition of the reference body, the shape and size of the measuring surface, and the measuring distance d or the radius of the hemisphere shall be described in the test report.
7.2 Hemispherical measuring surface
The center of the hemisphere is located at the center of the box formed by the reference body and its virtual image in the adjacent reflecting surface (origin Q in Figure 1). The radius r of the hemispherical measuring surface shall be greater than or equal to twice the size d of the characteristic sound source and shall not be less than 1m. The hemispherical radius shall be one of the following values (m): 1, 2, 4, 6, 8, 10, 12, 14, 16. When the radius is too large, the environmental conditions of Appendix A (Standard Appendix) are difficult to meet, and these radii should not be used. 7.2.1 Area of the hemispherical measuring surface and basic microphone position When there is only one reflecting surface, the area of the imaginary hemispherical surface where the microphone position is located is S=2 yuan2. When the sound source to be measured is located in front of a wall, S=2 yuan2. If it is located in a corner, S=0.5 yuanr2. The microphone positions on the hemispherical surface are shown in Figures B1 and B2 of Appendix B (Standard Appendix). Figure B1 shows four basic microphone positions, which are connected with equal areas on the hemispherical surface of radius r. If the sound source is placed close to more than one reflecting surface, the appropriate measurement surface and microphone positions should be set with reference to Figure B3 of Appendix B (Standard Appendix).
In special cases (i.e. for special types of machines, such as construction equipment or earth-moving machinery, which are measured in a moving state or driven mode), different numbers of microphones and arrays can be used, but it must be proved by preliminary investigation that the sound power level value deviates less than 1dB from the value measured using the given array in this standard.
7.2.2 Additional microphone positions on the hemispherical measurement surface Additional microphone positions are required on the measurement surface in the following cases: a) the range of sound pressure level values measured at the basic microphone positions (i.e. the difference in dB between the highest and lowest sound pressure levels) exceeds twice the number of basic measurement points;
b) the noise radiated by the sound source is highly directional; c) a loud source whose noise is radiated outward only through a small local area of the sound source, such as an opening in a machine that is enclosed around it.
For a hemispherical surface microphone array, the additional four measurement points are determined by rotating the original array of Figure B1 by 180° about the Z axis (see Table B1 and Figure B2). Note that the vertex on the Z axis of the new array coincides with the vertex of the original array, so that the total number of microphone positions increases from 4 to 7. For conditions b) or c), use additional microphone positions in areas of high noise radiation on the measurement surface (see 7.4.1). 7.3 Parallel hexagonal measuring surface
The measuring distance d is preferably 1m, at least 0.15m. d should be selected from one of the following values (m): 0.15, 0.25, 0.5, 1, 2, 4, 8. The measuring distance of the sound source should be greater than 1m. When selecting cl. 50
GB/T 3768--1996
First, the environmental requirements given in Appendix A (Annex of the standard) should be met. do= (1/2)*+(12/2)2+
a) Reference body on one reflection plane
do= V(/2)2++Number
b) Reference body on two reflection planes
d=V++Road
Reference body on three reflection planes
Figure 1 Reference body and characteristic sound source size d. Example 7.3.1. Area of parallelepiped measurement surface and microphone position. The measurement surface where the microphone is located is an imaginary surface with an area of S, enveloping the sound source, with each side parallel to the side of the reference body, and a distance d (measurement distance) from the reference body. The microphone positions on the parallelepiped measurement surface are shown in Figures C1 to C8 of Appendix C (Standard Appendix). According to Figures C1 to C6, the area S of the measurement surface is given by formula (3): S = 4(ab + ac + bc)
a=0.5l1+d;b=0.512+d;c=l3+d,
GB/T3768—1996
Where: 1, 2, —— are the length, width, and height of the reference body, respectively. If the sound source is placed close to more than one reflecting surface, an appropriate measurement surface should be set with reference to Figures C7 and C8 of Appendix C (Standard Appendix). The calculation of the measuring surface area S in this case is given in the respective figures. The microphone positions are arranged according to Figures C1 to C6. 7.3.2 Additional microphone positions on the parallelepiped measuring surface Additional microphone positions on the parallelepiped measuring surface are required in the following cases: a) the range of sound pressure level values (dB difference between the highest and lowest sound pressure levels) measured at the basic microphone positions exceeds twice the number of measuring points; b) the noise radiated by the sound source is highly directional; c) a loud sound source, the noise of which is radiated outward only through a small local part of the sound source, such as an opening in a machine that is enclosed around it.
For a), the method of increasing the number of measuring points is as shown in Figure C1 of Annex C (normative annex) by increasing the number of rectangular unit areas of equal size.
For b) or c), additional microphone positions are used in areas of high noise radiation on the measuring surface (see 7.4.1). 7.4 Additional method for selecting microphone positions
7.4.1 Additional microphone positions on the measurement surface according to 7.2.2 or 7.3.2. If additional microphone positions are required, a detailed investigation of the sound pressure level on a limited portion of the measurement surface must be carried out. The purpose of the investigation is to determine the highest and lowest sound pressure levels within the test frequency range. The additional microphone positions are not connected to the measurement surface with equal areas. In this case, the calculation procedure of GB6882 (non-equal area) should be used to determine Lw. 7.4.2 Reducing microphone positions
For special types of machines, if the investigation shows that the surface sound pressure level measured after the number of microphone positions is reduced is consistent with the surface sound pressure level measured according to 7.2 and 7.3 If the results measured at all microphone positions do not differ by more than ±1 dB, the number of microphone positions may be reduced. An example is a sound source with a symmetrical radiation pattern. Note 16: For safety reasons, the measurement point on the top of the sound source may be omitted. This shall be noted in the corresponding noise test specification. 7.5 Measurement
7.5.1 Environmental conditions
When environmental conditions have an impact on the measurement microphone (such as strong electricity, magnetic fields, wind, impact of air discharge of the equipment under test, high temperature or low temperature), the microphone should be appropriately selected or positioned to avoid it. The adverse environmental conditions indicated in the instruction manual of the measuring instrument should also be noted. 7.5.2 Measuring instrument
In addition to the provisions given in Chapter 5, the following requirements shall also be met. a) The orientation of the microphone shall be the same as the angle of incidence of the sound wave when it is calibrated. b) The time-averaged sound pressure level shall be measured using an integrating sound level meter that meets the requirements. When the sound pressure level fluctuation measured with the time characteristic s is less than ±1 dB, a sound level meter that meets the requirements is allowed. In the latter case, the average value of the maximum and minimum sound pressure levels during the measurement period is used to represent the time-averaged sound pressure level.
7.5.3 Measurement method
Observe the A-weighted sound pressure level during the typical period of the sound source's operation and read the A-weighted sound pressure level at each microphone position. Determine the following quantities:
a) The A-weighted sound pressure level L'pA during the operation of the sound source under test; b) The A-weighted sound pressure level L\pA of the background noise. Unless otherwise specified in the special noise test specification, the observation period should be at least 30s. For independent single sound events, the single event sound pressure level Lp is determined.18 For noise that changes with time, the observation period should be carefully specified. This is often related to the purpose of the measurement. For machines whose noise level changes with the change of working mode, an appropriate measurement period should be selected for each working mode and noted in the test report. 8 Calculation of A-weighted surface sound pressure level and A-weighted sound power level 8.1 Calculation of the average A-weighted sound pressure level of the measured surface 52
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