Detail of GB/T 3768-1996

Introduction of GB/T 3768-1996

1.1 General This Standard specifies a method for measuring the sound pressure levels on a measurement surface enveloping the source in order to calculate the sound power level produced by the noise source. It gives requirements for the test environment and instrumentation as well as techniques for obtaining the surface sound pressure level from which the sound power level of the source is calculated, leading to results which have grade 3 accuracy. It is important that specific noise test codes for various types of equipment be established and used in accordance with this International Standard. For each type of equipment, such noise test codes will give detailed requirements on mounting, loading and operating conditions for the equipment under test as well as a selection of the measurement surface and the microphone array as specified in this standard. Note 1: The noise test code for a particular type of equipment should give detailed information on the particular surface that is selected, as the use of differently shaped measurement surfaces may yield differing estimates of the sound power level of a source. 1.2 Types of noise and noise sources The methods specified in this Standard are suitable for all types of noise. Note 2: noise classification (steady, non-steady, fluctuating, etc.), is detailed ion GB/T 14259. This Standard is applicable to all types and sizes of noise source (e.g. installation, machine and component). This Standard is not applicable to very tall or very long sources such as chimneys, ducts, conveyors and multi-source industrial installation. 1.3 Test environment The test environments that are applicable for measurements made in accordance with this Standard may be located indoors or outdoors, with one or more sound-reflecting planes. 1.4 Measurement uncertainty For sources which radiate steady broad-band noise, determinations made in accordance with this standard result, with few exceptions, in standard deviations of reproducibility of the A-weighted sound power level equal to or less than 3 dB (K2A<5dB) or 4dB (5dB≤K2A≤7dB). For discrete-tone sources, the standard deviation of reproducibility is normally 1 dB larger (see Table 1). A single value of the sound power level of a noise source determined in accordance with the procedures given in this International Standard is likely to differ from the true value by an amount within the range of the measurement uncertainty. The uncertainty in determinations of the sound power level arises from several factors which affect the results, some associated with environmental conditions at the test site and others with experimental techniques. If a particular noise source were to be transported to each of a number of different test sites, and if, at each test site, the sound power level of that source were to be determined in accordance with this standard, the results would show a scatter. The standard deviation of the measured levels could be calculated (see examples in GB/T 14573.). With few exceptions, these standard deviations would not exceed those listed in table 1. The values standard deviations of reproducibility σR are given in table 1. The values of table 1 take into account the cumulative effects of measurement uncertainty in applying the procedures of this standard, but exclude variations in the sound power output caused by changes in operating conditions (e.g. rotational speed, power and voltage) or mounting conditions. The measurement uncertainty depends on the standard deviation of reproducibility tabulated in table 1 and on the degree of confidence that is desired. As examples, for a normal distribution of sound power levels, there is a 90 % confidence that the expected value of the sound power level of a source lies within the range ±1.6560σR of the measured value and a 95% confidence that it lies within the range ±1.960σR of the measured value. For further examples, reference should be made to GB/T 1457 and ISO 9296. Table 1: Upper values of the standard deviations of reproducibility of A-weighted sound power levels Application Highest standard deviation of reproducibility, σR dB For a source which emits noise with a relatively "flat" spectrum over the frequency range of interest 3 For a source which emits noise that contains predominant discrete tones 4 Notes: 3 If K2A is larger than or equal to 5 dB, σR may be 1 dB larger than the values given in Table 1. 4 A noise test code for a particular type of sound sources may have lower values of the standard deviation of reproducibility. 5 The standard deviations listed in table1 are associated with the test conditions and procedures defined in this International Standard and not with the noise source itself. They arise in part from variations between test sites, changes in atmospheric conditions if outdoors, the geometry of the test room or outdoor environment, the acoustical properties of the reflecting plane, absorption at the test room boundaries if indoors, background noise, and the type and calibration of instrumentation. They are also due to variations in experimental techniques, including the size and shape of the measurement surface, number and location of microphone positions, sound source location, integration times, and determination of environmental corrections, if any. The standard deviations are also affected by errors associated with measurements taken in the near field of the source; such errors depend upon the nature of the sound source, but generally increase for smaller measurement distances and lower frequencies (below 250 Hz). 6 If measurements are made at several test sites, the results of sound power determinations on a given source may be in better agreement than would be implied by the standard deviations of table 1. 7 For a particular family of sound sources, a similar size with similar sound power spectra and similar operating conditions, the standard deviations of reproducibility may be smaller than the values given in table I. A noise test code for a particular type of machinery or equipment making reference to this standard may state standard deviations smaller than those listed in Table 1, if substantiation is available from the result of suitable laboratory tests. 8 The standard deviations of reproducibility, as tabulated in table 1, include the uncertainty associated with repeated measurements on the same noise under the same conditions (standard deviation of repeatability). This uncertainty is usually much smaller than the uncertainty associated with variability from one test site to another. However, if it is difficult to maintain stable operating or mounting conditions for a particular source, the standard deviation of repeatability may not be small compared with the values given in table 1. In such cases, the fact that it was difficult to obtain repeatable sound power level data on the source should be recorded and stated in the test report. 9 The procedures of this standard and the standard deviations given in table 1 are applicable to measurements on an individual machine. Characterization of the sound power levels of batches of machines of the same family or type involves the use of random sampling techniques in which confidence intervals are specified, and the results are expressed in terms of statistical upper limits. In applying these techniques, the total standard deviation must be known or estimated, including the standard deviation of production, as defined in GB/T 14573, which is a measure of the variation in sound power output between individual machines within the batch. Statistical methods for the characterization of batches of machines are described in GB/T 14573.

Contents of GB/T 3768-1996

1 Scope 1.1 General 1.2 Types of noise and noise sources 1.3 Test environment 1.4 Measurement uncertainty 2 Normative References 3 Definitions 4 Acoustic Environment 4.1 General 4.2 Criterion for acoustic adequacy of test environment 4.3 Criterion for background noise 5 Instrumentation 5.1 General 5.2 Calibration 5.3 Microphone windscreen 6 Installation and operation conditions of noise source under test 6.1 General 6.2 Noise source location 6.3 Noise source installation and mounting 6.4 Auxiliary equipment 6.5 Operation of source during test 7 Measurement of sound pressure levels 7.1 Selection of the measurement surface 7.2 Hemispherical measurement surface 7.3 Parallelepiped measurement surface 7.4 Additional procedures for selection of microphone positions 7.5 Measurement 8 Calculation of A-weighted surface sound pressure level and A-weighted sound power level 8.1 Calculation of a-weighted sound pressure level averaged over the measurement surface 8.2 Corrections for background noise 8.3 Corrections for test environment 8.4 Calculation of the A-weighted surface sound pressure level 8.5 Calculation of the A-weighted sound power level 9 Information to be recorded 9.1 Noise source under test 9.2 Test conditions 9.3 Acoustic environment 9.4 Instrumentation 9.5 Acoustical data 9.6 Optional data 10 Information to be reported Annex A Annex B Annex C Annex D Annex E