GB/T 3767-1996 Acoustics—Determination of sound power levels of noise sources using sound pressure—Engineering merthod in an essentially free field over a reflecting plane
Acoustics—Determination of sound power levels of noise sources using sound pressure—Engineering merthod in an essentially free field over a reflecting plane
1.1 General provisions
This standard specifies the method of measuring the sound pressure level and calculating the sound power level of noise source on the measurement surface that envelopes the sound source at the essentially free-field near to one or more reflecting planes. Meanwhile, this standard also specifies the measuring environment, the requirements on measuring equipment, as well as the calculation methods of surface sound pressure levels and sound power levels. The accuracy rate of the measurement result of sound power level is of grade 2.
As for various types of equipments, it is very important to develop and apply the dedicated noise test specifications according to this standard. In the noise test specifications, detailed explanations on the selection of the installation, load, working conditions, and measurement surface of to-be-measured sound sources as well as the armament of microphones shall be specified.
Note 1: As for special type of equipment, its noise test specification shall specify the particulars of the selected specific measurement surface, because adopting the measurement surfaces of different forms may result in different evaluations on the sound power levels of sound sources.
1.2 Noise types and noise sources
The method specified in this standard is applicable to measuring various types of noises.
Note 2: See GB/T 14529 for the classification of noises (steady state, unsteady state, quasi-steady state and pulse).
This standard is applicable to the sound sources of various types and dimensions (such as equipments, machines, parts and components).
This standard is not applicable to the super-elevated or overlong sound sources, for example, the chimneys, pipes, conveying machines and the industrial equipments of multi-sound sources.
1.3 Measuring environment
This standard is applicable to the measuring environment at the essentially free field near to one or more indoor or outdoor reflecting planes.
1.4 Uncertainty of measurement
As for measurement result this standard, the standard deviation in the repeatability of the A-weighted sound power level shall be equal to or less than 1.5 dB (see Table 1), except for the exceptional cases.
The single sound power level value of the noise source measured according to this standard most likely may have one difference with its actual value within an uncertain range. The uncertainty of the measurement of sound power level is derived from the combined influence of the environmental condition and experimental technique in the laboratory that is adopted for measurement.
If the sound power level of one specific noise source is according to this standard in different laboratories, the measurement result may have discreteness. The calculation of the standard deviation of measurement result shall be made reference to GB/T 14573.4 and is related to the frequency. Except for the exceptional cases, the aforementioned standard deviation shall not exceed the values specified in Table 1. The standard deviation in repeatability specified in Table 1 is considered with the cumulative effect of uncertainty during the measuring process, however, the variation in the sound power output caused by the working conditions (rotation speed, power supply and voltage) or the variation in installation conditions are excluded.
The uncertainty of measurement is not only related to the standard deviation in repeatability, but also is related to the required confidence factor. For example, as for the sound power level of normal distribution, the actual value of the sound power level of sound source is within measured value ±1.645 when the confidence factor is 90%, and the actual value is within measured value ±1.960 when the confidence factor is 95%. Please see GB/T 14573 series and ISO 9296.
Table 1 Octave Band Center Frequency of the Estimated Value of Standard Deviation in Repeatability of Sound Power Level Measured According to this Standard
Center frequency of octave band
Hz Center frequency of 1/3 octave band
Hz Standard deviation of repeatability
dB
63 50~80 51)
125 100~160 3
250 200~315 2
500~4 000 400~5 000 1.5
8000 6300~10000 2.5
A-weighted 1 52)
1) It is generally applicable to the outdoor measurement, and many rooms can not meet this band.
2) It is applicable to the sound source with relatively "even" radiated noise spectra within 100~10000 Hz.
Notes
3 The standard deviation listed in Table 1 is the combined effect of the measuring condition and method specified in this standard but not include the influence of sound source itself. The standard deviation is caused by the following aspects: Variance in the measurement places, including the outdoor environment and climatic conditions, geometric shape of indoor room, sound absorption at the room boundary, and acoustic characteristics at the reflecting plane; background noise; calibration form of equipment; variance in experimental technique, including the shape and size of measurement surface, quantity of measuring points, orientation of microphone, position of sound source, integration time, and the correctional measurement of environment (if any). The standard deviation also is influenced by the error caused by the near field measurement. This error is related to the characteristics of sound source. Generally, this error will increase when the measuring distance and frequency are small (bellow 250 Hz).
4 If applying similar instruments in several laboratories, the consistency in the measurement results of the sound power levels measured in these laboratories may be better than the consistency embodied by the standard deviation in Table 1.
5 As for such specific types of sound sources that have similar size, similar power sound spectrum and similar working conditions, the standard deviation of in their repeatability may be less than the value specified in Table 1. When developing the noise test specifications according to this standard, the standard deviation smaller than the value listed in Table 1 may be marked out in the noise test specification if it is proved to be feasible through laboratory verification.
6 The standard deviation of repeatability listed in Table 1 includes the uncertainty of replicate measurement on one same noise source at identical conditions (the standard deviation in repetitiveness), and this uncertainty generally is much less than the uncertainty caused by the changing of laboratories. As for special sound sources, the standard deviation in repetitiveness may not be less than the value listed in Table 1 if it is difficult to keep stable working conditions and installation conditions, so that the reproducible sound power level will be difficult to obtain at such case, such condition shall be recorded and be explained in the test report.
7 The methods in this standard and the standard deviations specified in Table 1 are applicable to the measurement of single machine. As for the bulk of machines of one same type, the representation of the characteristics of their sound power levels is concerned in the random sampling technique within a specified confidence interval. And the result shall be represented in the upper limit of statistics. In the application of these techniques, the total uncertainty must be given or estimated, including the standard deviation of products as specified in GB/T 14573.2, which is the deviation among the sound power outputs of single machines in a batch of machines. See GB/T 14573.4 for the statistical method of batch characteristics of machines.
