Surface integration of the intensity component normal to the measurement surface is approximated by subdividing the measurement surface into contiguous partial surfaces, and scanning the intensity probe over each partial surface along a continuous path which covers the extent of the partial surface. The measurement instrument determines the averaged normal intensity component and averaged squared sound pressure over the duration of each scan. The scanning operation can be performed either manually or by means of a mechanical system.
The octave band or band-limited weighted sound power level is calculated from the measured one-third-octave- band values. The method is applicable to any source for which a physically stationary measurement surface can be defined, and on which the sound generated by the source under test and by other significant extraneous sources are stationary in time. The source is defined by the choice of measurement surface. The method is applicable in specific test environments fulfilling all relevant requirements of ISO 9614-3.
ISO 9614-3 specifies certain ancillary procedures, described in annex C, to be followed in conjunction with the sound power determination. The results are used to indicate the quality of the determination, and hence the grade of accuracy. If the quality of the determination does not meet the requirements of ISO 9614-3, the test procedure shall be modified in the manner indicated.
ISO 9614-3 is not applicable to any frequency band in which the sound power of the source is found to be negative on measurement.
It is applicable to sources situated in any environment which is neither so variable over time as to reduce the accuracy of the measurement of sound intensity to an unacceptable degree, nor subjects the intensity measurement probe to gas flows of unacceptable speed or unsteadiness.
In some cases it will be found that the test conditions are too adverse to allow the requirements of ISO 9614-3 to be met. For example, extraneous noise levels can exceed the dynamic capability of the measuring instrument or can vary to an excessive degree during the test. In such cases the method given in ISO 9614-3 is not suitable for the determination of the sound power level of the source.