ANSI S12.53.2-1999 pdf download Acoustics – Determination of sound power levels of noise sources using sound pressure – Engineering methods for small, movable sources in reverberant fields = Part 2: Methods for special reverberation test rooms (A Nationally Adopted International Standard)
The maximum size of the source under test and the lower limit of the frequency range for which the methods are applicable depend upon the size of the room used for the acoustical measurements. The volume of the noise sources should not exceed 1 % of the volume of the special reverberation test room. For the minimum test room volume of 70 m3, the recommended maximum size of the source is 0,7 m3. Measurements on sources emitting discrete- frequency components below 200 Hz are frequently difficult to make in such small rooms.
1.4 Measurement uncertainty
Determinations made in accordance with this part of I S0 3743 result, with few exceptions, in standard deviations of reproducibility equal to or less than 2.0 dB from 500 Hz to 4 O00 Hz, 3,O dB for 250 Hz and 8 O00 Hz, and 5, O di3 for 125 Hz (see table 1 ).
A single value of the sound power level of a noise source determined according to the procedures of this part of I S0 3743 is likely to differ from the true value by an amount within the range of the meacurement uncertainty. The uncertainty in determinations of the sound power level arises from several factors which affect the results, some associated with environmental conditions in the measurement laboratory and others with experimental techniques.
If a particular noise source were to be transported to each of a number of different laboratories, and if, at each laboratory, the sound power level of that source were to be determined in accordance with this part of ISO 3743, the results would show a scatter. The standard deviation of the measured levels could be calculated (see examples in ISO 7574-4:1985. annex B) and would vary with frequency. With few exceptions, these standard deviations would not ex- ceed those listed in table1. The values given in table 1 are standard deviations of reproducibility, σA, as defined in ISO 7574-1. The values of table 1 take into account the cumulative effects of measurement uncertainty in applying the procedures of this part of ISO 3743, but exclude variations in the sound power output caused by changes in operating conditions (e.g. rotational speed, line 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 true value of the sound power level of a source lies within the range 1 . 6 4 5 ~ ~ of the measured value and a 95 % confidence that it lies within the range f 1 ,960~ of the measured value. For further examples, reference may be made to the IS0 9296 and IS0 7574 series.