1.1 This International Standard specifies methods for measuring the sound pressure levels on a measurement surface enveloping a source, and for calculating the sound power level produced by the source. It gives requirements for the test environment and instrumentation, as well as techniques for obtaining the surface sound pressure level from which the A-weighted sound power level of the source and octave or one-third-octave band sound power levels are calculated. This method may be used to perform acceptance tests. 1.2 The aim of this International Standard is a grade 2 (engineering) result (see table 1). When the correction for background noise exceeds the limit of 1,3 dB but is less than 3 dB, and/or the correction for environment exceeds the limits of 2 dB but is less than 7 dB, then a grade 3 (survey) result is obtained. 1.3 This International Standard applies to gas turbines and gas turbine sets — for industrial applications (e.g. stationary), — for installation on board ships, or offshore installations, road and railway vehicles. It does not apply to gas turbines in aircraft applications. 1.4 The methods defined in this International Standard apply to the measurement of the noise emission of a gas turbine or gas turbine set under steady-state operating conditions. The results are expressed as sound pressure levels, and sound power levels in Aweighted and in octave bands. 1.5 Measurements made in conformity with this International Standard should result in standard deviations which are equal to or less than those given in table 3. The uncertainties in table 3 depend not only on the accuracies with which sound pressure levels and measurement surface areas are determined, but also on the"near-field error" which increases for smaller measurement distances and lower frequencies (i.e. those below 250 Hz). The near-field error always leads to sound power levels which are higher than the real sound power levels. NOTES 1 If the methods specified in this International Standard are used to compare the sound power levels of similar machines that are omnidirectional and radiate broad-band noise, the uncertainty in this comparison tends to result in standard deviations which are less than those given in table 3, provided that the measurements are performed in the same environment with the same shape of measurement surface. 2 The standard deviations given in table 3 reflect the cumulative effects of all causes of measurement uncertainty, excluding variations in the sound power levels from test to test which may be caused, for example, by changes in the mounting or operating conditions of the source. The reproducibility and repeatability of the test result may be considerably better (i.e. smaller standard deviations) than the uncertainties given in table 3 would indicate. Table 1 — International Standards specifying various methods for determining the sound power levels of machines and equipment Inter-nationalStandard Classificationof method1) Testenvironment2) Volume ofsource Character ofnoise Sound powerlevelsobtainable Optionalinformationavailable 1) See ISO 2204. 2) If the requirements for the test environment are not met, the sound power level of the source shall be determined using another method of measurement that shall be agreed to by the manufacturer and the customer. ISO 3741 Precision (grade 1) room meeting specified re-quirements Preferably less than 1 % of test room volume Steady, band broadReverberation In one-third- octave or octave bands A-weighted sound power level ISO 3742 Steady, discrete- frequency or narrow-band ISO 3743 Engineering (grade 2) Special rever- beration test room band, narrow- Steady, band or discrete- frequency A-weighted broadISO and in octave bands Other weighted sound power levels ISO 3744 Engineering 2) Outdoors or in large room Greatest sion less than 15 m dimenISO Any A-weighted and in one- third-octave or octave bands Directivity infor(grade mation; sound pressure levels as a function of time; other weighted sound power levels ISO 3745 Precision (grade 1) Anechoic or semi-anechoic room Preferably less than 0,5 % of test room vol- ume Any ISO 3746 Survey (grade 3) No special test environment No restrictions: limited only by available test environment Any A-weighted Sound pressure levels as a function of time; other weighted sound power levels Table 2 — Limits for correction Grade of accuracy Background noise correction Environment correction dB dB 1) For higher values of background noise and/or corrections, the real sound power level cannot be determined with acceptable uncertainty, but the results can be useful to estimate an upper limit of the noise emission of the gas turbine or the gas turbine set to be tested. Grade 2 ≤ 1,3 ≤ 2 Grade 3 > 1,3 to ≤ 3 > 2 to ≤ 7 Special case1) > 3 > 7 Table 3 — Uncertainty in determining sound power levels, expressed as the largest value of the standard deviation Grade of accuracy Octave band centre frequency A-weighted 31,5 Hz/63 Hz 125 Hz 250 Hz to 500 Hz 1 000 Hz to 4 000 Hz 8 000 Hz Grade 2 5 3 2 1,5 2,5 2 Grade 3 5 NOTE — The uncertainty in determining Sound power levels expressed as the largest value of the Standard deviation for air intake inlet and gas exhaust outlet may be higher.