BS EN ISO 6926:2016 pdf download – Acoustics — Requirements for the performance and calibration of reference sound sources used for the determination of sound power levels
It may be desirable for special sound sources to meet these criteria over a more limited frequency range or for a different spectrum shape. However, if a RSS does not comply with the requirements of this International Standard over at least the frequency range from 100 Hz to 10 000 Hz, it shall not be labelled or declared as being in compliance with the frequency range of this International Standard.
5.5 Directivity The highest value of the directivity index of the source in any one-third-octave band with midband frequency between 100 Hz and 10 000 Hz shall not exceed +6 dB when measured in a hemi-anechoic room complying with Clause 8. If the RSS is to be used exclusively in reverberation test rooms complying with ISO 3741, these requirements do not apply but in that case, the RSS shall be labelled “For use as a reference sound source in reverberation test rooms complying with ISO 3741”.
5.6.1 Whenever any mechanical damage has been inflicted on the reference sound source, it shall be recalibrated. For aerodynamic sound sources, measure the rotational speed (RPM) of the fanwheel. If the present RPM after a few minutes stabilization deviates less than 1% from that found during the last calibration, then the RSS is not electrically damaged.
5.6.2 The user manual for the reference sound source should provide the recommended maximum time interval between successive calibrations such that changes in the sound power levels of the specified model of the reference sound source is not expected to exceed 2,83 times the values in Table 1 (presumably from qualified historical tracking by the manufacturer). If this time interval is provided, it shall be considered a requirement of this International Standard. If it is not provided in the user manual, the maximum time interval shall be two years. NOTE If the difference between two consecutive calibrations is less than 2,83 times the standard deviation, there is, e.g. see ISO 5725-2  , a probability of 95 % that the temporal stability of the RSS complies with the values given in Table 1.
5.6.3 In order to determine whether or not recalibration of a reference sound source is necessary before the maximum time interval has expired, one-third-octave-band sound pressure levels shall be measured regularly at one or more fixed reference points with the source operating at a specific location in a specified test environment as a check of its temporal stability. The user manual for the reference sound source should specify the time interval between such regular measurements. If this time interval is provided, it shall be considered a requirement of this International Standard. If it is not provided in the user manual, the time interval shall be a maximum of six months. If, after using manufacturer-specified procedures to adjust the measured sound pressure levels to constant environmental conditions when necessary, changes in any one-third-octave-band sound pressure level between successive checks exceed 2,83 times the values in Table 1, the reference sound source shall be recalibrated.
NOTE If the above check indicates the need for recalibration, confidence may be diminished in any data that may have been taken with the reference sound source during the intervening time interval. In view of this, the laboratory may decide on a much shorter time interval, or select a variable time interval based on the number of tests it performs.
5.6.4 For laboratories using a reference sound source in a generally fixed location, in a controlled environment without transporting it, calibration intervals may be extended beyond the maximum time interval in 5.6.2 if changes in the surface time average band sound pressure levels measured at a maximum of one-year intervals over positions 1 to 10 of ISO 3744:2010, Table B.1, using a fixed measurement radius of at least 1 m, in an environment that has been qualified for broadband sound power level determination in accordance with ISO 3745:2012, Annex A do not exceed 2,83 times the values in Table 1. Alternatively, an equivalent test may be carried out with the RSS in one specified location in a reverberation room following the procedures specified for measurements of sound pressure levels from a RSS in ISO 3741. For this measurement, the reference sound source shall each time be measured at the same temperature.BS EN ISO 6926 pdf download.