ISO 6926:2016 specifies the acoustical performance requirements for reference sound sources:- temporal steadiness (stability) of the sound power output;- spectral characteristics;- directivity.Temporal steadiness is defined in terms of the standard deviation of repeatability (see 5.2). The spectral characteristics can be verified in either a hemi-anechoic room or a reverberation test room from measurements of the frequency band sound power levels in accordance with this International Standard (see 5.4). The performance requirements on directivity index can only be verified in a hemi-anechoic room (see 5.5.)ISO 6926:2016 also specifies procedures for providing level calibration data and uncertainty on a sound source intended for use as a reference sound source in terms of its sound power level under reference meteorological conditions as defined in Clause 4 in octave and in one-third-octave bands, and with frequency weighting A.ISO 6926:2016 is titled as a calibration standard even though the method is conducted in a testing laboratory and the level calibration results are not directly traceable to national standards of measure in a strict metrological sense. Testing laboratories performing this method are not expected to meet all requirements normally associated with a calibration laboratory.NOTE ISO/IEC 17025[15] specifies different requirements for the competence of testing laboratories and calibration laboratories respectively. Laboratories testing reference sound sources in accordance with this International Standard would typically comply with the requirements for testing laboratories but not necessarily with those for calibration laboratories.ISO 6926:2016 specifies methods to calibrate reference sound sources not only in a free field over a reflecting plane but also in reverberation test rooms at different distances from the boundary surfaces. For the position of the reference sound source on one reflecting plane, the two different test environments mentioned above are considered equivalent for frequency bands above or equal to 200 Hz. At 160 Hz and below, some systematic differences can occur (see 11.2). For frequencies below 100 Hz, an alternative calibration method using sound intensity is given.The sound source can either be placed directly on the floor or mounted on a stand to be used at a certain elevation above the floor. According to this International Standard, stand-mounted sources are calibrated in reverberation test rooms. Floor-mounted sources are either calibrated in hemi-anechoic or in reverberation test rooms. For floor-mounted sources in hemi-anechoic rooms, this International Standard is valid only for sources whose maximum vertical dimension is less than 0,5 m and whose maximum horizontal dimension is less than 0,8 m. According to this International Standard, only floor-mounted reference sound sources can be used when carrying out measurements on a measurement surface. For reference sound sources to be used or calibrated under reverberant conditions, no such restrictions on maximum dimensions apply.